Probem with ActiveFedora associations

gistfile1.rb

class Collection < ActiveFedora::Base
  has_many :contributors, inverse_of: :contributorable, as: 'contributorable'
  accepts_nested_attributes_for :contributors, allow_destroy: true, reject_if: proc { |attributes| attributes['contributor_name'].first.blank? }
end

class Contributor < ActiveFedora::Base
 belongs_to :contributorable, predicate: ::RDF::URI.new('http://collections.durham.ac.uk/ns#contributor_to'),
   class_name: 'ActiveFedora::Base'
end

Collection.new.association(:contributors).find_target

c1 = Collection.create
c2 = Collection.create
c3 = Collection.create
cont1 = Contributor.create
cont2 = Contributor.create
cont1.contributorable = c1
cont2.contributorable = c2
cont1.save
cont2.save

# This fails because it returns two results (cont1 & cont2)
expect(Collection.new.association(:contributors).find_target).to be_empty
=> [#<Contributor id: "41/cf/16/77/41cf1677-c74a-463a-97e0-1163a91d1658", contributorable_id: "70/5e/3d/ed/705e3ded-da2a-4118-99c1-4ed7ce194e81">, #<Contributor id: "8e/b9/f3/63/8eb9f363-7d8a-4fe3-8dc7-ac4a5d5f8f02", contributorable_id: "f2/cc/9f/f2/f2cc9ff2-44e4-451d-b01e-cc63131f180e">]