rom2.rb

class Users < ROM::Relation[:sql]
  schema(:users) do
    attribute :id, Types::Serial
    attribute :name, Types::String

    associate do
      many :posts
      many :labels, through: :posts
    end
  end
end

class Posts < ROM::Relation[:sql] do
  schema(:posts) do
    attribute :id, Types::Serial
    attribute :author_id, Types::ForeignKey(:users)
    attribute :title, Types::String
    attribute :body, Types::String

    associate do
      many :labels, through: :posts_labels
      belongs :author, relation: :users
    end
  end
end

class PostLabels < ROM::Relation[:sql]
  schema(:post_labels) do
    attribute :post_id, Types::ForeignKey(:posts)
    attribute :label_id, Types::ForeignKey(:labels)
    primary_key :post_id, :label_id

    associate do
      belongs :posts
      belongs :labels
    end
  end
end

class UserRepo < ROM::Repository[:users]
end

user_repo = UserRepo.new(rom_container)

user_repo.aggregate(:posts, :labels) # ╰(✿˙ᗜ˙)੭━☆ﾟ.*･｡ﾟ

__END__

#<ROM::Struct[User] id=1 name="Jane" posts=[#<ROM::Struct[Post] id=1 author_id=1 title="Hello From Jane" body="Jane Post">] labels=[#<ROM::Struct[Label] id=1 name="red" author_id=1>, #<ROM::Struct[Label] id=3 name="blue" author_id=1>]>