2 files changed, 18 insertions, 1 deletions
diff --git a/activerecord/lib/active_record/associations.rb b/activerecord/lib/active_record/associations.rb
index 4a637d5dab..2861a3940b 100755
--- a/activerecord/lib/active_record/associations.rb
+++ b/activerecord/lib/active_record/associations.rb
@@ -66,6 +66,16 @@ module ActiveRecord
     end
   end
 
+  class ReadOnlyAssociation < ActiveRecordError #:nodoc:
+    def initialize(reflection)
+      @reflection = reflection
+    end
+  
+    def message
+      "Can not add to a has_many :through association.  Try adding to #{@reflection.through_reflection.name.inspect}."
+    end
+  end
+
   module Associations # :nodoc:
     def self.append_features(base)
       super
diff --git a/activerecord/lib/active_record/associations/has_many_through_association.rb b/activerecord/lib/active_record/associations/has_many_through_association.rb
index 44054b42e1..8cafb26d44 100644
--- a/activerecord/lib/active_record/associations/has_many_through_association.rb
+++ b/activerecord/lib/active_record/associations/has_many_through_association.rb
@@ -8,7 +8,6 @@ module ActiveRecord
         construct_sql
       end
 
-
       def find(*args)
         options = Base.send(:extract_options_from_args!, args)
 
@@ -41,6 +40,14 @@ module ActiveRecord
         @loaded = false
       end
 
+      def <<(*args)
+        raise ActiveRecord::ReadOnlyAssociation, @reflection
+      end
+
+      [:push, :concat, :create, :build].each do |method|
+        alias_method method, :<<
+      end
+
       protected
         def method_missing(method, *args, &block)
           if @target.respond_to?(method) || (!@reflection.klass.respond_to?(method) && Class.respond_to?(method))