Adding unique: true for add_column and add_index in Active Record

Question

Even though my application isn't gonna allow a user to key in a location, I wanted to enforce a uniqueness on city in the database. Since my Rails app will be searching on the city column, I would like to add an index on the city column as well but was wondering if it matters adding unique: true on the index as well. Is this repetitive? If this doesn't make sense, I would really appreciate it if you could explain why.

class CreateLocations < ActiveRecord::Migration
  def change
    create_table :locations do |t|
      t.string :city, unique: true
      t.string :state

      t.timestamps
    end

    add_index :locations, :city, unique: true
  end
end

Answer 1

Using Rails 4, you can provide: index param a hash argument

class CreateLocations < ActiveRecord::Migration
  def change
    create_table :locations do |t|
      t.string :city, index: {unique: true}
      t.string :state

      t.timestamps
    end
  end
end

Answer 2

As far as I know the unique option in the create_table block is actually not supported and doesn't do anything, see TableDefinition. To create the unique index, you need to call the method add_index the way you do now. Note that a unique index is both for uniqueness and for searching etc., there's no need to add two indexes on the same column.

Answer 3

You can specify unique index while scaffolding:

rails generate model Locations city:string:uniq state:string

This will create Model, Spec, Factory and this migration:

class CreateLocations < ActiveRecord::Migration
  def change
    create_table :locations do |t|
      t.string :city
      t.string :state

      t.timestamps null: false
    end
    add_index :locations, :city, unique: true
  end
end

Rails knows what it's doing - nothing more is required.