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.:

<

Answer 1

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