Add function-based virtual fields.

[Dulmandakh]

PostgreSQL 12 added support for Generated Columns, see ​https://www.postgresql.org/docs/12/ddl-generated-columns.html. And I find it very interesting and useful, for example with SearchVectorField.

I imagine it would be called GeneratedField and accept base_field as ArrayField, then expression to generate a value for the field. For example,

class Album(models.Model):
  ...
  title = models.CharField(max_length=120)
  search = GeneratedField(
    SearchVectorField(),
    F('title')
  )

then generate SQL something like below.

CREATE TABLE album (
    ...
    title char(120),
    search tsvector GENERATED ALWAYS AS title STORED
);

I would like to work on this feature, but don't know how to pass expression and generate migration using the expression.

[Mariusz Felisiak]

We have an accepted ticket #30511 about identity columns on PostgreSQL, and I think we should stay with this. Generated/Function-based virtual columns are a huge topic that would require many changes and have many caveats, e.g. functions must be deterministic. They are feasible also on Oracle. This kind of features require few weeks (even months) of works, a precise plan, and should be preceded by a discussion on DevelopersMailingList and even DEP.

[Mariusz Felisiak]

Tentatively accepted, based on the ​feedback from the mailing list.

[Mariusz Felisiak]

​PR

[Mariusz Felisiak]

Jeremy, you cannot mark your own patches as RFC, check out ​docs.

[Paolo Melchiorre]

Mariusz if I have interpreted the documentation correctly, having just reviewed it again and not being the author of the PR, I have marked it as an RFC.

[Lily Foote]

One thing that might help this land is review of the Oracle support I added in ​https://github.com/django/django/pull/16860. There's a few open questions that could use thought from more people.

[Paolo Melchiorre]

I personally can't help with Oracle testing, as I believe many others here, but if there weren't other people able to help, as Adam has already suggested [1] in the PR comments, I wouldn't block the merge of this feature because it lacks the Oracle support given how little it is used.

[1] ​https://github.com/django/django/pull/16417#issuecomment-1508222534

[Mariusz Felisiak <felisiak.mariusz@…>]

In f333e35:

Fixed #31300 -- Added GeneratedField model field.

Thanks Adam Johnson and Paolo Melchiorre for reviews.

Co-Authored-By: Lily Foote <code@…>
Co-Authored-By: Mariusz Felisiak <felisiak.mariusz@…>

[GitHub <noreply@…>]

In 0b506bf:

Refs #31300 -- Added example to GeneratedField release notes.

[Mariusz Felisiak <felisiak.mariusz@…>]

In 7e4c1e8:

[5.0.x] Refs #31300 -- Added example to GeneratedField release notes.

Backport of 0b506bfe1ab9f1c38e439c77b3c3f81c8ac663ea from main

[Mariusz Felisiak <felisiak.mariusz@…>]

In 208870b:

Refs #31300 -- Allowed testing GeneratedField without collation.

CockroachDB and Snowflake don't support it.

[Mariusz Felisiak <felisiak.mariusz@…>]

In b735b90:

[5.0.x] Refs #31300 -- Allowed testing GeneratedField without collation.

CockroachDB and Snowflake don't support it.

Backport of 208870b6122c4cbc39c3b2432e13db54c920db51 from main