CanCan and polymorphic associations (Eager Loading error)

Question

I'm trying to define a user's ability to access something based on a column on an associated model (so something like can :read, Step, 'steppable' => {published: true}), the problem is that it's a polymorphic association so it can't find the steppable table because it doesn't exist.

I've got steps, and each step has a steppable (either a lecture, a quiz, or some other action). I need an activerecord query that will work. I've tried:

Step.includes(:steppable).where('steppable' => {published: true})

and

Step.joins(:steppable).where('steppable' => {published: true})

But both result in ActiveRecord::EagerLoadPolymorphicError: Can not eagerly load the polymorphic association :steppable

Models look like this:

class Step < ActiveRecord::Base
   ...
   belongs_to :steppable, polymorphic: true, dependent: :destroy
   ...
end

and

class Lecture
   ...
   has_one :step, as: :steppable, dependent: :destroy
   ...
end

Note: I'd like to be agnostic regarding the associated model, and in order for it to work for fetching records with CanCan, it has to be done using database columns (see github.com/ryanb/cancan/wiki/defining-abilities)

Answer 1

You should be able to do this:

can :read, Step, steppable_type: 'Lecture', steppable_id: Lecture.published.pluck(:id)
can :read, Step, steppable_type: 'OtherThing', steppable_id: OtherThing.published.pluck(:id)

You have to do it for each Steppable class, but it gets around the eager loading polymorphic associations problem. To dry this up a bit:

[Lecture, OtherThing].each do |klass|
  can :read, Step, steppable_type: klass.to_s, steppable_id: klass.published.pluck(:id)
end

In this case, as long as each steppable class has a scope published, you just add any steppable class into that array, even if published is defined differently in each class.