Add auto increment column in Postgres in Rails

class AddAutoIncrements < ActiveRecord::Migration
  def self.up
    add_column :designs, :custom_id, :integer
    execute <<-SQL
     CREATE SEQUENCE custom_id_seq START 1;
     ALTER SEQUENCE custom_id_seq OWNED BY designs.custom_id;
     ALTER TABLE designs ALTER COLUMN custom_id SET DEFAULT nextval('custom_id_seq');
    SQL
  end


  def self.down
    remove_column :designs, :custom_id
    execute <<-SQL
      DROP SEQUENCE custom_id_seq;
    SQL
  end


end

OR Added by Active record:

class AddAutoIncrements < ActiveRecord::Migration
 def self.up    
   add_column :users, :custom_id, :integer
  ActiveRecord::Base.connection.execute("CREATE SEQUENCE users_id_seq")
  ActiveRecord::Base.connection.execute("ALTER SEQUENCE users_id_seq OWNED BY users.id")
  ActiveRecord::Base.connection.execute("ALTER TABLE users ALTER COLUMN id SET DEFAULT nextval('users_id_seq')")   
    
   end
  end


  def self.down
    remove_column :users, :custom_id
    execute <<-SQL
      DROP SEQUENCE custom_id_seq;
    SQL
  end
end