mapper.rb

module Entities
end

rom = ROM.container(:sql, 'sqlite::memory') do |conf|
  conf.gateways[:default].use_logger(Logger.new($stdout))


  conf.default.create_table(:users) do
    primary_key :id
    column :login, String
  end

  conf.default.create_table(:employees_users) do
    foreign_key :verifable_id, :employees
    foreign_key :user_id, :users
  end

  conf.default.create_table(:employees) do
    primary_key :id
    column :name, String, null: false
  end

  conf.default.create_table(:roles_users) do
    foreign_key :role_id, :roles
    foreign_key :user_id, :users
  end

  conf.default.create_table(:roles) do
    primary_key :id
    column :name, String, null: false
  end

  conf.relation(:users) do
    schema(:users, infer: true) do
      associations do
        has_many :roles_users
        has_many :roles, through: :roles_users
        has_many :employees_users
        has_many :employees, through: :employees_users
      end
    end
  end

  conf.relation(:employees_users) do
    schema(:employees_users, infer: true) do
      associations do
        belongs_to :user, foreign_key: :user_id
        belongs_to :employee, foreign_key: :verifable_id
      end
    end
  end

  conf.relation(:employees) do
    schema(:employees, infer: true) do
      associations do
      end
    end
  end

  conf.relation(:roles_users) do
    schema(:roles_users, infer: true) do
      associations do
        belongs_to :user
        belongs_to :roles
      end
    end
  end

  conf.relation(:roles) do
    schema(:roles, infer: true) do
    end
  end
end

class UserRepo < ROM::Repository[:users]
  struct_namespace ::Entities
  def by_id(id)
    users.join(:employees_users).join(:employees, { id: :verifable_id }).select do |r|
      [
        r[:users][:id].as(:user_id),
        r[:employees][:id].as(:employee_id),
        r[:employees][:name]
      ]

    end.combine(:roles).to_a
  end
end

employee = rom.relations[:employees].changeset(:create, name: 'Jane', id: 5).commit
rom.relations[:employees].changeset(:create, name: 'Jane', id: 8).commit

user = rom.relations[:users]
  .changeset(:create, login: 'jane@doe.org')
  .commit

rom.relations[:employees_users]
  .changeset(:create, user_id: user[:id], verifable_id: employee[:id]).commit

role = rom.relations[:roles]
  .changeset(:create, name: 'a role').commit

rom.relations[:roles_users].changeset(
  :create,
  [
    { role_id: role[:id], user_id: user[:id] },
  ]
).commit

repo = UserRepo.new(rom)

user = repo.by_id(1)

p user


/work/.rvm/gems/ruby-2.4.1@test_stuff/gems/rom-core-4.1.0/lib/rom/relation/loaded.rb:96:in `fetch': key not found: :id (KeyError)
        from /work/.rvm/gems/ruby-2.4.1@test_stuff/gems/rom-core-4.1.0/lib/rom/relation/loaded.rb:96:in `block in pluck'
        from /work/.rvm/gems/ruby-2.4.1@test_stuff/gems/rom-core-4.1.0/lib/rom/relation/loaded.rb:48:in `block in each'
        from /work/.rvm/gems/ruby-2.4.1@test_stuff/gems/rom-core-4.1.0/lib/rom/relation/loaded.rb:48:in `each'
        from /work/.rvm/gems/ruby-2.4.1@test_stuff/gems/rom-core-4.1.0/lib/rom/relation/loaded.rb:48:in `each'
        from /work/.rvm/gems/ruby-2.4.1@test_stuff/gems/rom-core-4.1.0/lib/rom/relation/loaded.rb:96:in `map'
        from /work/.rvm/gems/ruby-2.4.1@test_stuff/gems/rom-core-4.1.0/lib/rom/relation/loaded.rb:96:in `pluck'
        from /work/.rvm/gems/ruby-2.4.1@test_stuff/gems/rom-sql-2.3.0/lib/rom/sql/associations/core.rb:12:in `preload'
        from /work/.rvm/gems/ruby-2.4.1@test_stuff/gems/rom-core-4.1.0/lib/rom/relation.rb:318:in `preload_assoc'
        from /work/.rvm/gems/ruby-2.4.1@test_stuff/gems/rom-core-4.1.0/lib/rom/auto_curry.rb:48:in `block (2 levels) in auto_curry'
        from /work/.rvm/gems/ruby-2.4.1@test_stuff/gems/rom-core-4.1.0/lib/rom/relation/curried.rb:59:in `call'
        from /work/.rvm/gems/ruby-2.4.1@test_stuff/gems/rom-core-4.1.0/lib/rom/relation/combined.rb:60:in `block in call'
        from /work/.rvm/gems/ruby-2.4.1@test_stuff/gems/rom-core-4.1.0/lib/rom/relation/combined.rb:60:in `map'
        from /work/.rvm/gems/ruby-2.4.1@test_stuff/gems/rom-core-4.1.0/lib/rom/relation/combined.rb:60:in `call'
        from /work/.rvm/gems/ruby-2.4.1@test_stuff/gems/rom-core-4.1.0/lib/rom/relation/materializable.rb:13:in `to_a'
        from mapper3.rb:86:in `by_id'
        from mapper3.rb:112:in `<main>'