Allow upserting rows with existing_pk even if they do not exist. (#889)

* Allow for __force_save__ in Model.upsert() method to save the models despite they already have a pk set. On integrity Error proceed to update the model, so in worst case two db calls will be made.

* Fix coverage

* Change implementation to checking if the row exists as postgres leaves hanging invalid transaction on integrity error. On force_save always check if row exists and then save/update (so always two queries).

ormar/models/mixins/save_mixin.py

@@ -305,7 +305,7 @@ class SavePrepareMixin(RelationMixin, AliasMixin):
         if (
             save_all or not instance.pk or not instance.saved
         ) and not instance.__pk_only__:
-            await instance.upsert()
+            await instance.upsert(__force_save__=True)
             if relation_field and relation_field.is_multi:
                 await instance._upsert_through_model(
                     instance=instance,

ormar/models/model.py

@@ -37,6 +37,15 @@ class Model(ModelRow):
         :return: saved Model
         :rtype: Model
         """
+
+        force_save = kwargs.pop("__force_save__", False)
+        if force_save:
+            expr = self.Meta.table.select().where(self.pk_column == self.pk)
+            row = await self.Meta.database.fetch_one(expr)
+            if not row:
+                return await self.save()
+            return await self.update(**kwargs)
+
         if not self.pk:
             return await self.save()
         return await self.update(**kwargs)