Association for polymorphic belongs_to of a particular type

Question

I'm relatively new to Rails. I would like to add an association to a model that uses the polymorphic association, but returns only models of a particular type, e.g.:

class Note < ActiveRecord::Base
  # The true polymorphic association
  belongs_to :subject, polymorphic: true

  # Same as subject but where subject_type is 'Volunteer'
  belongs_to :volunteer, source_association: :subject
  # Same as subject but where subject_type is 'Participation'
  belongs_to :participation, source_association: :subject
end

I've tried a vast array of combinations from reading about the associations on ApiDock but nothing seems to do exactly what I want. Here's the best I have so far:

class Note < ActiveRecord::Base
  belongs_to :subject, polymorphic: true
  belongs_to :volunteer, class_name: "Volunteer", foreign_key: :subject_id, conditions: {notes: {subject_type: "Volunteer"}}
  belongs_to :participation, class_name: "Participation", foreign_key: :subject_id, conditions: {notes: {subject_type: "Participation"}}
end

And I want it to pass this test:

describe Note do
  context 'on volunteer' do
    let!(:volunteer) { create(:volunteer) }
    let!(:note) { create(:note, subject: volunteer) }
    let!(:unrelated_note) { create(:note) }

    it 'narrows note scope to volunteer' do
      scoped = Note.scoped
      scoped = scoped.joins(:volunteer).where(volunteers: {id: volunteer.id})
      expect(scoped.count).to be 1
      expect(scoped.first.id).to eq note.id
    end

    it 'allows access to the volunteer' do
      expect(note.volunteer).to eq volunteer
    end

    it 'does not return participation' do
      expect(note.participation).to be_nil
    end

  end
end

The first test passes, but you can't call the relation directly:

  1) Note on volunteer allows access to the volunteer
     Failure/Error: expect(note.reload.volunteer).to eq volunteer
     ActiveRecord::StatementInvalid:
       PG::Error: ERROR:  missing FROM-clause entry for table "notes"
       LINE 1: ...."deleted" = 'f' AND "volunteers"."id" = 7798 AND "notes"."s...
                                                                    ^
       : SELECT  "volunteers".* FROM "volunteers"  WHERE "volunteers"."deleted" = 'f' AND "volunteers"."id" = 7798 AND "notes"."subject_type" = 'Volunteer' LIMIT 1
     # ./spec/models/note_spec.rb:10:in `block (3 levels) in <top (required)>'

---

Why?

The reason I want to do it this way is because I'm constructing a scope based on parsing a query string including joining to various models/etc; the code used to construct the scope is considerably more complex than that above - it uses collection.reflections, etc. My current solution works for this, but it offends me I can't call the relations directly from an instance of Note.

I could solve it by splitting it into two issues: using scopes directly

  scope :scoped_by_volunteer_id, lambda { |volunteer_id| where({subject_type: 'Volunteer', subject_id: volunteer_id}) }
  scope :scoped_by_participation_id, lambda { |participation_id| where({subject_type: 'Participation', subject_id: participation_id}) }

and then just using a getter for note.volunteer/note.participation that just returns note.subject if it has the right subject_type (nil otherwise) but I figured in Rails there must be a better way?

Answer 1

I had bump into the similar problem. and I finally ironed out the best and most robust solution by using a self reference association like below.

class Note < ActiveRecord::Base
  # The true polymorphic association
  belongs_to :subject, polymorphic: true

  # The trick to solve this problem
  has_one :self_ref, :class_name => self, :foreign_key => :id

  has_one :volunteer, :through => :self_ref, :source => :subject, :source_type => Volunteer
  has_one :participation, :through => :self_ref, :source => :subject, :source_type => Participation
end

Clean & simple, only tested on Rails 4.1, but I guess it should work for previous versions.

Answer 2

I have found a hackish way of getting around this issue. I have a similar use case in a project of mine, and I found this to work. In your Note model you can add associations like this:

class Note
  belongs_to :volunteer, 
    ->(note) {where('1 = ?', (note.subject_type == 'Volunteer')},
    :foreign_key => 'subject_id'
end

You will need to add one of these for each model that you wish to attach notes to. To make this process DRYer I would recommend creating a module like so:

 module Notable
   def self.included(other)
     Note.belongs_to(other.to_s.underscore.to_sym, 
       ->(note) {where('1 = ?', note.subject_type == other.to_s)},
       {:foreign_key => :subject_id})
   end
 end

Then include this in your Volunteer and Participation models.

[EDIT]

A slightly better lambda would be:

 ->(note) {(note.subject_type == "Volunteer") ? where('1 = 1') : none}

For some reason replacing the 'where' with 'all' does not seem to work. Also note that 'none' is only available in Rails 4.

[MOAR EDIT]

I'm not running rails 3.2 atm so I can't test, but I think you can achieve a similar result by using a Proc for conditions, something like:

belongs_to :volunteer, :foreign_key => :subject_id, 
  :conditions => Proc.new {['1 = ?', (subject_type == 'Volunteer')]}

Might be worth a shot

Answer 3

I was stuck on this sort of reverse association and in Rails 4.2.1 I finally discovered this. Hopefully this helps someone if they're using a newer version of Rails. Your question was the closest to anything I found in regard to the issue I was having.

belongs_to :volunteer, foreign_key: :subject_id, foreign_type: 'Volunteer'

Answer 4

You can do like so:

belongs_to :volunteer, -> {
  where(notes: { subject_type: 'Volunteer' }).includes(:notes)
}, foreign_key: :subject_id

The includes do the left join so you have the notes relation with all volunteer and participation. And you go through subject_id to find your record.

Answer 5

I believe I have figured out a decent way to handle this that also covers most use cases that one might need.

I will say, it is a hard problem to find an answer to, as it is hard to figure out how to ask the question, and also hard to weed out all the articles that are just standard Rails answers. I think this problem falls into the advanced ActiveRecord realm.

Essentially what we are trying to do is to add a relationship to the model and only use that association if certain prerequisites are met on the model where the association is made. For example, if I have class SomeModel, and it has belongs_to association called "some_association", we might want to apply some prerequisite conditions that must be true on the SomeModel record that influence whether :some_association returns a result or not. In the case of a polymorphic relationship, the prerequisite condition is that the polymorphic type column is a particular value, and if not that value, it should return nil.

The difficulty of solving this problem is compounded by the different ways. I know of three different modes of access: direct access on an instance (ex: SomeModel.first.some_association), :joins (Ex: SomeModel.joins(:some_association), and :includes (Ex: SomeModel.includes(:some_association)) (note: eager_load is just a variation on joins). Each of these cases needs to be handled in a specific way.

Today, as I've essentially been revisiting this problem, I came up with the following utility method that acts as a kind of wrapper method for belongs_to. I'm guessing a similar approach could be used for other association types.

  # WARNING: the joiner table must not be aliased to something else in the query,
  #  A parent / child relationship on the same table probably would not work here
  # TODO: figure out how to support a second argument scope being passed
  def self.belongs_to_with_prerequisites(name, prerequisites: {}, **options)
    base_class = self
    belongs_to name, -> (object=nil) {
      # For the following explanation, assume we have an ActiveRecord class "SomeModel" that has a belongs_to
      #   relationship on it called "some_association"
      # Object will be one of the following:
      #   * nil - when this association is loaded via an :includes.
      #     For example, SomeModel.includes(:some_association)
      #   * an model instance - when this association is called directly on the referring model
      #     For example: SomeModel.first.some_association, object will equal SomeModel.first
      #   * A JoinDependency - when we are joining this association
      #     For example, SomeModel.joins(:some_assocation)
      if !object.is_a?(base_class)
        where(base_class.table_name => prerequisites)
      elsif prerequisites.all? {|name, value| object.send(name) == value}
        self
      else
        none
      end
    },
    options
  end

That method would need to be injected into ActiveRecord::Base.

Then we could use it like:

  belongs_to_with_prerequisites :volunteer,
    prerequisites: { subject_type: 'Volunteer' },
    polymorphic: true,
    foreign_type: :subject_type,
    foreign_key: :subject_id

And it would allow us to do the following:

Note.first.volunteer
Note.joins(:volunteer)
Note.eager_load(:volunteer)

However, we'll get an error if we try to do this:

Note.includes(:volunteer)

If we run that last bit of code, it will tell us that the column subject_type does not exist on the volunteers table.

So we'd have to add a scope to the Notes class and use as follows:

class Note < ActiveRecord::Base
  belongs_to_with_prerequisites :volunteer,
        prerequisites: { subject_type: 'Volunteer' },
        polymorphic: true,
        foreign_type: :subject_type,
        foreign_key: :subject_id
  scope :with_volunteer, -> { includes(:volunteer).references(:volunteer) }
end
Note.with_volunteer

So at the end of the day, we don't have the extra join table that @stackNG's solution had, but that solution was definitely more eloquent and less hacky. Figured I'd post this anyway as it has been the result of a very thorough investigation and might help somebody else understand how this stuff works.