Add support for generated columns #196

Is your feature request related to a problem? Please describe.

I would like to efficiently query my model on a field which is calculated from another field.

I expect the following behaviour of this field:

• indices: i should be able to create an index on the calculated field
• db side filtering and aggregations: I should be able to filter and group by that field like any other DB column
• read-only: I should not be able to update the field
• follow updates from parent field: when the parent has changed, the calculated field changes too

Describe the solution you'd like

I would like to be able to define a field as generated.

import datetime
import ormar
from .base import BaseMeta


class Foo(ormar.Model):
    class Meta(BaseMeta):
        pass

    id: int = ormar.Integer(primary_key=True)
    dt: datetime.date = ormar.Date()
    year: int = ormar.Integer(generated_as="CAST(strftime('%Y', dt) AS INTEGER)", stored=False, index=True)

Describe alternatives you've considered

Some of this behaviour can be achieved by defining signals for pre-update and pre-save, which would ensure the calculated column is always stored correctly.

Anyways, this does not prevent me of changing the field value the model or in the DB.

I've also considered ormar.property_field, but this would require all filtering and grouping being implemented on application side.

Additional context

• [MySQL] CREATE TABLE and Generated Columns
• [SQLite] Generated Columns
• [PostgreSQL] Generated Columns