id	summary	reporter	owner	description	type	status	component	version	severity	resolution	keywords	cc	stage	has_patch	needs_docs	needs_tests	needs_better_patch	easy	ui_ux
32858	Syntax error when using an index transform on an ArrayField in an ExclusionConstraint	Lucidiot	nobody	"I am trying to create a GIST exclusion constraint on a PostgreSQL 12 database. One of the fields is an ArrayField, and I want to apply the constraint on its first item using `my_array__0`. When doing so, I get a syntax error from Postgres.

{{{#!python
from django.db import models
from django.contrib.postgres.constraints import ExclusionConstraint
from django.contrib.postgres.fields import ArrayField, RangeOperators


class MyModel(models.Model):

    my_array = models.ArrayField()

    class Meta:
        constraints = (
            ExclusionConstraint(
                name='foo',
                expressions=[
                    ('my_array__0', RangeOperators.EQUAL),
                ]
            )
        )
}}}

{{{
django.db.utils.ProgrammingError: syntax error at or near ""WITH""
LINE 1: ..."" ADD CONSTRAINT ""foo"" EXCLUDE USING GIST (""my_array""[1] WITH =)
                                                                    ^
}}}

It seems a similar issue had occurred with casts (#32392). The [https://www.postgresql.org/docs/current/sql-altertable.html docs for ALTER TABLE] mention this syntax for expressions in an EXCLUDE constraint:

{{{
exclude_element in an EXCLUDE constraint is:

{ column_name | ( expression ) } [ opclass ] [ ASC | DESC ] [ NULLS { FIRST | LAST } ]
}}}

This implies that parentheses around anything that is not a column name are mandatory. Maybe a more generic fix could be made to detect if something is only a column name, or just to add parentheses all the time? `EXCLUDE USING GIST ((""my_array""[1]) WITH =)` works without any trouble."	Bug	new	Database layer (models, ORM)	3.2	Normal				Unreviewed	0	0	0	0	0	0