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
30000	QuerySet constructed with .union() should raise an exception on unsupported filter() attempt.	Thomas Hamann	Hasan Ramezani	"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.
e.g.:

{{{
# 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
    else:
        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',
                                                    widget=forms.CheckboxSelectMultiple)
}}}

Do take note that the documentation for the .union() method ([https://docs.djangoproject.com/en/2.1/ref/models/querysets/#union 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."	Bug	closed	Database layer (models, ORM)	dev	Normal	duplicate	queryset union	Prabakaran Kumaresshan	Accepted	0	0	0	0	0	0