Add the ability to use database-level CHECK CONSTRAINTS

[Matthew Schinckel]

I mentioned adding database level CHECK CONSTRAINTS in a post on django-developers.

​http://groups.google.com/group/django-developers/browse_thread/thread/38937992972c7808

Attached to this ticket is a patch that provides the functionality listed in the first post of this thread. This is not complete: it only allows simple constraints, and is missing a complete test suite.

[Matthew Schinckel]

Contains patches against trunk that enable check constraints

[Russell Keith-Magee]

If we do this, it should be tied into the model validation framework in some way.

[Matthew Schinckel]

Agreed. The initial patch was made before the changes with model validation had hit. I will get back to this at some stage.

[Aymeric Augustin]

Change UI/UX from NULL to False.

[Aymeric Augustin]

Change Easy pickings from NULL to False.

[Rich Rauenzahn]

Could/should this feature request also handle this case of constraint I brought up in this thread?

https://code.djangoproject.com/ticket/11964

Basically unique_together doesn't consider a null value unique per SQL spec -- which makes perfect sense in single column indexes and is extended to multi column indexes.

So if someone wanted (A, B, NULL) to be a unique row there would need to be an index built on the first two columns "where <third column> is null"

These SO articles give a better overview:

​http://stackoverflow.com/questions/17510261/django-unique-together-constraint-failure
​http://dba.stackexchange.com/questions/9759/postgresql-multi-column-unique-constraint-and-null-values

[alexpirine]

I would also love this feature.

[Tim Graham <timograham@…>]

In 19b2dfd1:

Refs #11964, #26167 -- Made Expressions deconstructible.

[Tim Graham <timograham@…>]

In 508b5deb:

Refs #11964 -- Made Q objects deconstructible.

[Luke Plant]

This external project seems to implement this feature:

​https://github.com/rapilabs/django-db-constraints

It does some monkey patching that would be unnecessary if this were in core.

[Ian Foote]

Now that Django 2 is released, I'd like to see this land. I think I've resolved all the outstanding issues raised on the pull request before. Is the next step to rebase/squash my work in progress commits?

[Asif Saifuddin Auvi]

conflicts needed to be resolved and squash for final review I believe.

[Ian Foote]

Conflicts fixed and commits squashed.

[Tim Graham]

Please don't check "Patch needs improvement" only because there's a small merge conflict (as far as I can see, there's no outstanding review comments on the patch).

[Tim Graham <timograham@…>]

In 952f05a6:

Fixed #11964 -- Added support for database check constraints.

[Gavin Wahl]

Is it now possible to define a field that comes with a check constraint, or can they only be defined at the table level? I'd like to be able to define my own StrictlyPositiveIntegerField.

[Tim Graham <timograham@…>]

In 9a0e0d96:

Refs #11964 -- Renamed a database check constraint test.

[Tim Graham <timograham@…>]

In 9142bebf:

Refs #11964 -- Changed CheckConstraint() signature to use keyword-only arguments.

Also renamed the constraint argument to check to better represent which
part of the constraint the provided Q object represents.

[Tim Graham <timograham@…>]

In bc7e288:

Fixed #29745 -- Based Expression equality on detailed initialization signature.

The old implementation considered objects initialized with an equivalent
signature different if some arguments were provided positionally instead of
as keyword arguments.

Refs #11964, #26167.

[Scott Stevens]

It appears that adding multiple constraints to a table results in only the last being stored.

Reviewing the SQL (./manage.py sqlmigrate), I'm seeing each constraint being added by way of ALTER TABLE, CREATE TABLE, INSERT INTO ... SELECT, DROP TABLE, however only the most recent constraint is added each time, so the previous constraint is dropped with the old table when adding the new one.

Using 38f3de86bd0bfa4c9b57db1237fa55e9fa88bc6e, Python 3.6.6 (Win10x64) with SQLite database.

Should I file a new bug for this?

[Simon Charette]

Yeah please file a new ticket Scott and escalate it to release blocker as it's a bug in a yet unreleased version of Django.

This is probably due to the fact SQLite requires a table rebuild on most ALTER TABLE operations.

[Tim Graham <timograham@…>]

In 32da3cfd:

Refs #11964 -- Removed raw SQL from and cleaned up constraint operation tests.

[Carlton Gibson <carlton.gibson@…>]

In 95bda03:

Fixed #29868 -- Retained database constraints on SQLite table rebuilds.

Refs #11964.

Thanks Scott Stevens for testing this upcoming feature and the report.

[Balazs Endresz]

Is it now possible to define a field that comes with a check constraint, or can they only be defined at the table level? I'd like to be able to define my own StrictlyPositiveIntegerField.

Looks like positive integer fields have been already defined that way:
​https://github.com/django/django/blob/084536a/django/db/models/fields/__init__.py#L637
​https://github.com/django/django/blob/084536a/django/db/backends/postgresql/base.py#L95

Similarly, one can add a simple custom field (that has no additional arguments):

class StrictlyPositiveIntegerField(models.PositiveIntegerField):
    # NB the db_check method is not documented
    def db_check(self, connection):
        data = self.db_type_parameters(connection)
        return '"%(column)s" > 0' % data

With a few tweaks it probably could be made to work with custom field args and CheckConstraints too:

BaseDatabaseSchemaEditor.sql_create_check = (
    # need to drop the check first, otherwise it can't be modified:
    'ALTER TABLE %(table)s DROP CONSTRAINT IF EXISTS %(name)s'
    'ALTER TABLE %(table)s ADD CONSTRAINT %(name)s CHECK (%(check)s)'
)

class CustomFieldWithArgsAndChecks(models.Field):
    ... # handle custom args in __init__() and deconstruct() and define validators too
    def db_check(self, connection):
        constraint = models.CheckConstraint(check=models.Q(**{
            f'{self.name}__gte': self.gte,
            f'{self.name}__lte': self.lte,
        }), name='')
        with connection.schema_editor() as schema_editor:
            return constraint._get_check_sql(self.model, schema_editor)  

I wonder if there are any plans to officially support doing that kind of thing in the future.
Perhaps it isn't too far out of reach now with CheckConstraints in place.

[Mariusz Felisiak <felisiak.mariusz@…>]

In 8b3e1b6:

Refs #11964 -- Made constraint support check respect required_db_features.

This will notably silence the warnings issued when running the test
suite on MySQL.

[Mariusz Felisiak <felisiak.mariusz@…>]

In 306b6875:

Refs #11964 -- Removed SimpleCol in favor of Query(alias_cols).

This prevent having to pass simple_col through multiple function calls
by defining whether or not references should be resolved with aliases
at the Query level.