add fields method limiting the models columns fetched from db

tests/test_foreign_keys.py

@@ -1,9 +1,6 @@
-import asyncio
-
 import databases
 import pytest
 import sqlalchemy
-from pydantic import root_validator, validator
 
 import ormar
 from ormar.exceptions import NoMatch, MultipleMatches, RelationshipInstanceError
@@ -236,8 +233,8 @@ async def test_fk_filter():
 
             tracks = (
                 await Track.objects.select_related("album")
-                .filter(album__name="Fantasies")
-                .all()
+                    .filter(album__name="Fantasies")
+                    .all()
             )
             assert len(tracks) == 3
             for track in tracks:
@@ -245,8 +242,8 @@ async def test_fk_filter():
 
             tracks = (
                 await Track.objects.select_related("album")
-                .filter(album__name__icontains="fan")
-                .all()
+                    .filter(album__name__icontains="fan")
+                    .all()
             )
             assert len(tracks) == 3
             for track in tracks:
@@ -291,8 +288,8 @@ async def test_multiple_fk():
 
             members = (
                 await Member.objects.select_related("team__org")
-                .filter(team__org__ident="ACME Ltd")
-                .all()
+                    .filter(team__org__ident="ACME Ltd")
+                    .all()
             )
             assert len(members) == 4
             for member in members:
@@ -324,8 +321,8 @@ async def test_pk_filter():
 
             tracks = (
                 await Track.objects.select_related("album")
-                .filter(position=2, album__name="Test")
-                .all()
+                    .filter(position=2, album__name="Test")
+                    .all()
             )
             assert len(tracks) == 1
 

tests/test_many_to_many.py

@@ -89,7 +89,7 @@ async def test_assigning_related_objects(cleanup):
         # or from the other end:
         await news.posts.add(post)
 
-        # Creating related object from instance:
+        # Creating columns object from instance:
         await post.categories.create(name="Tips")
         assert len(post.categories) == 2
 
@@ -148,7 +148,7 @@ async def test_removal_of_the_relations(cleanup):
         await news.posts.remove(post)
         assert len(await news.posts.all()) == 0
 
-        # Remove all related objects:
+        # Remove all columns objects:
         await post.categories.add(news)
         await post.categories.clear()
         assert len(await post.categories.all()) == 0

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()