Make Relation#create_with always merge rather than overwrite, not just when merging two relations. If you wish to overwrite, you can do relation.create_with(nil), or for a specific attribute, relation.create_with(:attr => nil).

activerecord/lib/active_record/relation/query_methods.rb

@@ -128,7 +128,7 @@ def readonly(value = true)
 
     def create_with(value)
       relation = clone
-      relation.create_with_value = value
+      relation.create_with_value = value && (@create_with_value || {}).merge(value)
       relation
     end
 

activerecord/lib/active_record/relation/spawn_methods.rb

@@ -53,9 +53,7 @@ def merge(r)
 
       merged_relation.lock_value = r.lock_value unless merged_relation.lock_value
 
-      if r.create_with_value
-        merged_relation.create_with_value = (merged_relation.create_with_value || {}).merge(r.create_with_value)
-      end
+      merged_relation = merged_relation.create_with(r.create_with_value) if r.create_with_value
 
       # Apply scope extension modules
       merged_relation.send :apply_modules, r.extensions

activerecord/test/cases/relation_scoping_test.rb

@@ -491,5 +491,15 @@ def test_create_with_merge
              PoorDeveloperCalledJamis.create_with(:name => 'Aaron')).new
     assert_equal 20, aaron.salary
     assert_equal 'Aaron', aaron.name
+
+    aaron = PoorDeveloperCalledJamis.create_with(:name => 'foo', :salary => 20).
+                                     create_with(:name => 'Aaron').new
+    assert_equal 20, aaron.salary
+    assert_equal 'Aaron', aaron.name
+  end
+
+  def test_create_with_reset
+    jamis = PoorDeveloperCalledJamis.create_with(:name => 'Aaron').create_with(nil).new
+    assert_equal 'Jamis', jamis.name
   end
 end