Active Record and PostgreSQL

This guide covers PostgreSQL specific usage of Active Record.

After reading this guide, you will know:

  • How to use PostgreSQL's datatypes.
  • How to use UUID primary keys.
  • How to implement full text search with PostgreSQL.
  • How to back your Active Record models with database views.

Chapters

  1. Datatypes
  2. UUID Primary Keys
  3. Full Text Search
  4. Database Views

In order to use the PostgreSQL adapter you need to have at least version 9.3 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 and JSONB

# db/migrate/20131220144913_create_events.rb
# ... for json datatype:
create_table :events do |t|
  t.json 'payload'
end
# ... or for jsonb datatype:
create_table :events do |t|
  t.jsonb '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 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/user.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"

gen_random_uuid() (from pgcrypto) is assumed if no :default option was passed to create_table.

3 Full Text Search

# 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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