Support computed columns in SQLAlchemy ORM

[hammerhead]

Summary of the changes / Why this is an improvement

Fixes crate/sqlalchemy-cratedb#112

Checklist

• CLA is signed

[BaurzhanSakhariev]

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[amotl]

Hi Niklas,

thank you very much for this excellent contribution. Can I humbly ask you to add a short section about the usage of this feature to the documentation at docs/sqlalchemy.rst?

I think another subsection within the Table definition section ¹ would be the right place, just like the "``_id`` as primary key" section. While being at it, feel free to add corresponding anchors like .. _table-definition: to those subsections as well.

I think it will be enough to just demonstrate the example from the test case there, showing off the entity schema together with a few words of prose.

class DummyTable(self.Base):
    __tablename__ = 't'
    ts = sa.Column(sa.BigInteger, primary_key=True)
    p = sa.Column(sa.BigInteger, sa.Computed("date_trunc('day', ts)"))

Thank you in advance and with kind regards,
Andreas.

Footnotes

1. https://github.com/crate/crate-python/blob/master/docs/sqlalchemy.rst#id10 ↩

[amotl]

👍