Reset self.instance after each load

Fix #78

marshmallow_sqlalchemy/schema.py

@@ -183,7 +183,9 @@ def load(self, data, session=None, instance=None, *args, **kwargs):
         self.instance = instance or self.instance
         if not self.session:
             raise ValueError('Deserialization requires a session')
-        return super(ModelSchema, self).load(data, *args, **kwargs)
+        ret = super(ModelSchema, self).load(data, *args, **kwargs)
+        self.instance = None
+        return ret
 
     def validate(self, data, session=None, *args, **kwargs):
         self.session = session or self.session

tests/test_marshmallow_sqlalchemy.py

@@ -668,6 +668,23 @@ def test_model_schema_loading_custom_instance(self, models, schemas, student, se
         assert result.data is student
         assert result.data.current_school == student.current_school
 
+    # Regression test for https://github.com/marshmallow-code/marshmallow-sqlalchemy/issues/78
+    def test_model_schema_loading_resets_instance(self, models, schemas, student):
+        schema = schemas.StudentSchema()
+        result1 = schema.load({'full_name': 'new name'}, instance=student)
+        assert not result1.errors
+        data1 = result1.data
+        assert data1.id == student.id
+        assert data1.full_name == student.full_name
+
+        result2 = schema.load({'full_name': 'new name2'})
+        assert not result2.errors
+        data2 = result2.data
+        assert isinstance(data2, models.Student)
+        # loaded data is different from first instance (student)
+        assert data2 != student
+        assert data2.full_name == 'new name2'
+
     def test_model_schema_loading_no_instance_or_pk(self, models, schemas, student, session):
         schema = schemas.StudentSchema()
         dump_data = {'full_name': 'Terry Gilliam'}