Merge pull request rails#7569 from skorfmann/improve-has-many-through…

…-exception-message

Improve exception message for HasManyThroughAssociationPolymorphicSourceError

activerecord/lib/active_record/associations.rb

@@ -17,7 +17,7 @@ def initialize(owner_class_name, reflection)
 
   class HasManyThroughAssociationPolymorphicSourceError < ActiveRecordError #:nodoc:
     def initialize(owner_class_name, reflection, source_reflection)
-      super("Cannot have a has_many :through association '#{owner_class_name}##{reflection.name}' on the polymorphic object '#{source_reflection.class_name}##{source_reflection.name}'.")
+      super("Cannot have a has_many :through association '#{owner_class_name}##{reflection.name}' on the polymorphic object '#{source_reflection.class_name}##{source_reflection.name}' without 'source_type'. Try adding 'source_type: \"#{reflection.name.to_s.classify}\"' to 'has_many :through' definition.")
     end
   end