discourse/db/migrate/20171220181249_change_user_emails_primary_index.rb

class ChangeUserEmailsPrimaryIndex < ActiveRecord::Migration[5.1]
  def up
    remove_index :user_emails, [:user_id, :primary]
    add_index :user_emails, [:user_id, :primary], unique: true, where: '"primary"'
  end

  def down
    remove_index :user_emails, [:user_id, :primary]
    add_index :user_emails, [:user_id, :primary], unique: true
  end
end