[Django] #36607: Improve the documentation by mentioning that "filter(a,b)" may not be the same as "filter(a).filter(b)" on querysets.

[Django]

#36607: Improve the documentation by mentioning that "filter(a,b)" may not be the
same as "filter(a).filter(b)" on querysets.
-------------------------------------+-------------------------------------
Reporter: Aaron | Type:
| Uncategorized
Status: new | Component: Database
| layer (models, ORM)
Version: 5.2 | Severity: Normal
Keywords: | Triage Stage:
| Unreviewed
Has patch: 0 | Needs documentation: 0
Needs tests: 0 | Patch needs improvement: 0
Easy pickings: 0 | UI/UX: 0
-------------------------------------+-------------------------------------
Dear community,

I just got informed about the fact that "filter(a,b)" might not be the
same as
"filter(a).filter(b)". The following example may be useful for
illustrating
this. It uses some code that is inspired by our code and won't run without
context, but I hope it is illustrating enough.

The intend is to filter for MaintenanceOperations. Therefore, we create
one:

{{{#!python
op = MaintenanceOperationFactory()
}}}


These objects are children of the MaintenanceOperation:
{{{#!python
maint_a = UnscheduledMaintenanceFactory(operation=op,
execution_start="2030-03-10T12:00:00Z")
maint_b = UnscheduledMaintenanceFactory(operation=op,
execution_start="2030-05-10T12:00:00Z")
}}}

We construct a first queryset like this:
{{{#!python
qs1 =
MaintenanceOperation.objects.filter(maintenances__execution_start__gt="2030-04-10T12:00:00Z",
maintenances__execution_start__lt="2030-04-20T12:00:00Z",)
}}}

This qs evaluates to: <MaintenanceOperationQuerySet []> (aka the empty
list). This happens because
neither UnscheduledMaintenance meets both criteria. The underlying sql is
this:

{{{#!sql
SELECT "maintenance_maintenanceoperation"."id" FROM
"maintenance_maintenanceoperation" INNER JOIN "maintenance_maintenance" ON
("maintenance_maintenanceoperation"."id" =
"maintenance_maintenance"."operation_id") WHERE
("maintenance_maintenanceoperation"."deleted_at" IS NULL AND
"maintenance_maintenance"."execution_start" > 2030-04-10 12:00:00+00:00
AND
"maintenance_maintenance"."execution_start" < 2030-04-20 12:00:00+00:00)
}}}

To demonstrate the second case, we build the following queryset:

{{{#!python
qs2 = MaintenanceOperation.objects.filter(
maintenances__execution_start__gt="2030-04-10T12:00:00Z",).filter(
maintenances__execution_start__lt="2030-04-20T12:00:00Z",)
}}}

This qs evaluates to:<MaintenanceOperationQuerySet [<MaintenanceOperation:
#1:
beauftragt>]>(This list contains the MaintenanceOperation we created at
the start)
This is different from the first one, because the sql is done with another
join:

{{{#!sql
SELECT "maintenance_maintenanceoperation"."id"
FROM "maintenance_maintenanceoperation" INNER JOIN
"maintenance_maintenance" ON
("maintenance_maintenanceoperation"."id" =
"maintenance_maintenance"."operation_id") INNER JOIN
"maintenance_maintenance"
T3 ON ("maintenance_maintenanceoperation"."id" = T3."operation_id") WHERE
("maintenance_maintenanceoperation"."deleted_at" IS NULL AND
"maintenance_maintenance"."execution_start" > 2030-04-10 12:00:00+00:00
AND
T3."execution_start" < 2030-04-20 12:00:00+00:00)
}}}

I think this should be mentioned in the filter documentation, particularly
in the
section on joining filters. If this is met with approval I am open to
sketching a
suggestion.
--
Ticket URL: <https://code.djangoproject.com/ticket/36607>
Django <https://code.djangoproject.com/>
The Web framework for perfectionists with deadlines.

[Django]

#36607: Improve the documentation by mentioning that "filter(a,b)" may not be the
same as "filter(a).filter(b)" on querysets.
-------------------------------------+-------------------------------------

Reporter: Aaron | Owner: (none)
Type: Uncategorized | Status: new
Component: Database layer | Version: 5.2
(models, ORM) |
Severity: Normal | Resolution:

Keywords: | Triage Stage:
| Unreviewed
Has patch: 0 | Needs documentation: 0
Needs tests: 0 | Patch needs improvement: 0
Easy pickings: 0 | UI/UX: 0
-------------------------------------+-------------------------------------

Changes (by Egor R):

* cc: Egor R (added)

--
Ticket URL: <https://code.djangoproject.com/ticket/36607#comment:1>

[Django]

#36607: Improve the documentation by mentioning that "filter(a,b)" may not be the
same as "filter(a).filter(b)" on querysets.
-------------------------------------+-------------------------------------
Reporter: Aaron | Owner: (none)
Type: Uncategorized | Status: new
Component: Database layer | Version: 5.2
(models, ORM) |
Severity: Normal | Resolution:
Keywords: | Triage Stage:
| Unreviewed
Has patch: 0 | Needs documentation: 0
Needs tests: 0 | Patch needs improvement: 0
Easy pickings: 0 | UI/UX: 0
-------------------------------------+-------------------------------------

Comment (by Tanishq):

Hey Aaron,

the example you shared does a great job highlighting the difference
between {{{filter(a, b)}}} and {{{filter(a).filter(b)}}}
—especially how SQL joins come into play. I agree it’s worth documenting
this nuance, probably in the “Joining and Filtering” section of
{{{docs/topics/db/queries.txt}}}. A quick note about how chained filters
can trigger extra joins and potentially change result sets (like in your
{{{UnscheduledMaintenance}}}example) would be super helpful.
I can work on this Issue and even update the docs to perfectly cover this
section.
--
Ticket URL: <https://code.djangoproject.com/ticket/36607#comment:2>

[Django]

#36607: Improve the documentation by mentioning that "filter(a,b)" may not be the
same as "filter(a).filter(b)" on querysets.
-------------------------------------+-------------------------------------
Reporter: Aaron | Owner: (none)
Type: Uncategorized | Status: new
Component: Database layer | Version: 5.2
(models, ORM) |
Severity: Normal | Resolution:
Keywords: | Triage Stage:
| Unreviewed
Has patch: 0 | Needs documentation: 0
Needs tests: 0 | Patch needs improvement: 0
Easy pickings: 0 | UI/UX: 0
-------------------------------------+-------------------------------------

Comment (by Aaron):

Hi Tanishq,

I appreciate the feedback and the interest to work on this. Let me know
when you have done something and I will have a look at it, looking forward
to it :)
--
Ticket URL: <https://code.djangoproject.com/ticket/36607#comment:3>

[Django]

#36607: Improve the documentation by mentioning that "filter(a,b)" may not be the
same as "filter(a).filter(b)" on querysets.
-------------------------------------+-------------------------------------
Reporter: Aaron | Owner: (none)

Type: Uncategorized | Status: closed

Component: Database layer | Version: 5.2
(models, ORM) |

Severity: Normal | Resolution: invalid

Keywords: | Triage Stage:
| Unreviewed
Has patch: 0 | Needs documentation: 0
Needs tests: 0 | Patch needs improvement: 0
Easy pickings: 0 | UI/UX: 0
-------------------------------------+-------------------------------------

Changes (by Simon Charette):

* resolution: => invalid
* status: new => closed

Comment:

Hello Aaron, thank you for your report.

The behaviour difference between chaining and combining `filter` calls
when spanning multi-valued relationships
[https://docs.djangoproject.com/en/5.2/topics/db/queries/#spanning-multi-
valued-relationships is already by the documented] under the ''Making
queries > Spanning multi-valued relationships'' section with a concrete
example so I'm going to assume you missed it before opening the ticket.
--
Ticket URL: <https://code.djangoproject.com/ticket/36607#comment:4>