How do I make a column unique and index it in a Ruby on Rails migration?

rails generate migration add_index_to_table_name column_name:uniq

or

rails generate migration add_column_name_to_table_name column_name:string:uniq:index

generates

class AddIndexToModerators < ActiveRecord::Migration
  def change
    add_column :moderators, :username, :string
    add_index :moderators, :username, unique: true
  end
end

If you’re adding an index to an existing column, remove or comment the add_column line, or put in a check

add_column :moderators, :username, :string unless column_exists? :moderators, :username

If you are creating a new table, you can use the inline shortcut:

  def change
    create_table :posts do |t|
      t.string :title, null: false, index: { unique: true }
      t.timestamps
    end
  end

Since this hasn’t been mentioned yet but answers the question I had when I found this page, you can also specify that an index should be unique when adding it via t.references or t.belongs_to:

create_table :accounts do |t|
  t.references :user, index: { unique: true } # or t.belongs_to

  # other columns...
end

(as of at least Rails 4.2.7)

I’m using Rails 5 and the above answers work great; here’s another way that also worked for me (the table name is :people and the column name is :email_address)

class AddIndexToEmailAddress < ActiveRecord::Migration[5.0]
  def change
    change_table :people do |t|
      t.index :email_address, unique: true
    end
  end
end

You might want to add name for the unique key as many times the default unique_key name by rails can be too long for which the DB can throw the error.

To add name for your index just use the name: option.
The migration query might look something like this –

add_index :table_name, [:column_name_a, :column_name_b, ... :column_name_n], unique: true, name: 'my_custom_index_name'

More info – http://apidock.com/rails/ActiveRecord/ConnectionAdapters/SchemaStatements/add_index

add_index :table_name, :column_name, unique: true

To index multiple columns together, you pass an array of column names instead of a single column name.

If you have missed to add unique to DB column, just add this validation in model to check if the field is unique:

class Person < ActiveRecord::Base
  validates_uniqueness_of :user_name
end

refer here
Above is for testing purpose only, please add index by changing DB column as suggested by @Nate

please refer this with index for more information