2 files changed, 10 insertions, 0 deletions
diff --git a/activerecord/lib/active_record/associations.rb b/activerecord/lib/active_record/associations.rb
index cdc8f25119..c0cd222244 100644
--- a/activerecord/lib/active_record/associations.rb
+++ b/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
diff --git a/activerecord/lib/active_record/reflection.rb b/activerecord/lib/active_record/reflection.rb
index 70165c600e..7a7f7812df 100644
--- a/activerecord/lib/active_record/reflection.rb
+++ b/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