Hi there,

First of all, I would like to thank you all for making Mongoid, it is a really well executed project. We had an issue when upgrading from Mongoid 2.4.12 to 3.0.4. In our project we have an entity that references another entity multiple times, one for each concern it represents, like the one below:

# Users can act both as resource owners and resource collaborators
class User
  include Mongoid::Document
  include Owner
  include Collaborator

end

The concerns are represented as follows:

module Collaborator
  extend ActiveSupport::Concern

  included do
    has_many :tasks, :as => :collaborator, :dependent => :destroy
    has_many :collaborator_resources, :as => :collaborator, :class_name => 'SharedResource'
  end

end

module Owner
  extend ActiveSupport::Concern

  included do
    has_many :permissions, :as => :owner, :dependent => :destroy
    has_many :owner_resources, :as => :owner, :class_name => 'SharedResource'
  end

end

Each concern associates the parent again to the following entities:

class Task
  include Mongoid::Document

  belongs_to :collaborator, :polymorphic => true
  validates :collaborator, :presence => true

end

class Permission
  include Mongoid::Document

  belongs_to :owner, :polymorphic => true
  validates :owner, :presence => true

end

The problem is, Mongoid 3.x is raising the following error in this scenario:

> Mongoid::Errors::InvalidSetPolymorphicRelation:
>
> Problem:
> The owner attribute can't be set to an instance of User as User has multiple relations referencing SharedResource as owner.
> Summary:
> If the parent class of a polymorphic relation has multiple definitions for the same relation, the values must be set from the parent side and not the child side since Mongoid cannot determine from the child side which relation to go in.
> Resolution:
> Set the values from the parent, or redefine the relation with only a single definition in the parent.

The same relations work fine in Mongoid 2.4.x, also I can't see where the above example might be wrong by definition. I wonder if there is an issue with Mongoid 3.x and multiple polymorphic associations.

To better illustrate the issue, I created two sample repositories with this example and specs to check this behavior, one for each branch of Mongoid. The samples are on:

erichmachado/issue-sample-mongoid-3
erichmachado/issue-sample-mongoid-2

I really appreciate your help and feedback on this. Thank you,
Erich