add exclude_fields queryset method

tests/test_excluding_subset_of_columns.py

@@ -0,0 +1,121 @@
+from typing import Optional
+
+import databases
+import pydantic
+import pytest
+import sqlalchemy
+
+import ormar
+from tests.settings import DATABASE_URL
+
+database = databases.Database(DATABASE_URL, force_rollback=True)
+metadata = sqlalchemy.MetaData()
+
+
+class Company(ormar.Model):
+    class Meta:
+        tablename = "companies"
+        metadata = metadata
+        database = database
+
+    id: int = ormar.Integer(primary_key=True)
+    name: str = ormar.String(max_length=100, nullable=False)
+    founded: int = ormar.Integer(nullable=True)
+
+
+class Car(ormar.Model):
+    class Meta:
+        tablename = "cars"
+        metadata = metadata
+        database = database
+
+    id: int = ormar.Integer(primary_key=True)
+    manufacturer: Optional[Company] = ormar.ForeignKey(Company)
+    name: str = ormar.String(max_length=100)
+    year: int = ormar.Integer(nullable=True)
+    gearbox_type: str = ormar.String(max_length=20, nullable=True)
+    gears: int = ormar.Integer(nullable=True)
+    aircon_type: str = ormar.String(max_length=20, nullable=True)
+
+
+@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_selecting_subset():
+    async with database:
+        async with database.transaction(force_rollback=True):
+            toyota = await Company.objects.create(name="Toyota", founded=1937)
+            await Car.objects.create(
+                manufacturer=toyota,
+                name="Corolla",
+                year=2020,
+                gearbox_type="Manual",
+                gears=5,
+                aircon_type="Manual",
+            )
+            await Car.objects.create(
+                manufacturer=toyota,
+                name="Yaris",
+                year=2019,
+                gearbox_type="Manual",
+                gears=5,
+                aircon_type="Manual",
+            )
+            await Car.objects.create(
+                manufacturer=toyota,
+                name="Supreme",
+                year=2020,
+                gearbox_type="Auto",
+                gears=6,
+                aircon_type="Auto",
+            )
+
+            all_cars = (
+                await Car.objects.select_related("manufacturer")
+                .exclude_fields(["gearbox_type", "gears", "aircon_type", "year", "company__founded"])
+                .all()
+            )
+            for car in all_cars:
+                assert all(
+                    getattr(car, x) is None
+                    for x in ["year", "gearbox_type", "gears", "aircon_type"]
+                )
+                assert car.manufacturer.name == "Toyota"
+                assert car.manufacturer.founded is None
+
+            all_cars = (
+                await Car.objects.select_related("manufacturer")
+                .exclude_fields("year")
+                .exclude_fields(["gearbox_type", "gears"])
+                .exclude_fields("aircon_type")
+                .all()
+            )
+            for car in all_cars:
+                assert all(
+                    getattr(car, x) is None
+                    for x in ["year", "gearbox_type", "gears", "aircon_type"]
+                )
+                assert car.manufacturer.name == "Toyota"
+                assert car.manufacturer.founded == 1937
+
+            all_cars_check = await Car.objects.select_related("manufacturer").all()
+            for car in all_cars_check:
+                assert all(
+                    getattr(car, x) is not None
+                    for x in ["year", "gearbox_type", "gears", "aircon_type"]
+                )
+                assert car.manufacturer.name == "Toyota"
+                assert car.manufacturer.founded == 1937
+
+            with pytest.raises(pydantic.error_wrappers.ValidationError):
+                # cannot exclude mandatory model columns - company__name in this example
+                await Car.objects.select_related("manufacturer").exclude_fields(
+                    ["company__name"]
+                ).all()