How to Map A Many to Many Join in Ruby Object Mapper

Question

Given:

create_table(:foos) do
  primary_key(:id)
  String(:name)
end

create_table(:bars) do
  primary_key(:id)
  String(:name)
end

create_table(:foos_bars) do
  primary_key(:id)
  foreign_key(:foo_id, :foos)
  foreign_key(:bar_id, :bars)
  String(:name)
end

class Foos < ROM::Relation[:sql]
  dataset :foos

  def with_bars(id)
    prefix('foos').qualified.select(
      :foos__id, :foos__name
    ).select_append(
      :bars__id, :bars__name
    ).left_join(
      :foos_bars, foos_bars__bars_id: :foos__id
    ).left_join(
      :bars, bars__id: :foos_bars__bars_id
    ).where(foos__id: id)
  end
end

class FoosModel
  include Virtus.model

  attribute :id
  attribute :name
  attribute :bars
end

class BarsModel
  include Virtus.model

  attribute :id
  attribute :name
  attribute :foos
end

I've tried many, many random variations on the keywords shown in (but not explained in) the ROM docs to no avail. Here's a literal interpretation of the docs into a mapper for Foos, which doesn't work:

class FoosMapper < ROM::Mapper
  relation :foos
  register_as :foos
  model Foo

  prefix('foos')
  attribute :id
  attribute :name

  group :bars do
    model Bar
    attribute :id, from: :bar
    attribute :name, from: :bar
  end
end

How does one write a mapper (or rework the relation to work with the mapper) to get the simple result of a foo with a bars attribute having all the bars linked by the foos_bars table?

Answer 1

Building up complex joins like that is not recommended unless you have good reasons like performance. It's much simpler to use repositories to compose relations by defining relation views that you need for reusability and composing those in various way inside repos. Defining custom mappers should also be avoided unless you have some unique requirements.

I made a gist that illustrates that right here: https://gist.github.com/solnic/9307e1b2e3428718dd12

We're working on a new set of docs that will properly explain those things.