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Active Record and PostgreSQL

This guide covers PostgreSQL specific usage of Active Record.

After reading this guide, you will know:

In order to use the PostgreSQL adapter you need to have at least version 9.1 installed. Older versions are not supported.

To get started with PostgreSQL have a look at the configuring Rails guide. It describes how to properly setup Active Record for PostgreSQL.

1 Datatypes

PostgreSQL offers a number of specific datatypes. Following is a list of types, that are supported by the PostgreSQL adapter.

1.1 Bytea

# db/migrate/20140207133952_create_documents.rb
create_table :documents do |t|
  t.binary 'payload'
end

# app/models/document.rb
class Document < ApplicationRecord
end

# Usage
data = File.read(Rails.root + "tmp/output.pdf")
Document.create payload: data

1.2 Array

# db/migrate/20140207133952_create_books.rb
create_table :books do |t|
  t.string 'title'
  t.string 'tags', array: true
  t.integer 'ratings', array: true
end
add_index :books, :tags, using: 'gin'
add_index :books, :ratings, using: 'gin'

# app/models/book.rb
class Book < ApplicationRecord
end

# Usage
Book.create title: "Brave New World",
            tags: ["fantasy", "fiction"],
            ratings: [4, 5]

## Books for a single tag
Book.where("'fantasy' = ANY (tags)")

## Books for multiple tags
Book.where("tags @> ARRAY[?]::varchar[]", ["fantasy", "fiction"])

## Books with 3 or more ratings
Book.where("array_length(ratings, 1) >= 3")

1.3 Hstore

You need to enable the hstore extension to use hstore.

# db/migrate/20131009135255_create_profiles.rb
ActiveRecord::Schema.define do
  enable_extension 'hstore' unless extension_enabled?('hstore')
  create_table :profiles do |t|
    t.hstore 'settings'
  end
end

# app/models/profile.rb
class Profile < ApplicationRecord
end

# Usage
Profile.create(settings: { "color" => "blue", "resolution" => "800x600" })

profile = Profile.first
profile.settings # => {"color"=>"blue", "resolution"=>"800x600"}

profile.settings = {"color" => "yellow", "resolution" => "1280x1024"}
profile.save!

Profile.where("settings->'color' = ?", "yellow")
#=> #<ActiveRecord::Relation [#<Profile id: 1, settings: {"color"=>"yellow", "resolution"=>"1280x1024"}>]>

1.4 JSON

# db/migrate/20131220144913_create_events.rb
create_table :events do |t|
  t.json 'payload'
end

# app/models/event.rb
class Event < ApplicationRecord
end

# Usage
Event.create(payload: { kind: "user_renamed", change: ["jack", "john"]})

event = Event.first
event.payload # => {"kind"=>"user_renamed", "change"=>["jack", "john"]}

## Query based on JSON document
# The -> operator returns the original JSON type (which might be an object), whereas ->> returns text
Event.where("payload->>'kind' = ?", "user_renamed")

1.5 Range Types

This type is mapped to Ruby Range objects.

# db/migrate/20130923065404_create_events.rb
create_table :events do |t|
  t.daterange 'duration'
end

# app/models/event.rb
class Event < ApplicationRecord
end

# Usage
Event.create(duration: Date.new(2014, 2, 11)..Date.new(2014, 2, 12))

event = Event.first
event.duration # => Tue, 11 Feb 2014...Thu, 13 Feb 2014

## All Events on a given date
Event.where("duration @> ?::date", Date.new(2014, 2, 12))

## Working with range bounds
event = Event.
  select("lower(duration) AS starts_at").
  select("upper(duration) AS ends_at").first

event.starts_at # => Tue, 11 Feb 2014
event.ends_at # => Thu, 13 Feb 2014

1.6 Composite Types

Currently there is no special support for composite types. They are mapped to normal text columns:

CREATE TYPE full_address AS
(
  city VARCHAR(90),
  street VARCHAR(90)
);

# db/migrate/20140207133952_create_contacts.rb
execute <<-SQL
 CREATE TYPE full_address AS
 (
   city VARCHAR(90),
   street VARCHAR(90)
 );
SQL
create_table :contacts do |t|
  t.column :address, :full_address
end

# app/models/contact.rb
class Contact < ApplicationRecord
end

# Usage
Contact.create address: "(Paris,Champs-Élysées)"
contact = Contact.first
contact.address # => "(Paris,Champs-Élysées)"
contact.address = "(Paris,Rue Basse)"
contact.save!

1.7 Enumerated Types

Currently there is no special support for enumerated types. They are mapped as normal text columns:

# db/migrate/20131220144913_create_articles.rb
def up
  execute <<-SQL
    CREATE TYPE article_status AS ENUM ('draft', 'published');
  SQL
  create_table :articles do |t|
    t.column :status, :article_status
  end
end

# NOTE: It's important to drop table before dropping enum.
def down
  drop_table :articles

  execute <<-SQL
    DROP TYPE article_status;
  SQL
end

# app/models/article.rb
class Article < ApplicationRecord
end

# Usage
Article.create status: "draft"
article = Article.first
article.status # => "draft"

article.status = "published"
article.save!

To add a new value before/after existing one you should use ALTER TYPE:

# db/migrate/20150720144913_add_new_state_to_articles.rb
# NOTE: ALTER TYPE ... ADD VALUE cannot be executed inside of a transaction block so here we are using disable_ddl_transaction!
disable_ddl_transaction!

def up
  execute <<-SQL
    ALTER TYPE article_status ADD VALUE IF NOT EXISTS 'archived' AFTER 'published';
  SQL
end

ENUM values can't be dropped currently. You can read why here.

Hint: to show all the values of the all enums you have, you should call this query in bin/rails db or psql console:

SELECT n.nspname AS enum_schema,
       t.typname AS enum_name,
       e.enumlabel AS enum_value
  FROM pg_type t
      JOIN pg_enum e ON t.oid = e.enumtypid
      JOIN pg_catalog.pg_namespace n ON n.oid = t.typnamespace

1.8 UUID

You need to enable the pgcrypto (only PostgreSQL >= 9.4) or uuid-ossp extension to use uuid.

# db/migrate/20131220144913_create_revisions.rb
create_table :revisions do |t|
  t.uuid :identifier
end

# app/models/revision.rb
class Revision < ApplicationRecord
end

# Usage
Revision.create identifier: "A0EEBC99-9C0B-4EF8-BB6D-6BB9BD380A11"

revision = Revision.first
revision.identifier # => "a0eebc99-9c0b-4ef8-bb6d-6bb9bd380a11"

You can use uuid type to define references in migrations:

# db/migrate/20150418012400_create_blog.rb
enable_extension 'pgcrypto' unless extension_enabled?('pgcrypto')
create_table :posts, id: :uuid, default: 'gen_random_uuid()'

create_table :comments, id: :uuid, default: 'gen_random_uuid()' do |t|
  # t.belongs_to :post, type: :uuid
  t.references :post, type: :uuid
end

# app/models/post.rb
class Post < ApplicationRecord
  has_many :comments
end

# app/models/comment.rb
class Comment < ApplicationRecord
  belongs_to :post
end

See this section for more details on using UUIDs as primary key.

1.9 Bit String Types

# db/migrate/20131220144913_create_users.rb
create_table :users, force: true do |t|
  t.column :settings, "bit(8)"
end

# app/models/device.rb
class User < ApplicationRecord
end

# Usage
User.create settings: "01010011"
user = User.first
user.settings # => "01010011"
user.settings = "0xAF"
user.settings # => 10101111
user.save!

1.10 Network Address Types

The types inet and cidr are mapped to Ruby IPAddr objects. The macaddr type is mapped to normal text.

# db/migrate/20140508144913_create_devices.rb
create_table(:devices, force: true) do |t|
  t.inet 'ip'
  t.cidr 'network'
  t.macaddr 'address'
end

# app/models/device.rb
class Device < ApplicationRecord
end

# Usage
macbook = Device.create(ip: "192.168.1.12",
                        network: "192.168.2.0/24",
                        address: "32:01:16:6d:05:ef")

macbook.ip
# => #<IPAddr: IPv4:192.168.1.12/255.255.255.255>

macbook.network
# => #<IPAddr: IPv4:192.168.2.0/255.255.255.0>

macbook.address
# => "32:01:16:6d:05:ef"

1.11 Geometric Types

All geometric types, with the exception of points are mapped to normal text. A point is casted to an array containing x and y coordinates.

2 UUID Primary Keys

You need to enable the pgcrypto (only PostgreSQL >= 9.4) or uuid-ossp extension to generate random UUIDs.

# db/migrate/20131220144913_create_devices.rb
enable_extension 'pgcrypto' unless extension_enabled?('pgcrypto')
create_table :devices, id: :uuid, default: 'gen_random_uuid()' do |t|
  t.string :kind
end

# app/models/device.rb
class Device < ApplicationRecord
end

# Usage
device = Device.create
device.id # => "814865cd-5a1d-4771-9306-4268f188fe9e"

uuid_generate_v4() (from uuid-ossp) is assumed if no :default option was passed to create_table.

# db/migrate/20131220144913_create_documents.rb
create_table :documents do |t|
  t.string 'title'
  t.string 'body'
end

add_index :documents, "to_tsvector('english', title || ' ' || body)", using: :gin, name: 'documents_idx'

# app/models/document.rb
class Document < ApplicationRecord
end

# Usage
Document.create(title: "Cats and Dogs", body: "are nice!")

## all documents matching 'cat & dog'
Document.where("to_tsvector('english', title || ' ' || body) @@ to_tsquery(?)",
                 "cat & dog")

4 Database Views

Imagine you need to work with a legacy database containing the following table:

rails_pg_guide=# \d "TBL_ART"
                                        Table "public.TBL_ART"
   Column   |            Type             |                         Modifiers
------------+-----------------------------+------------------------------------------------------------
 INT_ID     | integer                     | not null default nextval('"TBL_ART_INT_ID_seq"'::regclass)
 STR_TITLE  | character varying           |
 STR_STAT   | character varying           | default 'draft'::character varying
 DT_PUBL_AT | timestamp without time zone |
 BL_ARCH    | boolean                     | default false
Indexes:
    "TBL_ART_pkey" PRIMARY KEY, btree ("INT_ID")

This table does not follow the Rails conventions at all. Because simple PostgreSQL views are updateable by default, we can wrap it as follows:

# db/migrate/20131220144913_create_articles_view.rb
execute <<-SQL
CREATE VIEW articles AS
  SELECT "INT_ID" AS id,
         "STR_TITLE" AS title,
         "STR_STAT" AS status,
         "DT_PUBL_AT" AS published_at,
         "BL_ARCH" AS archived
  FROM "TBL_ART"
  WHERE "BL_ARCH" = 'f'
  SQL

# app/models/article.rb
class Article < ApplicationRecord
  self.primary_key = "id"
  def archive!
    update_attribute :archived, true
  end
end

# Usage
first = Article.create! title: "Winter is coming",
                        status: "published",
                        published_at: 1.year.ago
second = Article.create! title: "Brace yourself",
                         status: "draft",
                         published_at: 1.month.ago

Article.count # => 2
first.archive!
Article.count # => 1

This application only cares about non-archived Articles. A view also allows for conditions so we can exclude the archived Articles directly.

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