Don't try to autosave nested assocs. Fixes #2961.

activerecord/lib/active_record/autosave_association.rb

@@ -343,7 +343,7 @@ def save_collection_association(reflection)
               if autosave
                 saved = association.insert_record(record, false)
               else
-                association.insert_record(record)
+                association.insert_record(record) unless reflection.nested?
               end
             elsif autosave
               saved = record.save(:validate => false)

activerecord/lib/active_record/reflection.rb

@@ -262,6 +262,10 @@ def chain
         [self]
       end
 
+      def nested?
+        false
+      end
+
       # An array of arrays of conditions. Each item in the outside array corresponds to a reflection
       # in the #chain. The inside arrays are simply conditions (and each condition may itself be
       # a hash, array, arel predicate, etc...)

activerecord/test/cases/associations/nested_through_associations_test.rb

@@ -545,6 +545,20 @@ def test_nested_has_many_through_with_foreign_key_option_on_the_source_reflectio
     assert_equal [organizations(:nsa)], organizations
   end
 
+  def test_nested_has_many_through_should_not_be_autosaved
+    welcome_general, thinking_general = taggings(:welcome_general), taggings(:thinking_general)
+
+    assert_equal [welcome_general, thinking_general],
+                 categorizations(:david_welcome_general).post_taggings.order('taggings.id')
+
+    c = Categorization.new
+    c.author = authors(:david)
+    c.post_taggings.to_a
+    assert !c.post_taggings.empty?
+    c.save
+    assert !c.post_taggings.empty?
+  end
+
   private
 
     def assert_includes_and_joins_equal(query, expected, association)