Allow FilteredModelChoiceField to accept a callable or relation name as well as a property / method name for filter_accessor

wagtail/admin/filters.py

@@ -1,4 +1,5 @@
 import django_filters
+from django.db import models
 from django.utils.translation import gettext_lazy as _
 from django_filters.widgets import SuffixedMultiWidget
 
@@ -38,6 +39,16 @@ class FilteredModelChoiceIterator(django_filters.fields.ModelChoiceIterator):
 
 
 class FilteredModelChoiceField(django_filters.fields.ModelChoiceField):
+    """
+    A ModelChoiceField that uses FilteredSelect to dynamically show/hide options based on another
+    ModelChoiceField of related objects; an option will be shown whenever the selected related
+    object is present in the result of filter_accessor for that option.
+
+    filter_field - the HTML `id` of the related ModelChoiceField
+    filter_accessor - either the name of a relation, property or method on the model instance which
+        returns a queryset of related objects, or a function which accepts the model instance and
+        returns such a queryset.
+    """
     widget = FilteredSelect
     iterator = FilteredModelChoiceIterator
 
@@ -48,13 +59,19 @@ class FilteredModelChoiceField(django_filters.fields.ModelChoiceField):
         self.widget.filter_field = filter_field
 
     def get_filter_value(self, obj):
-        # filter_accessor identifies a property or method on the instances being listed here,
-        # which gives us a queryset of related objects. Turn this queryset into a list of IDs
-        # that will become the 'data-filter-value' used to filter this listing
-        queryset = getattr(obj, self.filter_accessor)
-        if callable(queryset):
-            queryset = queryset()
+        # Use filter_accessor to obtain a queryset of related objects
+        if callable(self.filter_accessor):
+            queryset = self.filter_accessor(obj)
+        else:
+            # treat filter_accessor as a method/property name of obj
+            queryset = getattr(obj, self.filter_accessor)
+            if isinstance(queryset, models.Manager):
+                queryset = queryset.all()
+            elif callable(queryset):
+                queryset = queryset()
 
+        # Turn this queryset into a list of IDs that will become the 'data-filter-value' used to
+        # filter this listing
         return queryset.values_list('pk', flat=True)
 
 

wagtail/admin/tests/test_filters.py

@@ -0,0 +1,75 @@
+from django import forms
+from django.contrib.auth import get_user_model
+from django.contrib.auth.models import Group
+from django.test import TestCase
+
+from wagtail.admin.filters import FilteredModelChoiceField
+
+
+User = get_user_model()
+
+
+class TestFilteredModelChoiceField(TestCase):
+    def setUp(self):
+        self.musicians = Group.objects.create(name="Musicians")
+        self.actors = Group.objects.create(name="Actors")
+
+        self.david = User.objects.create_user(
+            'david', 'david@example.com', 'kn1ghtr1der', first_name="David", last_name="Hasselhoff"
+        )
+        self.david.groups.set([self.musicians, self.actors])
+
+        self.kevin = User.objects.create_user(
+            'kevin', 'kevin@example.com', '6degrees', first_name="Kevin", last_name="Bacon"
+        )
+        self.kevin.groups.set([self.actors])
+
+        self.morten = User.objects.create_user(
+            'morten', 'morten@example.com', 't4ke0nm3', first_name="Morten", last_name="Harket"
+        )
+        self.morten.groups.set([self.musicians])
+
+    def test_with_relation(self):
+
+        class UserForm(forms.Form):
+            users = FilteredModelChoiceField(
+                queryset=User.objects.order_by('username'), filter_field='id_group', filter_accessor='groups'
+            )
+
+        form = UserForm()
+        html = str(form['users'])
+        expected_html = """
+            <select name="users" data-widget="filtered-select" data-filter-field="id_group" required id="id_users">
+                <option value="" selected>---------</option>
+                <option value="%(david)s" data-filter-value="%(musicians)s,%(actors)s">david</option>
+                <option value="%(kevin)s" data-filter-value="%(actors)s">kevin</option>
+                <option value="%(morten)s" data-filter-value="%(musicians)s">morten</option>
+            </select>
+        """ % {
+            'david': self.david.pk, 'kevin': self.kevin.pk, 'morten': self.morten.pk,
+            'musicians': self.musicians.pk, 'actors': self.actors.pk,
+        }
+        self.assertHTMLEqual(html, expected_html)
+
+    def test_with_callable(self):
+
+        class UserForm(forms.Form):
+            users = FilteredModelChoiceField(
+                queryset=User.objects.order_by('username'), filter_field='id_group',
+                filter_accessor=lambda user: user.groups.all()
+            )
+
+        form = UserForm()
+        html = str(form['users'])
+        expected_html = """
+            <select name="users" data-widget="filtered-select" data-filter-field="id_group" required id="id_users">
+                <option value="" selected>---------</option>
+                <option value="%(david)s" data-filter-value="%(musicians)s,%(actors)s">david</option>
+                <option value="%(kevin)s" data-filter-value="%(actors)s">kevin</option>
+                <option value="%(morten)s" data-filter-value="%(musicians)s">morten</option>
+            </select>
+        """ % {
+            'david': self.david.pk, 'kevin': self.kevin.pk, 'morten': self.morten.pk,
+            'musicians': self.musicians.pk, 'actors': self.actors.pk,
+        }
+        self.assertHTMLEqual(html, expected_html)