Fix assigning expressions to Mapped attributes.

[MaicoTimmerman]

Description

Allow for assigning expressions to mapped attributes. Fixes #68.

I tried to use ColumnElement[_T], however since sqlalchemy.sql.operators.ColumnOperators all return ClauseElement, this is not possible. Perhaps the operators need to return ColumnElement?

Used test-file:

from sqlalchemy import Column, DateTime, create_engine
from sqlalchemy import Integer
from sqlalchemy.orm import registry, Session
from sqlalchemy.sql.functions import now

mapper_registry: registry = registry()

e = create_engine('sqlite:///database.db')


@mapper_registry.mapped
class A:
    __tablename__ = "a"
    id = Column(Integer, primary_key=True)
    integer = Column(Integer)
    date_time = Column(DateTime())


with Session(e) as s:
    a = A(integer=1)
    a.integer = A.integer + 1
    a.date_time = now()
    s.add(a)
    s.commit()

Checklist

This pull request is:

• A documentation / typographical error fix
  • Good to go, no issue or tests are needed
• A short code fix
  • please include the issue number, and create an issue if none exists, which
    must include a complete example of the issue. one line code fixes without an
    issue and demonstration will not be accepted.
  • Please include: Fixes: #<issue number> in the commit message
  • please include tests. one line code fixes without tests will not be accepted.
• A new feature implementation
  • please include the issue number, and create an issue if none exists, which must
    include a complete example of how the feature would look.
  • Please include: Fixes: #<issue number> in the commit message
  • please include tests.

Have a nice day!