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).

tests/test_model_methods/test_upsert.py

@@ -0,0 +1,64 @@
+from typing import Optional
+
+import databases
+import pytest
+import sqlalchemy
+
+import ormar
+from tests.settings import DATABASE_URL
+
+database = databases.Database(DATABASE_URL, force_rollback=True)
+metadata = sqlalchemy.MetaData()
+
+
+class Director(ormar.Model):
+    class Meta:
+        tablename = "directors"
+        metadata = metadata
+        database = database
+
+    id: int = ormar.Integer(primary_key=True)
+    name: str = ormar.String(max_length=100, nullable=False, name="first_name")
+    last_name: str = ormar.String(max_length=100, nullable=False, name="last_name")
+
+
+class Movie(ormar.Model):
+    class Meta:
+        tablename = "movies"
+        metadata = metadata
+        database = database
+
+    id: int = ormar.Integer(primary_key=True)
+    name: str = ormar.String(max_length=100, nullable=False, name="title")
+    year: int = ormar.Integer()
+    profit: float = ormar.Float()
+    director: Optional[Director] = ormar.ForeignKey(Director)
+
+
+@pytest.fixture(autouse=True, scope="module")
+def create_test_database():
+    engine = sqlalchemy.create_engine(DATABASE_URL)
+    metadata.drop_all(engine)
+    metadata.create_all(engine)
+    yield
+    metadata.drop_all(engine)
+
+
+@pytest.mark.asyncio
+async def test_updating_selected_columns():
+    async with database:
+        director1 = await Director(name="Peter", last_name="Jackson").save()
+
+        await Movie(
+            id=1, name="Lord of The Rings", year=2003, director=director1, profit=1.212
+        ).upsert()
+
+        with pytest.raises(ormar.NoMatch):
+            await Movie.objects.get()
+
+        await Movie(
+            id=1, name="Lord of The Rings", year=2003, director=director1, profit=1.212
+        ).upsert(__force_save__=True)
+        lotr = await Movie.objects.get()
+        assert lotr.year == 2003
+        assert lotr.name == "Lord of The Rings"