For the record I haven' tested the above myself.
It might only work on Django 3.0+ and require you to pass
`output_field=BooleanField()` to Func.
Le jeudi 14 novembre 2019 09:46:48 UTC-5, Simon Charette a écrit :
>
> 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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