Active Record Validations

1. Validations Overview

Here's an example of a very simple validation:

class Person < ApplicationRecord
  validates :name, presence: true
end

irb> Person.new(name: "John Doe").valid?
=> true
irb> Person.new(name: nil).valid?
=> false

As you can see, the Person is not valid without a name attribute.

Before we dig into more details, let's talk about how validations fit into the big picture of your application.

1.1. Why Use Validations?

Validations are used to ensure that only valid data is saved into your database. For example, it may be important to your application to ensure that every user provides a valid email address and mailing address. Model-level validations are the best way to ensure that only valid data is saved into your database. They can be used with any database, cannot be bypassed by end users, and are convenient to test and maintain. Rails provides built-in helpers for common needs, and allows you to create your own validation methods as well.

1.2. Alternate Ways to Validate

There are several other ways to validate data before it is saved into your database, including native database constraints, client-side validations and controller-level validations. Here's a summary of the pros and cons:

• Database constraints and/or stored procedures make the validation mechanisms database-dependent and can make testing and maintenance more difficult. However, if your database is used by other applications, it may be a good idea to use some constraints at the database level. Additionally, database-level validations can safely handle some things (such as uniqueness in heavily-used tables) that can be difficult to implement otherwise.
• Client-side validations can be useful, but are generally unreliable if used alone. If they are implemented using JavaScript, they may be bypassed if JavaScript is turned off in the user's browser. However, if combined with other techniques, client-side validation can be a convenient way to provide users with immediate feedback as they use your site.
• Controller-level validations can be tempting to use, but often become unwieldy and difficult to test and maintain. Whenever possible, it's a good idea to keep your controllers simple, as it will make working with your application easier in the long run.

Rails recommends using model-level validations in most circumstances, however there may be specific cases where you want to complement them with alternate validations.

1.3. Validation Triggers

There are two kinds of Active Record objects - those that correspond to a row inside your database and those that do not. When you instantiate a new object, using the new method, the object does not get saved in the database as yet. Once you call save on that object then will it be saved into the appropriate database table. Active Record uses an instance method called persisted? (and its inverse new_record?) to determine whether an object is already in the database or not. Consider the following Active Record class:

class Person < ApplicationRecord
end

We can see how it works by looking at some bin/rails console output:

irb> p = Person.new(name: "Jane Doe")
=> #<Person id: nil, name: "Jane Doe", created_at: nil, updated_at: nil>

irb> p.new_record?
=> true

irb> p.persisted?
=> false

irb> p.save
=> true

irb> p.new_record?
=> false

irb> p.persisted?
=> true

Saving a new record will send an SQL INSERT operation to the database, whereas updating an existing record will send an SQL UPDATE operation. Validations are typically run before these commands are sent to the database. If any validations fail, the object will be marked as invalid and Active Record will not perform the INSERT or UPDATE operation. This helps to avoid storing an invalid object in the database. You can choose to have specific validations run when an object is created, saved, or updated.

The following methods trigger validations, and will save the object to the database only if the object is valid:

• create
• create!
• save
• save!
• update
• update!

The bang versions (methods that end with an exclamation mark, like save!) raise an exception if the record is invalid. The non-bang versions - save and update returns false, and create returns the object.

1.4. Skipping Validations

The following methods skip validations, and will save the object to the database regardless of its validity. They should be used with caution. Refer to the method documentation to learn more.

• decrement!
• decrement_counter
• increment!
• increment_counter
• insert
• insert!
• insert_all
• insert_all!
• toggle!
• touch
• touch_all
• update_all
• update_attribute
• update_attribute!
• update_column
• update_columns
• update_counters
• upsert
• upsert_all
• save(validate: false)

1.5. Checking Validity

Before saving an Active Record object, Rails runs your validations, and if these validations produce any validation errors, then Rails will not save the object.

You can also run the validations on your own. valid? triggers your validations and returns true if no errors are found in the object, and false otherwise. As you saw above:

class Person < ApplicationRecord
  validates :name, presence: true
end

irb> Person.new(name: "John Doe").valid?
=> true
irb> Person.new(name: nil).valid?
=> false

After Active Record has performed validations, any failures can be accessed through the errors instance method, which returns a collection of errors. By definition, an object is valid if the collection is empty after running validations.

class Person < ApplicationRecord
  validates :name, presence: true
end

irb> person = Person.new
=> #<Person id: nil, name: nil, created_at: nil, updated_at: nil>
irb> person.errors.size
=> 0

irb> person.valid?
=> false
irb> person.errors.objects.first.full_message
=> "Name can't be blank"

irb> person.save
=> false

irb> person.save!
ActiveRecord::RecordInvalid: Validation failed: Name can't be blank

irb> Person.create!
ActiveRecord::RecordInvalid: Validation failed: Name can't be blank

invalid? is the inverse of valid?. It triggers your validations, returning true if any errors were found in the object, and false otherwise.

1.6. Inspecting and Handling Errors

To verify whether or not a particular attribute of an object is valid, you can use errors[:attribute]. It returns an array of all the error messages for :attribute. If there are no errors on the specified attribute, an empty array is returned. This allows you to easily determine whether there are any validation issues with a specific attribute.

Here’s an example illustrating how to check for errors on an attribute:

class Person < ApplicationRecord
  validates :name, presence: true
end

irb> new_person = Person.new
irb> new_person.errors[:name]
=> [] # no errors since validations are not run until saved
irb> new_person.errors[:name].any?
=> false

irb> create_person = Person.create
irb> create_person.errors[:name]
=> ["can't be blank"] # validation error because `name` is required
irb> create_person.errors[:name].any?
=> true

Additionally, you can use the errors.add method to manually add error messages for specific attributes. This is particularly useful when defining custom validation scenarios.

class Person < ApplicationRecord
  validate do |person|
    errors.add :name, :too_short, message: "is not long enough"
  end
end

2. Validations

Active Record offers many predefined validations that you can use directly inside your class definitions. These predefined validations provide common validation rules. Each time a validation fails, an error message is added to the object's errors collection, and this error is associated with the specific attribute being validated.

When a validation fails, the error message is stored in the errors collection under the attribute name that triggered the validation. This means you can easily access the errors related to any specific attribute. For instance, if you validate the :name attribute and the validation fails, you will find the error message under errors[:name].

In modern Rails applications, the more concise validate syntax is commonly used, for example:

validate :name, presence: true

However, older versions of Rails used "helper" methods, such as:

validates_presence_of :name

Both notations perform the same function, but the newer form is recommended for its readability and alignment with Rails' conventions.

Each validation accepts an arbitrary number of attribute names, allowing you to apply the same type of validation to multiple attributes in a single line of code.

Additionally, all validations accept the :on and :message options. The :on option specifies when the validation should be triggered, with possible values being :create or :update. The :message option allows you to define a custom error message that will be added to the errors collection if the validation fails. If you do not specify a message, Rails will use a default error message for that validation.

Below we outline the most commonly used validations.

2.1. absence

This validator validates that the specified attributes are absent. It uses the Object#present? method to check if the value is neither nil nor a blank string - that is, a string that is either empty or consists of whitespace only.

#absence is commonly used for conditional validations. For example:

class Person < ApplicationRecord
  validates :phone_number, :address, absence: true, if: :invited?
end

irb> person = Person.new(name: "Jane Doe", invitation_sent_at: Time.current)
irb> person.valid?
=> true # absence validation passes

If you want to be sure that an association is absent, you'll need to test whether the associated object itself is absent, and not the foreign key used to map the association.

class LineItem < ApplicationRecord
  belongs_to :order, optional: true
  validates :order, absence: true
end

irb> line_item = LineItem.new
irb> line_item.valid?
=> true # absence validation passes

order = Order.create
irb> line_item_with_order = LineItem.new(order: order)
irb> line_item_with_order.valid?
=> false # absence validation fails

Rails will usually infer the inverse association automatically. In cases where you use a custom :foreign_key or a :through association, it's important to explicitly set the :inverse_of option to optimize the association lookup. This helps avoid unnecessary database queries during validation.

For more details, check out the Bi-directional Associations documentation.

If you validate the absence of an object associated via a has_one or has_many relationship, it will check that the object is neither present? nor marked_for_destruction?.

Since false.present? is false, if you want to validate the absence of a boolean field you should use:

validates :field_name, exclusion: { in: [true, false] }

The default error message is "must be blank".

2.2. acceptance

This method validates that a checkbox on the user interface was checked when a form was submitted. This is typically used when the user needs to agree to your application's terms of service, confirm that some text is read, or any similar concept.

class Person < ApplicationRecord
  validates :terms_of_service, acceptance: true
end

This check is performed only if terms_of_service is not nil. The default error message for this validation is "must be accepted". You can also pass in a custom message via the message option.

class Person < ApplicationRecord
  validates :terms_of_service, acceptance: { message: "must be agreed to" }
end

It can also receive an :accept option, which determines the allowed values that will be considered as acceptable. It defaults to ['1', true] and can be easily changed.

class Person < ApplicationRecord
  validates :terms_of_service, acceptance: { accept: "yes" }
  validates :eula, acceptance: { accept: ["TRUE", "accepted"] }
end

This validation is very specific to web applications and this 'acceptance' does not need to be recorded anywhere in your database. If you don't have a field for it, the validator will create a virtual attribute. If the field does exist in your database, the accept option must be set to or include true or else the validation will not run.

2.3. confirmation

You should use this validator when you have two text fields that should receive exactly the same content. For example, you may want to confirm an email address or a password. This validation creates a virtual attribute whose name is the name of the field that has to be confirmed with "_confirmation" appended.

class Person < ApplicationRecord
  validates :email, confirmation: true
end

In your view template you could use something like

<%= text_field :person, :email %>
<%= text_field :person, :email_confirmation %>

class Person < ApplicationRecord
  validates :email, confirmation: true
  validates :email_confirmation, presence: true
end

There is also a :case_sensitive option that you can use to define whether the confirmation constraint will be case sensitive or not. This option defaults to true.

class Person < ApplicationRecord
  validates :email, confirmation: { case_sensitive: false }
end

The default error message for this validator is "doesn't match confirmation". You can also pass in a custom message via the message option.

Generally when using this validator, you will want to combine it with the :if option to only validate the "_confirmation" field when the initial field has changed and not every time you save the record. More on conditional validations later.

class Person < ApplicationRecord
  validates :email, confirmation: true
  validates :email_confirmation, presence: true, if: :email_changed?
end

2.4. comparison

This validator will validate a comparison between any two comparable values.

class Promotion < ApplicationRecord
  validates :end_date, comparison: { greater_than: :start_date }
end

The default error message for this validator is "failed comparison". You can also pass in a custom message via the message option.

These options are all supported:

2.5. format

This validator validates the attributes' values by testing whether they match a given regular expression, which is specified using the :with option.

class Product < ApplicationRecord
  validates :legacy_code, format: { with: /\A[a-zA-Z]+\z/,
    message: "only allows letters" }
end

Inversely, by using the :without option instead you can require that the specified attribute does not match the regular expression.

In either case, the provided :with or :without option must be a regular expression or a proc or lambda that returns one.

The default error message is "is invalid".

2.6. inclusion and exclusion

Both of these validators validate whether an attribute’s value is included or excluded from a given set. The set can be any enumerable object such as an array, range, or a dynamically generated collection using a proc, lambda, or symbol.

• inclusion ensures that the value is present in the set.
• exclusion ensures that the value is not present in the set.

In both cases, the option :in receives the set of values, and :within can be used as an alias. For full options on customizing error messages, see the message documentation.

If the enumerable is a numerical, time, or datetime range, the test is performed using Range#cover?, otherwise, it uses include?. When using a proc or lambda, the instance under validation is passed as an argument, allowing for dynamic validation.

2.6.1. Examples

For inclusion:

class Coffee < ApplicationRecord
  validates :size, inclusion: { in: %w(small medium large),
    message: "%{value} is not a valid size" }
end

For exclusion:

class Account < ApplicationRecord
  validates :subdomain, exclusion: { in: %w(www us ca jp),
    message: "%{value} is reserved." }
end

Both validators allow the use of dynamic validation through methods that return an enumerable. Here’s an example using a proc for inclusion:

class Coffee < ApplicationRecord
  validates :size, inclusion: { in: ->(coffee) { coffee.available_sizes } }

  def available_sizes
    %w(small medium large extra_large)
  end
end

Similarly, for exclusion:

class Account < ApplicationRecord
  validates :subdomain, exclusion: { in: ->(account) { account.reserved_subdomains } }

  def reserved_subdomains
    %w(www us ca jp admin)
  end
end

2.7. length

This validator validates the length of the attributes' values. It provides a variety of options, so you can specify length constraints in different ways:

class Person < ApplicationRecord
  validates :name, length: { minimum: 2 }
  validates :bio, length: { maximum: 500 }
  validates :password, length: { in: 6..20 }
  validates :registration_number, length: { is: 6 }
end

The possible length constraint options are:

The default error messages depend on the type of length validation being performed. You can customize these messages using the :wrong_length, :too_long, and :too_short options and %{count} as a placeholder for the number corresponding to the length constraint being used. You can still use the :message option to specify an error message.

class Person < ApplicationRecord
  validates :bio, length: { maximum: 1000,
    too_long: "%{count} characters is the maximum allowed" }
end

2.8. numericality

This validator validates that your attributes have only numeric values. By default, it will match an optional sign followed by an integer or floating point number.

To specify that only integer numbers are allowed, set :only_integer to true. Then it will use the following regular expression to validate the attribute's value.

/\A[+-]?\d+\z/

Otherwise, it will try to convert the value to a number using Float. Floats are converted to BigDecimal using the column's precision value or a maximum of 15 digits.

class Player < ApplicationRecord
  validates :points, numericality: true
  validates :games_played, numericality: { only_integer: true }
end

The default error message for :only_integer is "must be an integer".

Besides :only_integer, this validator also accepts the :only_numeric option which specifies the value must be an instance of Numeric and attempts to parse the value if it is a String.

The default error message when no options are specified is "is not a number".

There are also many options that can be used to add constraints to acceptable values:

2.9. presence

This validator validates that the specified attributes are not empty. It uses the Object#blank? method to check if the value is either nil or a blank string - that is, a string that is either empty or consists of whitespace.

class Person < ApplicationRecord
  validates :name, :login, :email, presence: true
end

person = Person.new(name: "Alice", login: "alice123", email: "alice@example.com")
person.valid?
=> true # presence validation passes

invalid_person = Person.new(name: "", login: nil, email: "bob@example.com")
invalid_person.valid?
=> false # presence validation fails

To check that an association is present, you'll need to test that the associated object is present, and not the foreign key used to map the association. Testing the association will help you to determine that the foreign key is not empty and also that the referenced object exists.

class Supplier < ApplicationRecord
  has_one :account
  validates :account, presence: true
end

irb> account = Account.create(name: "Account A")

irb> supplier = Supplier.new(account: account)
irb> supplier.valid?
=> true # presence validation passes

irb> invalid_supplier = Supplier.new
irb> invalid_supplier.valid?
=> false # presence validation fails

In cases where you use a custom :foreign_key or a :through association, it's important to explicitly set the :inverse_of option to optimize the association lookup. This helps avoid unnecessary database queries during validation.

For more details, check out the Bi-directional Associations documentation.

If you validate the presence of an object associated via a has_one or has_many relationship, it will check that the object is neither blank? nor marked_for_destruction?.

Since false.blank? is true, if you want to validate the presence of a boolean field you should use one of the following validations:

# Value _must_ be true or false
validates :boolean_field_name, inclusion: [true, false]
# Value _must not_ be nil, aka true or false
validates :boolean_field_name, exclusion: [nil]

By using one of these validations, you will ensure the value will NOT be nil which would result in a NULL value in most cases.

The default error message is "can't be blank".

2.10. uniqueness

This validator validates that the attribute's value is unique right before the object gets saved.

class Account < ApplicationRecord
  validates :email, uniqueness: true
end

The validation happens by performing an SQL query into the model's table, searching for an existing record with the same value in that attribute.

There is a :scope option that you can use to specify one or more attributes that are used to limit the uniqueness check:

class Holiday < ApplicationRecord
  validates :name, uniqueness: { scope: :year,
    message: "should happen once per year" }
end

In order to add a uniqueness database constraint on your database, use the add_index statement in a migration and include the unique: true option.

If you are using the :scope option in your uniqueness validation, and you wish to create a database constraint to prevent possible violations of the uniqueness validation, you must create a unique index on both columns in your database. See the MySQL manual and the MariaDB manual for more details about multiple column indexes, or the PostgreSQL manual for examples of unique constraints that refer to a group of columns.

There is also a :case_sensitive option that you can use to define whether the uniqueness constraint will be case sensitive, case insensitive, or if it should respect the default database collation. This option defaults to respecting the default database collation.

class Person < ApplicationRecord
  validates :name, uniqueness: { case_sensitive: false }
end

A :conditions option can be used to specify additional conditions as a WHERE SQL fragment to limit the uniqueness constraint lookup:

validates :name, uniqueness: { conditions: -> { where(status: "active") } }

The default error message is "has already been taken".

See validates_uniqueness_of for more information.

2.11. validates_associated

You should use this validator when your model has associations that always need to be validated. Every time you try to save your object, valid? will be called on each one of the associated objects.

class Library < ApplicationRecord
  has_many :books
  validates_associated :books
end

This validation will work with all of the association types.

The default error message for validates_associated is "is invalid". Note that each associated object will contain its own errors collection; errors do not bubble up to the calling model.

2.12. validates_each

This validator validates attributes against a block. It doesn't have a predefined validation function. You should create one using a block, and every attribute passed to validates_each will be tested against it.

In the following example, we will reject names and surnames that begin with lowercase.

class Person < ApplicationRecord
  validates_each :name, :surname do |record, attr, value|
    record.errors.add(attr, "must start with upper case") if /\A[[:lower:]]/.match?(value)
  end
end

The block receives the record, the attribute's name, and the attribute's value.

You can do anything you like to check for valid data within the block. If your validation fails, you should add an error to the model, therefore making it invalid.

2.13. validates_with

This validator passes the record to a separate class for validation.

class AddressValidator < ActiveModel::Validator
  def validate(record)
    if record.house_number.blank?
      record.errors.add :house_number, "is required"
    end

    if record.street.blank?
      record.errors.add :street, "is required"
    end

    if record.postcode.blank?
      record.errors.add :postcode, "is required"
    end
  end
end

class Invoice < ApplicationRecord
  validates_with AddressValidator
end

There is no default error message for validates_with. You must manually add errors to the record's errors collection in the validator class.

To implement the validate method, you must accept a record parameter in the method definition, which is the record to be validated.

If you want to add an error on a specific attribute, you can pass it as the first argument to the add method.

def validate(record)
  if record.some_field != "acceptable"
    record.errors.add :some_field, "this field is unacceptable"
  end
end

We will cover validation errors in greater detail later.

The validates_with validator takes a class, or a list of classes to use for validation.

class Person < ApplicationRecord
  validates_with MyValidator, MyOtherValidator, on: :create
end

Like all other validations, validates_with takes the :if, :unless and :on options. If you pass any other options, it will send those options to the validator class as options:

class AddressValidator < ActiveModel::Validator
  def validate(record)
    options[:fields].each do |field|
      if record.send(field).blank?
        record.errors.add field, "is required"
      end
    end
  end
end

class Invoice < ApplicationRecord
  validates_with AddressValidator, fields: [:house_number, :street, :postcode, :country]
end

If your validator is complex enough that you want instance variables, you can easily use a plain old Ruby object instead:

class Invoice < ApplicationRecord
  validate do |invoice|
    AddressValidator.new(invoice).validate
  end
end

class AddressValidator
  def initialize(invoice)
    @invoice = invoice
  end

  def validate
    validate_field(:house_number)
    validate_field(:street)
    validate_field(:postcode)
  end

  private
    def validate_field(field)
      if @invoice.send(field).blank?
        @invoice.errors.add field, "#{field.to_s.humanize} is required"
      end
    end
end

We will cover custom validations more later.

3. Validation Options

There are several common options supported by the validators. These options are:

• :allow_nil: Skip validation if the attribute is nil.
• :allow_blank: Skip validation if the attribute is blank.
• :message: Specify a custom error message.
• :on: Specify the contexts where this validation is active.
• :strict: Raise an exception when the validation fails.
• :if and :unless: Specify when the validation should or should not occur.

3.1. :allow_nil

The :allow_nil option skips the validation when the value being validated is nil.

class Coffee < ApplicationRecord
  validates :size, inclusion: { in: %w(small medium large),
    message: "%{value} is not a valid size" }, allow_nil: true
end

irb> Coffee.create(size: nil).valid?
=> true
irb> Coffee.create(size: "mega").valid?
=> false

For full options to the message argument please see the message documentation.

3.2. :allow_blank

The :allow_blank option is similar to the :allow_nil option. This option will let validation pass if the attribute's value is blank?, like nil or an empty string for example.

class Topic < ApplicationRecord
  validates :title, length: { is: 5 }, allow_blank: true
end

irb> Topic.create(title: "").valid?
=> true
irb> Topic.create(title: nil).valid?
=> true
irb> Topic.create(title: "short").valid?
=> false # 'short' is not of length 5, so validation fails even though it's not blank

3.3. :message

As you've already seen, the :message option lets you specify the message that will be added to the errors collection when validation fails. When this option is not used, Active Record will use the respective default error message for each validation.

The :message option accepts either a String or Proc as its value.

A String :message value can optionally contain any/all of %{value}, %{attribute}, and %{model} which will be dynamically replaced when validation fails. This replacement is done using the i18n gem, and the placeholders must match exactly, no spaces are allowed.

class Person < ApplicationRecord
  # Hard-coded message
  validates :name, presence: { message: "must be given please" }

  # Message with dynamic attribute value. %{value} will be replaced
  # with the actual value of the attribute. %{attribute} and %{model}
  # are also available.
  validates :age, numericality: { message: "%{value} seems wrong" }
end

A Proc :message value is given two arguments: the object being validated, and a hash with :model, :attribute, and :value key-value pairs.

class Person < ApplicationRecord
  validates :username,
    uniqueness: {
      # object = person object being validated
      # data = { model: "Person", attribute: "Username", value: <username> }
      message: ->(object, data) do
        "Hey #{object.name}, #{data[:value]} is already taken."
      end
    }
end

To translate error messages, see the I18n guide.

3.4. :on

The :on option lets you specify when the validation should happen. The default behavior for all the built-in validations is to be run on save (both when you're creating a new record and when you're updating it). If you want to change it, you can use on: :create to run the validation only when a new record is created or on: :update to run the validation only when a record is updated.

class Person < ApplicationRecord
  # it will be possible to update email with a duplicated value
  validates :email, uniqueness: true, on: :create

  # it will be possible to create the record with a non-numerical age
  validates :age, numericality: true, on: :update

  # the default (validates on both create and update)
  validates :name, presence: true
end

You can also use :on to define custom contexts. Custom contexts need to be triggered explicitly by passing the name of the context to valid?, invalid?, or save.

class Person < ApplicationRecord
  validates :email, uniqueness: true, on: :account_setup
  validates :age, numericality: true, on: :account_setup
end

irb> person = Person.new(age: 'thirty-three')
irb> person.valid?
=> true
irb> person.valid?(:account_setup)
=> false
irb> person.errors.messages
=> {:email=>["has already been taken"], :age=>["is not a number"]}

person.valid?(:account_setup) executes both the validations without saving the model. person.save(context: :account_setup) validates person in the account_setup context before saving.

Passing an array of symbols is also acceptable.

class Book
  include ActiveModel::Validations

  validates :title, presence: true, on: [:update, :ensure_title]
end

irb> book = Book.new(title: nil)
irb> book.valid?
=> true
irb> book.valid?(:ensure_title)
=> false
irb> book.errors.messages
=> {:title=>["can't be blank"]}

When triggered by an explicit context, validations are run for that context, as well as any validations without a context.

class Person < ApplicationRecord
  validates :email, uniqueness: true, on: :account_setup
  validates :age, numericality: true, on: :account_setup
  validates :name, presence: true
end

irb> person = Person.new
irb> person.valid?(:account_setup)
=> false
irb> person.errors.messages
=> {:email=>["has already been taken"], :age=>["is not a number"], :name=>["can't be blank"]}

You can read more about use-cases for :on in the Custom Contexts section.

4. Conditional Validations

Sometimes it will make sense to validate an object only when a given condition is met. You can do that by using the :if and :unless options, which can take a symbol, a Proc or an Array. You may use the :if option when you want to specify when the validation should happen. Alternatively, if you want to specify when the validation should not happen, then you may use the :unless option.

4.1. Using a Symbol with :if and :unless

You can associate the :if and :unless options with a symbol corresponding to the name of a method that will get called right before validation happens. This is the most commonly used option.

class Order < ApplicationRecord
  validates :card_number, presence: true, if: :paid_with_card?

  def paid_with_card?
    payment_type == "card"
  end
end

4.2. Using a Proc with :if and :unless

It is possible to associate :if and :unless with a Proc object which will be called. Using a Proc object gives you the ability to write an inline condition instead of a separate method. This option is best suited for one-liners.

class Account < ApplicationRecord
  validates :password, confirmation: true,
    unless: Proc.new { |a| a.password.blank? }
end

As lambda is a type of Proc, it can also be used to write inline conditions taking advantage of the shortened syntax.

validates :password, confirmation: true, unless: -> { password.blank? }

4.3. Grouping Conditional Validations

Sometimes it is useful to have multiple validations use one condition. It can be easily achieved using with_options.

class User < ApplicationRecord
  with_options if: :is_admin? do |admin|
    admin.validates :password, length: { minimum: 10 }
    admin.validates :email, presence: true
  end
end

All validations inside of the with_options block will automatically have if: :is_admin? merged into its options.

4.4. Combining Validation Conditions

On the other hand, when multiple conditions define whether or not a validation should happen, an Array can be used. Moreover, you can apply both :if and :unless to the same validation.

class Computer < ApplicationRecord
  validates :mouse, presence: true,
                    if: [Proc.new { |c| c.market.retail? }, :desktop?],
                    unless: Proc.new { |c| c.trackpad.present? }
end

The validation only runs when all the :if conditions and none of the :unless conditions are evaluated to true.

5. Strict Validations

You can also specify validations to be strict and raise ActiveModel::StrictValidationFailed when the object is invalid.

class Person < ApplicationRecord
  validates :name, presence: { strict: true }
end

irb> Person.new.valid?
=> ActiveModel::StrictValidationFailed: Name can't be blank

Strict validations ensure that an exception is raised immediately when validation fails, which can be useful in situations where you want to enforce immediate feedback or halt processing when invalid data is encountered. For example, you might use strict validations in a scenario where invalid input should prevent further operations, such as when processing critical transactions or performing data integrity checks.

There is also the ability to pass a custom exception to the :strict option.

class Person < ApplicationRecord
  validates :token, presence: true, uniqueness: true, strict: TokenGenerationException
end

irb> Person.new.valid?
=> TokenGenerationException: Token can't be blank

6. Listing Validators

If you want to find out all of the validators for a given object, you can use validators.

For example, if we have the following model using a custom validator and a built-in validator:

class Person < ApplicationRecord
  validates :name, presence: true, on: :create
  validates :email, format: URI::MailTo::EMAIL_REGEXP
  validates_with MyOtherValidator, strict: true
end

We can now use validators on the "Person" model to list all validators, or even check a specific field using validators_on.

irb> Person.validators
#=> [#<ActiveRecord::Validations::PresenceValidator:0x10b2f2158
      @attributes=[:name], @options={:on=>:create}>,
     #<MyOtherValidatorValidator:0x10b2f17d0
      @attributes=[:name], @options={:strict=>true}>,
     #<ActiveModel::Validations::FormatValidator:0x10b2f0f10
      @attributes=[:email],
      @options={:with=>URI::MailTo::EMAIL_REGEXP}>]
     #<MyOtherValidator:0x10b2f0948 @options={:strict=>true}>]

irb> Person.validators_on(:name)
#=> [#<ActiveModel::Validations::PresenceValidator:0x10b2f2158
      @attributes=[:name], @options={on: :create}>]

7. Performing Custom Validations

When the built-in validations are not enough for your needs, you can write your own validators or validation methods as you prefer.

7.1. Custom Validators

Custom validators are classes that inherit from ActiveModel::Validator. These classes must implement the validate method which takes a record as an argument and performs the validation on it. The custom validator is called using the validates_with method.

class MyValidator < ActiveModel::Validator
  def validate(record)
    unless record.name.start_with? "X"
      record.errors.add :name, "Provide a name starting with X, please!"
    end
  end
end

class Person < ApplicationRecord
  validates_with MyValidator
end

The easiest way to add custom validators for validating individual attributes is with the convenient ActiveModel::EachValidator. In this case, the custom validator class must implement a validate_each method which takes three arguments: record, attribute, and value. These correspond to the instance, the attribute to be validated, and the value of the attribute in the passed instance.

class EmailValidator < ActiveModel::EachValidator
  def validate_each(record, attribute, value)
    unless URI::MailTo::EMAIL_REGEXP.match?(value)
      record.errors.add attribute, (options[:message] || "is not an email")
    end
  end
end

class Person < ApplicationRecord
  validates :email, presence: true, email: true
end

As shown in the example, you can also combine standard validations with your own custom validators.

7.2. Custom Methods

You can also create methods that verify the state of your models and add errors to the errors collection when they are invalid. You must then register these methods by using the validate class method, passing in the symbols for the validation methods' names.

You can pass more than one symbol for each class method and the respective validations will be run in the same order as they were registered.

The valid? method will verify that the errors collection is empty, so your custom validation methods should add errors to it when you wish validation to fail:

class Invoice < ApplicationRecord
  validate :expiration_date_cannot_be_in_the_past,
    :discount_cannot_be_greater_than_total_value

  def expiration_date_cannot_be_in_the_past
    if expiration_date.present? && expiration_date < Date.today
      errors.add(:expiration_date, "can't be in the past")
    end
  end

  def discount_cannot_be_greater_than_total_value
    if discount > total_value
      errors.add(:discount, "can't be greater than total value")
    end
  end
end

By default, such validations will run every time you call valid? or save the object. But it is also possible to control when to run these custom validations by giving an :on option to the validate method, with either: :create or :update.

class Invoice < ApplicationRecord
  validate :active_customer, on: :create

  def active_customer
    errors.add(:customer_id, "is not active") unless customer.active?
  end
end

See the section above for more details about :on.

7.3. Custom Contexts

You can define your own custom validation contexts for callbacks, which is useful when you want to perform validations based on specific scenarios or group certain callbacks together and run them in a specific context. A common scenario for custom contexts is when you have a multi-step form and want to perform validations per step.

For instance, you might define custom contexts for each step of the form:

class User < ApplicationRecord
  validate :personal_information, on: :personal_info
  validate :contact_information, on: :contact_info
  validate :location_information, on: :location_info

  private
    def personal_information
      errors.add(:base, "Name must be present") if first_name.blank?
      errors.add(:base, "Age must be at least 18") if age && age < 18
    end

    def contact_information
      errors.add(:base, "Email must be present") if email.blank?
      errors.add(:base, "Phone number must be present") if phone.blank?
    end

    def location_information
      errors.add(:base, "Address must be present") if address.blank?
      errors.add(:base, "City must be present") if city.blank?
    end
end

In these cases, you may be tempted to skip callbacks altogether, but defining a custom context can be a more structured approach. You will need to combine a context with the :on option to define a custom context for a callback.

Once you've defined the custom context, you can use it to trigger the validations:

irb> user = User.new(name: "John Doe", age: 17, email: "jane@example.com", phone: "1234567890", address: "123 Main St")
irb> user.valid?(:personal_info) # => false
irb> user.valid?(:contact_info) # => true
irb> user.valid?(:location_info) # => false

You can also use the custom contexts to trigger the validations on any method that supports callbacks. For example, you could use the custom context to trigger the validations on save:

irb> user = User.new(name: "John Doe", age: 17, email: "jane@example.com", phone: "1234567890", address: "123 Main St")
irb> user.save(context: :personal_info) # => false
irb> user.save(context: :contact_info) # => true
irb> user.save(context: :location_info) # => false

8. Working with Validation Errors

The valid? and invalid? methods only provide a summary status on validity. However you can dig deeper into each individual error by using various methods from the errors collection.

The following is a list of the most commonly used methods. Please refer to the ActiveModel::Errors documentation for a list of all the available methods.

8.1. errors

The errors method is the starting point through which you can drill down into various details of each error.

This returns an instance of the class ActiveModel::Errors containing all errors, each error is represented by an ActiveModel::Error object.

class Person < ApplicationRecord
  validates :name, presence: true, length: { minimum: 3 }
end

irb> person = Person.new
irb> person.valid?
=> false
irb> person.errors.full_messages
=> ["Name can't be blank", "Name is too short (minimum is 3 characters)"]

irb> person = Person.new(name: "John Doe")
irb> person.valid?
=> true
irb> person.errors.full_messages
=> []

irb> person = Person.new
irb> person.valid?
=> false
irb> person.errors.first.details
=> {:error=>:too_short, :count=>3}

8.2. errors[]

errors[] is used when you want to check the error messages for a specific attribute. It returns an array of strings with all error messages for the given attribute, each string with one error message. If there are no errors related to the attribute, it returns an empty array.

This method is only useful after validations have been run, because it only inspects the errors collection and does not trigger validations itself. It's different from the ActiveRecord::Base#invalid? method explained above because it doesn't verify the validity of the object as a whole. errors[] only checks to see whether there are errors found on an individual attribute of the object.

class Person < ApplicationRecord
  validates :name, presence: true, length: { minimum: 3 }
end

irb> person = Person.new(name: "John Doe")
irb> person.valid?
=> true
irb> person.errors[:name]
=> []

irb> person = Person.new(name: "JD")
irb> person.valid?
=> false
irb> person.errors[:name]
=> ["is too short (minimum is 3 characters)"]

irb> person = Person.new
irb> person.valid?
=> false
irb> person.errors[:name]
=> ["can't be blank", "is too short (minimum is 3 characters)"]

8.3. errors.where and Error Object

Sometimes we may need more information about each error besides its message. Each error is encapsulated as an ActiveModel::Error object, and the where method is the most common way of access.

where returns an array of error objects filtered by various degrees of conditions.

Given the following validation:

class Person < ApplicationRecord
  validates :name, presence: true, length: { minimum: 3 }
end

We can filter for just the attribute by passing it as the first parameter to errors.where(:attr). The second parameter is used for filtering the type of error we want by calling errors.where(:attr, :type).

irb> person = Person.new
irb> person.valid?
=> false

irb> person.errors.where(:name)
=> [ ... ] # all errors for :name attribute

irb> person.errors.where(:name, :too_short)
=> [ ... ] # :too_short errors for :name attribute

Lastly, we can filter by any options that may exist on the given type of error object.

irb> person = Person.new
irb> person.valid?
=> false

irb> person.errors.where(:name, :too_short, minimum: 3)
=> [ ... ] # all name errors being too short and minimum is 3

You can read various information from these error objects:

irb> error = person.errors.where(:name).last

irb> error.attribute
=> :name
irb> error.type
=> :too_short
irb> error.options[:count]
=> 3

You can also generate the error message:

irb> error.message
=> "is too short (minimum is 3 characters)"
irb> error.full_message
=> "Name is too short (minimum is 3 characters)"

The full_message method generates a more user-friendly message, with the capitalized attribute name prepended. (To customize the format that full_message uses, see the I18n guide.)

8.4. errors.add

The add method creates the error object by taking the attribute, the error type and additional options hash. This is useful when writing your own validator, as it lets you define very specific error situations.

class Person < ApplicationRecord
  validate do |person|
    errors.add :name, :too_plain, message: "is not cool enough"
  end
end

irb> person = Person.new
irb> person.errors.where(:name).first.type
=> :too_plain
irb> person.errors.where(:name).first.full_message
=> "Name is not cool enough"

8.5. errors[:base]

You can add errors that are related to the object's state as a whole, instead of being related to a specific attribute. To do this you must use :base as the attribute when adding a new error.

class Person < ApplicationRecord
  validate do |person|
    errors.add :base, :invalid, message: "This person is invalid because ..."
  end
end

irb> person = Person.new
irb> person.errors.where(:base).first.full_message
=> "This person is invalid because ..."

8.6. errors.size

The size method returns the total number of errors for the object.

class Person < ApplicationRecord
  validates :name, presence: true, length: { minimum: 3 }
end

irb> person = Person.new
irb> person.valid?
=> false
irb> person.errors.size
=> 2

irb> person = Person.new(name: "Andrea", email: "andrea@example.com")
irb> person.valid?
=> true
irb> person.errors.size
=> 0

8.7. errors.clear

The clear method is used when you intentionally want to clear the errors collection. Of course, calling errors.clear upon an invalid object won't actually make it valid: the errors collection will now be empty, but the next time you call valid? or any method that tries to save this object to the database, the validations will run again. If any of the validations fail, the errors collection will be filled again.

class Person < ApplicationRecord
  validates :name, presence: true, length: { minimum: 3 }
end

irb> person = Person.new
irb> person.valid?
=> false
irb> person.errors.empty?
=> false

irb> person.errors.clear
irb> person.errors.empty?
=> true

irb> person.save
=> false

irb> person.errors.empty?
=> false

9. Displaying Validation Errors in Views

Once you've defined a model and added validations, you'll want to display an error message when a validation fails during the creation of that model via a web form.

Since every application handles displaying validation errors differently, Rails does not include any view helpers for generating these messages. However, Rails gives you a rich number of methods to interact with validations that you can use to build your own. In addition, when generating a scaffold, Rails will put some generated ERB into the _form.html.erb that displays the full list of errors on that model.

Assuming we have a model that's been saved in an instance variable named @article, it looks like this:

<% if @article.errors.any? %>
  <div id="error_explanation">
    <h2><%= pluralize(@article.errors.count, "error") %> prohibited this article from being saved:</h2>

    <ul>
      <% @article.errors.each do |error| %>
        <li><%= error.full_message %></li>
      <% end %>
    </ul>
  </div>
<% end %>

Furthermore, if you use the Rails form helpers to generate your forms, when a validation error occurs on a field, it will generate an extra <div> around the entry.

<div class="field_with_errors">
  <input id="article_title" name="article[title]" size="30" type="text" value="">
</div>

You can then style this div however you'd like. The default scaffold that Rails generates, for example, adds this CSS rule:

.field_with_errors {
  padding: 2px;
  background-color: red;
  display: table;
}

This means that any field with an error ends up with a 2 pixel red border.

9.1. Customizing Error Field Wrapper

Rails uses the field_error_proc configuration option to wrap fields with errors in HTML. By default, this option wraps the erroneous form fields in a <div> with a field_with_errors class, as seen in the example above:

config.action_view.field_error_proc = Proc.new { |html_tag, instance| content_tag :div, html_tag, class: "field_with_errors" }

You can customize this behavior by modifying the field_error_proc setting in your application configuration, allowing you to change how errors are presented in your forms. For more details,refer to the Configuration Guide on field_error_proc.