Don't allow a has_one association to go :through a collection association [#2976 state:resolved]

activerecord/lib/active_record/associations.rb

@@ -32,6 +32,12 @@ module ActiveRecord
     end
   end
 
+  class HasOneThroughCantAssociateThroughCollection < ActiveRecordError #:nodoc:
+    def initialize(owner_class_name, reflection, through_reflection)
+      super("Cannot have a has_one :through association '#{owner_class_name}##{reflection.name}' where the :through association '#{owner_class_name}##{through_reflection.name}' is a collection. Specify a has_one or belongs_to association in the :through option instead.")
+    end
+  end
+
   class HasManyThroughSourceAssociationNotFoundError < ActiveRecordError #:nodoc:
     def initialize(reflection)
       through_reflection      = reflection.through_reflection

activerecord/lib/active_record/reflection.rb

@@ -388,6 +388,10 @@ module ActiveRecord
           raise HasManyThroughSourceAssociationMacroError.new(self)
         end
 
+        if macro == :has_one && through_reflection.collection?
+          raise HasOneThroughCantAssociateThroughCollection.new(active_record.name, self, through_reflection)
+        end
+
         check_validity_of_inverse!
       end
 

activerecord/test/cases/associations/has_one_through_associations_test.rb

@@ -25,10 +25,6 @@ class HasOneThroughAssociationsTest < ActiveRecord::TestCase
     assert_equal clubs(:boring_club), @member.club
   end
 
-  def test_has_one_through_with_has_many
-    assert_equal clubs(:moustache_club), @member.favourite_club
-  end
-
   def test_creating_association_creates_through_record
     new_member = Member.create(:name => "Chris")
     new_member.club = Club.create(:name => "LRUG")
@@ -235,6 +231,12 @@ class HasOneThroughAssociationsTest < ActiveRecord::TestCase
   end
 
   def test_has_one_through_with_default_scope_on_join_model
-    assert_equal posts(:welcome).comments.order('id').first, authors(:david).comment_on_first_posts
+    assert_equal posts(:welcome).comments.order('id').first, authors(:david).comment_on_first_post
+  end
+
+  def test_has_one_through_many_raises_exception
+    assert_raise(ActiveRecord::HasOneThroughCantAssociateThroughCollection) do
+      members(:groucho).club_through_many
+    end
   end
 end

activerecord/test/models/author.rb

@@ -28,7 +28,9 @@ class Author < ActiveRecord::Base
 
   has_many :first_posts
   has_many :comments_on_first_posts, :through => :first_posts, :source => :comments, :order => 'posts.id desc, comments.id asc'
-  has_one  :comment_on_first_posts,  :through => :first_posts, :source => :comments, :order => 'posts.id desc, comments.id asc'
+
+  has_one :first_post
+  has_one :comment_on_first_post,  :through => :first_post, :source => :comments, :order => 'posts.id desc, comments.id asc'
 
   has_many :thinking_posts, :class_name => 'Post', :conditions => { :title => 'So I was thinking' }, :dependent => :delete_all
   has_many :welcome_posts,  :class_name => 'Post', :conditions => { :title => 'Welcome to the weblog' }

activerecord/test/models/member.rb

@@ -1,12 +1,15 @@
 class Member < ActiveRecord::Base
   has_one :current_membership
-  has_many :memberships
+  has_one :membership
   has_many :fellow_members, :through => :club, :source => :members
   has_one :club, :through => :current_membership
-  has_one :favourite_club, :through => :memberships, :conditions => ["memberships.favourite = ?", true], :source => :club
+  has_one :favourite_club, :through => :membership, :conditions => ["memberships.favourite = ?", true], :source => :club
   has_one :sponsor, :as => :sponsorable
   has_one :sponsor_club, :through => :sponsor
   has_one :member_detail
   has_one :organization, :through => :member_detail
   belongs_to :member_type
-end
+
+  has_many :current_memberships
+  has_one :club_through_many, :through => :current_memberships, :source => :club
+end