Introduced NullableModelMultipleChoiceField to allow null filtering without causing introspection issues during database migrations

netbox/utilities/filters.py

@@ -1,15 +1,65 @@
 import django_filters
+import itertools
 
+from django import forms
 from django.db.models import Q
+from django.utils.encoding import force_text
 
 
-class NullableModelMultipleChoiceFilter(django_filters.MultipleChoiceFilter):
+class NullableModelMultipleChoiceField(forms.ModelMultipleChoiceField):
+    """
+    This field operates like a normal ModelMultipleChoiceField except that it allows for one additional choice which is
+    used to represent a value of Null. This is accomplished by creating a new iterator which first yields the null
+    choice before entering the queryset iterator, and by ignoring the null choice during cleaning. The effect is similar
+    to defining a MultipleChoiceField with:
+
+        choices = [(0, 'None')] + [(x.id, x) for x in Foo.objects.all()]
+
+    However, the above approach forces immediate evaluation of the queryset, which can cause issues when calculating
+    database migrations.
+    """
+    iterator = forms.models.ModelChoiceIterator
+
+    def __init__(self, null_value=0, null_label='None', *args, **kwargs):
+        self.null_value = null_value
+        self.null_label = null_label
+        super(NullableModelMultipleChoiceField, self).__init__(*args, **kwargs)
+
+    def _get_choices(self):
+        if hasattr(self, '_choices'):
+            return self._choices
+        # Prepend the null choice to the queryset iterator
+        return itertools.chain(
+            [(self.null_value, self.null_label)],
+            self.iterator(self),
+        )
+    choices = property(_get_choices, forms.ChoiceField._set_choices)
+
+    def clean(self, value):
+        # Strip all instances of the null value before cleaning
+        if value is not None:
+            stripped_value = [x for x in value if x != force_text(self.null_value)]
+        else:
+            stripped_value = value
+        super(NullableModelMultipleChoiceField, self).clean(stripped_value)
+        return value
+
+
+class NullableModelMultipleChoiceFilter(django_filters.ModelMultipleChoiceFilter):
+    """
+    This class extends ModelMultipleChoiceFilter to accept an additional value which implies "is null". The default
+    queryset filter argument is:
+
+        .filter(fieldname=value)
+
+    When filtering by the value representing "is null" ('0' by default) the argument is modified to:
+
+        .filter(fieldname__isnull=True)
+    """
+    field_class = NullableModelMultipleChoiceField
 
     def __init__(self, *args, **kwargs):
-        # Convert the queryset to a list of choices prefixed with a "None" option
-        queryset = kwargs.pop('queryset')
-        self.to_field_name = kwargs.pop('to_field_name', 'pk')
-        kwargs['choices'] = [(0, 'None')] + [(getattr(o, self.to_field_name), o) for o in queryset]
+        self.null_value = kwargs.get('null_value', 0)
         super(NullableModelMultipleChoiceFilter, self).__init__(*args, **kwargs)
 
     def filter(self, qs, value):
@@ -24,13 +74,13 @@ class NullableModelMultipleChoiceFilter(django_filters.MultipleChoiceFilter):
 
         q = Q()
         for v in set(value):
-            # Filtering on NULL
-            if v == str(0):
+            # Filtering by "is null"
+            if v == force_text(self.null_value):
                 arg = {'{}__isnull'.format(self.name): True}
-            # Filtering on a related field (e.g. slug)
-            elif self.to_field_name != 'pk':
-                arg = {'{}__{}'.format(self.name, self.to_field_name): v}
-            # Filtering on primary key
+            # Filtering by a related field (e.g. slug)
+            elif self.field.to_field_name is not None:
+                arg = {'{}__{}'.format(self.name, self.field.to_field_name): v}
+            # Filtering by primary key (default)
             else:
                 arg = {self.name: v}
             if self.conjoined:

netbox/utilities/forms.py

@@ -4,7 +4,6 @@ import re
 
 from django import forms
 from django.core.urlresolvers import reverse_lazy
-from django.db.models import Count
 from django.utils.encoding import force_text
 from django.utils.html import format_html
 from django.utils.safestring import mark_safe