reset @arel when modifying a Relation in place.

/cc @tenderlove

activerecord/CHANGELOG.md

@@ -1,3 +1,8 @@
+*   Reset the cache when modifying a Relation with cached Arel.
+    Additionally display a warning message to make the user aware.
+
+    *Yves Senn*
+
 *   PostgreSQL should internally use `:datetime` consistently for TimeStamp. Assures
     different spellings of timestamps are treated the same.
 

activerecord/lib/active_record/associations/preloader/through_association.rb

@@ -84,7 +84,7 @@ def through_scope
             end
 
             scope.references! reflection_scope.values[:references]
-            scope.order! reflection_scope.values[:order] if scope.eager_loading?
+            scope = scope.order reflection_scope.values[:order] if scope.eager_loading?
           end
 
           scope

activerecord/lib/active_record/relation/query_methods.rb

@@ -64,6 +64,7 @@ def #{name}_values                   # def select_values
                                              #
         def #{name}_values=(values)          # def select_values=(values)
           raise ImmutableRelation if @loaded #   raise ImmutableRelation if @loaded
+          check_cached_relation
           @values[:#{name}] = values         #   @values[:select] = values
         end                                  # end
       CODE
@@ -81,11 +82,22 @@ def #{name}_value                    # def readonly_value
       class_eval <<-CODE, __FILE__, __LINE__ + 1
         def #{name}_value=(value)            # def readonly_value=(value)
           raise ImmutableRelation if @loaded #   raise ImmutableRelation if @loaded
+          check_cached_relation
           @values[:#{name}] = value          #   @values[:readonly] = value
         end                                  # end
       CODE
     end
 
+    def check_cached_relation # :nodoc:
+      if defined?(@arel) && @arel
+        @arel = nil
+        ActiveSupport::Deprecation.warn <<-WARNING
+Modifying already cached Relation. The cache will be reset.
+Use a cloned Relation to prevent this warning.
+WARNING
+      end
+    end
+
     def create_with_value # :nodoc:
       @values[:create_with] || {}
     end