Replace index with unique index

class AddUniquenessConstraintToUsersGuid < ActiveRecord::Migration
  def up
    # Delete all but the first user for users with duplicate :guid
    duplicate_guids = User.select(:guid).group(:guid).having('count(*) > 1').pluck(:guid)
    duplicate_guids.each do |duplicate_guid|
      next unless duplicate_guid.present?
      users_to_be_deleted = User.where(guid: duplicate_guid).order('id ASC')[1..-1]
      users_to_be_deleted.each(&:destroy)
    end

    change_table(:users, { bulk: true }) do |t|
      t.remove_index :guid
      t.index :guid, name: :index_users_on_unique_guid, unique: true
    end
  end

  def down
    change_table(:users, { bulk: true }) do |t|
      t.remove_index name: :index_users_on_unique_guid
      t.index :guid
    end
  end
end