Question

Im having a bit of an issue setting up a Rails has_many :through :polymorphic relationship.

I’m aware this subject is well documented on SO but i think my problem is down to my model and foreign_key names as opposed to syntax i.e i think this is a “i’ve been looking at code too long” issue that just requires another set of eyes.

Anyway i have the following setup:

class Milestone < ActiveRecord::Base
  has_many :responsible_items, :as => :responsibility
  has_many :responsible, :through => :responsible_items
end

class ResponsibleItem < ActiveRecord::Base
  belongs_to :responsible, :class_name => "User"
  belongs_to :responsibility, :polymorphic => true
end

class User < ActiveRecord::Base
  has_many :responsible_items, :foreign_key => :responsible_id
  has_many :responsibilities, :through => :responsible_items
end

This seems to work fine, without error, from the Milestone side of things. For example in terminal i can write:

Milestone.first.responsible

…and get an empty collection as i’d expect.

However, from the User side of things, running:

User.first.responsibilities

…is returning an AR error:

ActiveRecord::HasManyThroughAssociationPolymorphicSourceError: Cannot have a has_many :through association 'User#responsibilities' on the polymorphic object 'Responsibility#responsibility'.

I’m assuming the issue is something to do with the fact that i am referring to the User relationship as :responsible. Is this right?

Any help would be much appreciated, thanks.

Answer 1

Thanks to @Abid’s comment i got thinking about the logistics of pulling in all responsibilities for a user, which wasn’t going to be feasible. I needed to be more specific about what i wanted from the relationship and as result defining the following worked:

class User < ActiveRecord::Base
  has_many :responsible_items, :foreign_key => :responsible_id
  has_many :milestone_responsibilities, :through => :responsible_items, :source => :responsibility, :source_type => 'Milestone'
end

I can then extend this as i add further polymorphic relationships on other models for example:

class User < ActiveRecord::Base
  has_many :responsible_items, :foreign_key => :responsible_id
  has_many :milestone_responsibilities, :through => :responsible_items, :source => :responsibility, :source_type => 'Milestone'
  has_many :task_responsibilities, :through => :responsible_items, :source => :responsibility, :source_type => 'Task'
end