add fields method limiting the models columns fetched from db

tests/test_selecting_subset_of_columns.py

@@ -0,0 +1,82 @@
+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: ormar.Integer(primary_key=True)
+    name: ormar.String(max_length=100)
+    founded: ormar.Integer(nullable=True)
+
+
+class Car(ormar.Model):
+    class Meta:
+        tablename = "cars"
+        metadata = metadata
+        database = database
+
+    id: ormar.Integer(primary_key=True)
+    manufacturer: ormar.ForeignKey(Company)
+    name: ormar.String(max_length=100)
+    year: ormar.Integer(nullable=True)
+    gearbox_type: ormar.String(max_length=20, nullable=True)
+    gears: ormar.Integer(nullable=True)
+    aircon_type: 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').fields(['id', 'name', 'company__name']).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').fields('id').fields(
+                ['name', 'manufacturer']).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').fields(
+                    ['id', 'name', 'company__founded']).all()