Hello there,
I guess your example is not the best since this check could be defined on
Author.Meta.constraints
directly but if you wanted to define such a check on Book anyway you'll
have to define a function
and refer to it in the check constraint using Func.
Using RunSQL in your migrations
RunSQL("""
create or replace function author_age_gt(int, int) returns boolean as $$
select exists (
select 1
from "author"
where id = $1 and age > $2
);
$$ language sql;
""")
And then you could define your CheckConstraint as
CheckConstraint(Func(F('author'), 18, function='author_age_gt'),
name='age_check')
Cheers,
Simon
Le jeudi 14 novembre 2019 02:45:32 UTC-5, Olivier a écrit :
>
> Hello,
>
> Let say I want to enforce a database constraint saying a "book's author
> age is over 18".
> As you may guess, books and authors are each described with a Model
> subclass with a foreign key (in this example each book is written by a
> single author) linking both.
>
> class Author(models.Model):
> age = models.SmallIntegerField()
>
>
> class Book(models.Model):
> author = models.ForeignKey(Author, on_delete=models.PROTECT)
>
> class Meta:
> constraints = [
> models.CheckConstraint(check=models.Q(<WHATEVER>__gt=18),
> name='age_check'),
> ]
>
>
> I've tried replacing <WHATEVER> above with author_age or author__age.
> In both cases, makemigrations works but migrate fails with:
> django.core.exceptions.FieldError: Joined field references are not
> permitted in this query
>
>
> Can I work around this ?
> Any working example ?
>
> Best regards
>
>
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