update readme, bump version

README.md

@@ -408,6 +408,83 @@ await Product.objects.create(name="Mars", company="Mars")
 
 ```
 
+Since version >=0.3.6 Ormar supports selecting subset of model columns to limit the data load
+Warning - mandatory fields cannot be excluded as it will raise validation error, to exclude a field it has to be nullable.
+pkcolumn cannot be excluded - it's always auto added
+```python
+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)
+
+
+
+# build some sample data
+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')
+
+# select manufacturer but only name - to include related models use notation {model_name}__{column}
+all_cars = await Car.objects.select_related('manufacturer').fields(['id', 'name', 'company__name']).all()
+for car in all_cars:
+    # excluded columns will yield None
+    assert all(getattr(car, x) is None for x in ['year', 'gearbox_type', 'gears', 'aircon_type'])
+    # included column on related models will be available, pk column is always included
+    # even if you do not include it in fields list
+    assert car.manufacturer.name == 'Toyota'
+    # also in the nested related models - you cannot exclude pk - it's always auto added 
+    assert car.manufacturer.founded is None
+
+# fields() can be called several times, building up the columns to select
+# models selected in select_related but with no columns in fields list implies all fields
+all_cars = await Car.objects.select_related('manufacturer').fields('id').fields(
+                ['name']).all()
+# all fiels from company model are selected
+assert all_cars[0].manufacturer.name == 'Toyota' 
+assert all_cars[0].manufacturer.founded ==  1937
+
+# cannot exclude mandatory model columns - company__name in this example
+await Car.objects.select_related('manufacturer').fields(['id', 'name', 'company__founded']).all()
+# will raise pydantic ValidationError as company.name is required
+
+```
+
 
 ## Data types
 

ormar/__init__.py

@@ -28,7 +28,7 @@ class UndefinedType:  # pragma no cover
 
 Undefined = UndefinedType()
 
-__version__ = "0.3.5"
+__version__ = "0.3.6"
 __all__ = [
     "Integer",
     "BigInteger",

tests/test_selecting_subset_of_columns.py

@@ -50,33 +50,69 @@ 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,
+            await Car.objects.create(
-                                     aircon_type='Manual')
+                manufacturer=toyota,
-            await Car.objects.create(manufacturer=toyota, name="Yaris", year=2019, gearbox_type='Manual', gears=5,
+                name="Corolla",
-                                     aircon_type='Manual')
+                year=2020,
-            await Car.objects.create(manufacturer=toyota, name="Supreme", year=2020, gearbox_type='Auto', gears=6,
+                gearbox_type="Manual",
-                                     aircon_type='Auto')
+                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()
+            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 all(
-                assert car.manufacturer.name == 'Toyota'
+                    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(
+            all_cars = (
-                ['name', 'manufacturer']).all()
+                await Car.objects.select_related("manufacturer")
+                .fields("id")
+                .fields(["name"])
+                .all()
+            )
             for car in all_cars:
-                assert all(getattr(car, x) is None for x in ['year', 'gearbox_type', 'gears', 'aircon_type'])
+                assert all(
-                assert car.manufacturer.name == 'Toyota'
+                    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()
+            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 all(
-                assert car.manufacturer.name == 'Toyota'
+                    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(
+                await Car.objects.select_related("manufacturer").fields(
-                    ['id', 'name', 'company__founded']).all()
+                    ["id", "name", "company__founded"]
+                ).all()