ActiveRecord, has_many :through, and Polymorphic Associations

Question

Folks,

Want to make sure I understand this correctly. And please disregard the case for inheritance here (SentientBeing), trying to instead focus on polymorphic models in has_many :through relationships. That said, consider the following...

class Widget < ActiveRecord::Base
  has_many :widget_groupings

  has_many :people, :through => :widget_groupings, :source => :person, :conditions => "widget_groupings.grouper_type = 'Person'"
  has_many :aliens, :through => :widget_groupings, :source => :alien, :conditions => "video_groupings.grouper_type = 'Alien'"
end

class Person < ActiveRecord::Base
  has_many :widget_groupings, :as => grouper
  has_many :widgets, :through => :widget_groupings
end

class Alien < ActiveRecord::Base
  has_many :widget_groupings, :as => grouper
  has_many :widgets, :through => :widget_groupings  
end

class WidgetGrouping < ActiveRecord::Base
  belongs_to :widget
  belongs_to :grouper, :polymorphic => true
end

In a perfect world, I'd like to, given a Widget and a Person, do something like:

widget.people << my_person

However, when I do this, I've noticed the 'type' of the 'grouper' is always null in widget_groupings. However, if I to something like the following:

widget.widget_groupings << WidgetGrouping.new({:widget => self, :person => my_person}) 

Then all works as I would have normally expected. I don't think I've ever seen this occur with non polymorphic associations and just wanted to know if this was something specific to this use case or if I'm potentially staring at a bug.

Thanks for any help!

Answer 1

There is a known issue with Rails 3.1.1 that breaks this functionality. If you are having this problem first try upgrading, it's been fixed in 3.1.2

You're so close. The problem is you're misusing the :source option. :source should points to the polymorphic belongs_to relationship. Then all you need to do is specify :source_type for the relationship you're trying to define.

This fix to the Widget model should allow you do exactly what you're looking for.

class Widget < ActiveRecord::Base
  has_many :widget_groupings

  has_many :people, :through => :widget_groupings, :source => :grouper, :source_type => 'Person'
  has_many :aliens, :through => :widget_groupings, :source => :grouper, :source_type => 'Alien'
end

Answer 2

As mentioned above, this doesn't work with rails 3.1.1 due to a bug on :source, but it's fixed in Rails 3.1.2

Answer 3

For example: locations have many trains, copters, trucks, ships. And trains, copters, trucks, ships have many locations.

Different locations can have similar transports(store in moveable polymorphic columns).

#db/migrations/create_moveable_locations.rb

class CreateMoveableLocations < ActiveRecord::Migration
  def change
    create_table :moveable_locations do |t|
      t.references :moveable, polymorphic: true
      t.references :location

      t.timestamps
    end
  end
end

#app/models/moveable_location.rb

class MoveableLocation < ActiveRecord::Base
  belongs_to :moveable, polymorphic: true
  belongs_to :location
end

#app/models/location.rb

class Location < ActiveRecord::Base
  has_many :moveable_locations, dependent: :destroy

  has_many :trains, through: :moveable_locations, source: :moveable, source_type: 'Train'
  has_many :copters, through: :moveable_locations, source: :moveable, source_type: 'Copter'
  has_many :trucks, through: :moveable_locations, source: :moveable, source_type: 'Truck'
  has_many :ships, through: :moveable_locations, source: :moveable, source_type: 'Ship'
end

#app/models/train.rb

class Train < ActiveRecord::Base
  has_many :moveable_locations, as: :moveable, dependent: :destroy
  has_many :locations, through: :moveable_locations
end

#app/models/copter.rb

class Copter < ActiveRecord::Base
  has_many :moveable_locations, as: :moveable, dependent: :destroy
  has_many :locations, through: :moveable_locations
end

#app/models/truck.rb

class Truck < ActiveRecord::Base
  has_many :moveable_locations, as: :moveable, dependent: :destroy
  has_many :locations, through: :moveable_locations
end

#app/models/ship.rb

class Ship < ActiveRecord::Base
  has_many :moveable_locations, as: :moveable, dependent: :destroy
  has_many :locations, through: :moveable_locations
end

Using:

train1 = Train.create(title: 'Train1')
train2 = Train.create(title: 'Train2')
location1 = Location.create(title: 'Location1', train_ids: [train1.id, train2.id])
location2 = Location.create(title: 'Location2', trains: [train1, train2])
location3 = Location.create(title: 'Location3')
location3.train_ids << [train1., train2.id]
location4 = Location.create(title: 'Location34')
location4.trains << [train1, train2]
copter1 = Copter.create(title: 'Copter1', location_ids: [location1.id, location2.id]
copter2 = Copter.create(title: 'Copter1')
copter2.location_ids << [location1.id, location2.id, location3.id]

Answer 4

has many :through and polymorphic don't work together. If you try to access them directly, it should throw an error. If i am not mistaken, you have to hand write widget.people and the push routine.

I don't think it is a bug, it is just something which hasn't been implemented yet. I would imagine we see it in the feature, because everyone has a case in which they could use it.