Provide an API for only fetching certain database fields

Consider the following:

class Employee(Model):
    id = fields.IntField(pk=True)
    name = fields.CharField(max_length=1024)
    ... 100+ other fields

async def get_employee_names():
    employees = await Employee.all()
    return [employee.name for employee in employees]

In this case, get_employee_names only needs a single field from the employee table, which is name. If Employee has a lot of fields in the table (as often occurs with mature databases), we serialize all those fields into memory even when we only need a single field. That’s wasteful.

An improved API might be

async def get_employee_names():
    employees = await Employee.only('name').all()
    return [employee.name for employee in employees]

Django implements this API as documented here: https://docs.djangoproject.com/en/2.2/ref/models/querysets/#only

[grigi]

I have a working implementation in #350 Just need to write docs for it as the interactions are quite complex, as I allow partial updates.

Please have a look.

[grigi]

Ok, let’s just do this. I could use it for the pydantic serialisation to fetch less data as well.