Can't find null objects in a ManyToMany that may be null

[ph@…]

Model B defined a ManyToMany relationship to Model A that can be null. There's no way in the ORM to look up all the Bs where it's null. There might be a workaround with a custom SQL query.

Jacobkm in IRC told me to post a ticket so he doesn't forget to fix this.

[ph@…]

Yeah, workaround is the SQL: SELECT app_as.id FROM app_as LEFT JOIN app_as_bs ON app_as.id = app_as_bs.a_id WHERE app_as_bs.a_id IS NULL

[bo@…]

I think the 'easiest' way to allow for this case is to allow one to change the JOIN type .. currently, using INNER JOINS is the issue .. a LEFT JOIN would allow you to do this

MyModel.objects.filter(manytomany__id__isnull = True)

however, there is no method to 'detect' the isnull on the primary key ... i.e. change this

SELECT  ...  FROM app_mymodel INNER JOIN app_manytomany AS app_app_manytomany ON app_mymodel.id = app_app_manytomany.asset_id WHERE (app_app_manytomany.id IS NULL)

to

SELECT  ...  FROM app_mymodel LEFT JOIN app_manytomany AS app_app_manytomany ON app_mymodel.id = app_app_manytomany.mymodel_id WHERE (app_app_manytomany.id IS NULL)

the former will always return nothing (unless of course the id can be null, but since when should a primary key autoinc field be null? :)

so maybe it can detect something like

MyModel.objects.filter(manytomany__pk__isnull = True)

(the pk flag to force this this to be true) ..

the issue with not supporting this, means the usual filter chaining cannot be done as it currently needs be a special static query

[Ben Slavin <benjamin.slavin@…>]

Howdy folks.

I just ran into this and came-up with a monkey patch around the Q object to enable this. It's useful for me, so I figured I'd post it here.

This is not a long term fix, but should help to hold people over until the QuerySet refactor.

Again... this isn't something I intend to be included in trunk (and recommend against it)

[Ben Slavin <benjamin.slavin@…>]

Monkey patch as a temporary work-around

[Malcolm Tredinnick]

(In [7477]) Merged the queryset-refactor branch into trunk.

This is a big internal change, but mostly backwards compatible with existing
code. Also adds a couple of new features.

Fixed #245, #1050, #1656, #1801, #2076, #2091, #2150, #2253, #2306, #2400, #2430, #2482, #2496, #2676, #2737, #2874, #2902, #2939, #3037, #3141, #3288, #3440, #3592, #3739, #4088, #4260, #4289, #4306, #4358, #4464, #4510, #4858, #5012, #5020, #5261, #5295, #5321, #5324, #5325, #5555, #5707, #5796, #5817, #5987, #6018, #6074, #6088, #6154, #6177, #6180, #6203, #6658