SQL How To » Order with attribute of an associated record

This article assumes:

  • You have a users table
  • You have a teams table

We have Users, each users is associated to a Team. Let's imagine we want to get all the users, but we want them ordered by Team name, and if they're in the same team, we want them to be ordered by creation date. Here are our relations:

module Relations
  class Users < ROM::Relation[:sql]
    attribute :created_at, ROM::Types::DateTime

    schema(:users) do
      associations do
        belongs_to :team
      end
    end
  end

  class Teams < ROM::Relation[:sql]
    schema(:teams) do
      attribute :name, ROM::Types::String

      associations do
        has_many :users
      end
    end
  end
end

And here is the code to get such ordering:

2.5.3 :003 > users.join(:teams).order(teams[:name], :created_at)
 => #<Relations::Users name=ROM::Relation::Name(users) dataset=#<Sequel::Mysql2::Dataset: "SELECT `users`.`id`, 
 `users`.`name`, `users`.`created_at`, `users`.`team_id` FROM `users` INNER JOIN `teams` ON (`users`.`team_id` = 
 `teams`.`id`) ORDER BY `teams`.`name`, `created_at`">>
2.5.3 :004 > users.combine(:teams).join(:teams).order(teams[:name], :created_at).to_a
 => [#<ROM::Struct::User id=1 name="John Doe" created_at=2019-07-07 15:54:14 +0200 team_id=1 team=#<ROM::Struct::Team 
 id=1 name="A team">>, #<ROM::Struct::User id=3 name="Jack Doe" created_at=2019-07-07 15:54:27 +0200 team_id=1 
 team=#<ROM::Struct::Team id=1 name="A team">>, #<ROM::Struct::User id=2 name="Jane Doe" created_at=2019-07-07 15:54:22 
 +0200 team_id=2 team=#<ROM::Struct::Team id=2 name="B team">>]

If you want to know more about how you can use "#order" method, you can take a look at the API docs.