Support annotated check constraints.

[Tonye Jack]

With the current Implementation of ​https://docs.djangoproject.com/en/2.2/ref/models/constraints/#checkconstraint there's currently no documented support for using annotated fields directly in the check constraint definition.

Created a sample repo ​https://github.com/jackton1/django-check-constraint which describes more on the possible use case to enable more complex check constraint definition which can be used directly.

Use Case.

UDF

CREATE OR REPLACE FUNCTION public.non_null_count(VARIADIC arg_array ANYARRAY)
  RETURNS BIGINT AS
  $$
    SELECT COUNT(x) FROM UNNEST($1) AS x
  $$ LANGUAGE SQL IMMUTABLE;

Expected SQL

ALTER TABLE app_name_test_modoel ADD CONSTRAINT app_name_test_model_optional_field_provided
    CHECK(non_null_count(amount::integer , amount_off::integer, percentage::integer) = 1);

[Simon Charette]

CheckConstraint accepts any boolean expression since Django 3.1+ so this particular one can be expressed using RawSQL

CheckConstraint(
    check=RawSQL(
        'non_null_count(amount::integer , amount_off::integer, percentage::integer) = 1',
        output_field=models.BooleanField(),
     )
)

Or event Func, Cast, and Exact.

non_null_count = Func(Cast('amount', models.IntegerField()), Cast('amount_off', models.IntegerField()), Cast('percentage', models.IntegerField()), function='non_null_count')
CheckConstraint(
    check=Exact(non_null_count, 1),
)