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
30628	order_by() on union() querysets results with wrong ordering when the same field type is presented multiple times.	Julien Enselme	Simon Charette	"When doing a union of 2 querysets and then doing an `order_by` on the resulting queryset, if we order on a field whose type is present multiple time, the ordering will be incorrect if the field we sort on is not the 1st field of the query. Explicitly settings values with `.values('field1', 'field2')` on the base querysets or after the union has no effect.

For instance, with this model, with 2 `DecimalField`s and one `BooleanField`:

{{{
class Listing(models.Model):
    sale_price = models.DecimalField('Sale price', max_digits=10, decimal_places=2)
    yearly_rent = models.DecimalField('Yearly rent', max_digits=10, decimal_places=2)
    toto = models.BooleanField()
}}}

{{{
# Create 2 qs.
qs1 = Listing.objects.all()
qs2 = Listing.objects.all()

# Create the union QS.
qs3 = qs1.union(qs2)

# Order on the 1st decimal field. This prints (which is correct) :
# SELECT ""union_listing"".""id"", ""union_listing"".""sale_price"", ""union_listing"".""yearly_rent"" FROM ""union_listing"" UNION SELECT ""union_listing"".""id"", ""union_listing"".""sale_price"", ""union_listing"".""yearly_rent"" FROM ""union_listing"" ORDER BY (2) ASC
print(qs3.order_by('sale_price').query)
# Order on the 2nd deciamal field. This will print the same query as above which is incorrect.
print(qs3.order_by('yearly_rent').query)
# Not ordering on a DecimalField. This is correct again.
print(qs3.order_by('toto').query)
}}}

From the debugging I did, it seems to come from [https://github.com/django/django/commit/bc7e288ca9554ac1a0a19941302dea19df1acd21 this commit]: If I revert `def __eq__` back to what it was in Django 2.1 (https://github.com/django/django/blob/stable/2.1.x/django/db/models/expressions.py#L363) it works as normal again. The difference between the two methods that can explain this is that in Django 2.1, we have a check on the actual field instances thanks to `other_args[1]`, but in 2.2, because of `identity[2][1]` which is the class of the field, we can't distinguish two fields of the same type (please refer to the respective implementations to know the values of `other_args` and `identity`).

[https://github.com/Jenselme/dj-test-unions Sample projet to reproduce] (sqlite db included) ([https://github.com/Jenselme/dj-test-unions/blob/master/union/models.py Model], [https://github.com/Jenselme/dj-test-unions/blob/master/test-script.py test file]). Steps:
1. Install Django 2.2
2. Run `DJANGO_SETTINGS_MODULE=testunion.settings python test-script.py`
3. You will see the queries for `qs3.order_by('sale_price')` and `qs3.order_by('yearly_rent')` They are exactly the same whereas they should be different (one with `ORDER BY (1)` and the other with `ORDER BY (2)`). "	Bug	closed	Database layer (models, ORM)	2.2	Release blocker	fixed		Simon Charette	Accepted	1	0	0	0	0	0