Opened 20 months ago

Last modified 12 months ago

#30000 closed Bug

Passing QuerySet constructed with .union() causes ModelMultipleChoiceField to return wrong values — at Version 1

Reported by: thoha Owned by: nobody
Component: Database layer (models, ORM) Version: master
Severity: Normal Keywords: queryset union
Cc: Prabakaran Kumaresshan Triage Stage: Accepted
Has patch: no Needs documentation: no
Needs tests: no Patch needs improvement: no
Easy pickings: no UI/UX: no

Description (last modified by thoha)

I have a form that displays large lists of selectable records using ModelMultipleChoiceFields. These fields need to show a predetermined list of records, obtained through some database queries performed within the Form's class init block.
The QuerySet required is constructed by reaching through a few ForeignKey relationships to retrieve the required records and then using the .union() method to assemble the QuerySet.

# Retrieve original record:
 base = BaseModel.objects.filter(source_name=source_name).first()
# Get related record via ForeignKey relationship:
related_record = base.related_record
# related_record.other_related_things has ManyToMany relationship to another Model (one or more records).
# So get them:
other_related_things = related_record.other_related_things.all()
# Loop through these:
union_queryset = None 
for other_related_thing in other_related_things:
    # Each other_related_thing has a set of SelectableRecords associated with it, via a ForeignKey relationship from the latter model to the former one. This gets that QuerySet:
    other_related_thing_queryset = other_related_thing.selectablerecords_set.all()
    # Build union_queryset:
    if union_queryset is None:
        union_queryset = other_related_thing_queryset
        union_queryset = union_queryset_object.union(other_related_thing_queryset)
        # The resulting union_queryset is a unified queryset containing all of the required fields. 
        # If only one other_related_thing exists, the result of evaluating the union_queryset QuerySet ought to be the 
        # same as the result of evaluating SelectableRecords.objects.all()

However, when I pass the union_queryset to my ModelMultipleChoiceFields, validating the form returns all of the records contained in union_queryset instead of the records selected on the form. The form displays correctly.

The code used for this is:

self.fields['selectable_records'] = forms.ModelMultipleChoiceField(queryset=union_queryset, required=False,
                                                    label='Selectable records',

Do take note that the documentation for the .union() method (here) indicates there are some limits to the QuerySet returned, and I assume my problem may have to do with this. Still, it would be welcome if this were explained somewhere (should this behaviour be normal), as it is rather unexpected.

Change History (1)

comment:1 Changed 20 months ago by thoha

Description: modified (diff)
Note: See TracTickets for help on using tickets.
Back to Top