#35575: Add support for constraint validation on GeneratedFields
-------------------------------------+-------------------------------------
Reporter: Mark Gensler | Owner: Mark
| Gensler
Type: New feature | Status: assigned
Component: Database layer | Version: 5.0
(models, ORM) |
Severity: Normal | Resolution:
Keywords: generatedfield | Triage Stage: Accepted
uniqueconstraint checkconstraint |
Has patch: 0 | Needs documentation: 0
Needs tests: 0 | Patch needs improvement: 0
Easy pickings: 0 | UI/UX: 0
-------------------------------------+-------------------------------------
Comment (by Simon Charette):
I feel like we should avoid the naive approach of calling
`refresh_from_db(fields=generated_fields)` as that will mutate the in-
memory instance and leave it in this form as well as perform an extra
query per constraint.
What should be done IMO is for the query performed in `UniqueConstraint`
to call `replace_expressions({F(gfield.name):
gfield.expression.replace_expressions(....) for gfield in
generated_field})` so that will turn things like
{{{#!python
class Contributor(models.Model):
first_name = models.TextField()
last_name = models.TextField()
full_name = models.GeneratedField(
Lower(Concat("first_name", models.Value(" "), "last_name"))
)
class Meta:
constraints = {
UniqueConstraint(names="unique_full_name", MD5("full_name"))
}
}}}
Then the query to full clean the unique constraint, assuming `first_name`
and `last_name` are available, would be
{{{#!sql
SELECT 1 FROM contributor
WHERE md5(lower("first_name" || ' ' || "last_name")) = md5(lower('Mark' ||
' ' || 'Gensler')
}}}
The more we push to the database the less likely we are to run into race
conditions and serde roudtrip issues.
--
Ticket URL: <https://code.djangoproject.com/ticket/35575#comment:6>
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