How to Create a New Table With a Unique Index in an Active Record / Rails 4 Migration

How do I create a new table, through a rails migration, and add an unique index to it?

In the docs I found how to add a index to a table after it's been created, but how do you do both -- create the table, and add the unique index -- in the same migration file?

Here's the full process:

Generate a migration ( rails generate migration CreateFoos bar:string or rails g migration CreateFoos bar:string )

Modify your migration to look something like this:

class CreateFoos < ActiveRecord::Migration
  def change
    create_table :foos do |t|
      t.string :bar, :null => false

      t.index :bar, unique: true
    end
  end
end

Run rake db:migrate

A more compact way:

class CreateFoobars < ActiveRecord::Migration
  def change
    create_table :foobars do |t|
      t.string :name, index: {unique: true}
    end
  end
end

After generating a migration rails generate migration CreateBoards name:string description:string

In the migration file, add index as shown below:

class CreateBoards < ActiveRecord::Migration
  def change
   create_table :boards do |t|
     t.string :name
     t.string :description
 
     t.timestamps
   end

   add_index :boards, :name, unique: true

 end
end

You can create the table and index with the generator without changing the migration file

For a unique index

rails generate model CreateFoos bar:string:uniq

For a non-unique index

rails generate model CreateFoos bar:string:index

In Rails 5, you can provide index options along with column definition.

create_table :table_name do |t|
  t.string :key, null: false, index: {unique: true}
  t.jsonb :value

  t.timestamps
end


Column   |            Type             | Collation | Nullable |                 Default
------------+-----------------------------+-----------+----------+-----------------------------------------
id         | bigint                      |           | not null | nextval('table_name_id_seq'::regclass)
key        | character varying           |           | not null |
value      | jsonb                       |           |          |
created_at | timestamp without time zone |           | not null |
updated_at | timestamp without time zone |           | not null |
Indexes:
  "table_name_pkey" PRIMARY KEY, btree (id)
  "index_table_name_on_key" UNIQUE, btree (key)