Add support for custom to_field names for foreign keys (#486)

Django's ForeignKey supports custom to_fields. The to field is the
primary key by default, but can be modified. The to field is also
used by forms to reduce database lookups.

This patch add support for custom to_field names on both model or
form layer.

Author: codingjoe

django_select2/forms.py

@@ -444,11 +444,12 @@ def optgroups(self, name, value, attrs=None):
             c for c in selected_choices
             if c not in self.choices.field.empty_values
         }
-        choices = (
-            (obj.pk, self.label_from_instance(obj))
-            for obj in self.choices.queryset.filter(pk__in=selected_choices)
-        )
-        for option_value, option_label in choices:
+        field_name = self.choices.field.to_field_name or 'pk'
+        query = Q(**{'%s__in' % field_name: selected_choices})
+        for obj in self.choices.queryset.filter(query):
+            option_value = self.choices.choice(obj)[0]
+            option_label = self.label_from_instance(obj)
+
             selected = (
                 str(option_value) in value and
                 (has_selected is False or self.allow_multiple_selected)

tests/test_forms.py

@@ -22,7 +22,7 @@
 from tests.testapp.forms import (
     NUMBER_CHOICES, HeavySelect2MultipleWidgetForm, TitleModelSelect2Widget
 )
-from tests.testapp.models import City, Country, Genre
+from tests.testapp.models import Artist, City, Country, Genre, Groupie
 
 try:
     from django.urls import reverse
@@ -362,6 +362,12 @@ def test_get_url(self):
         widget = ModelSelect2Widget(queryset=Genre.objects.all(), search_fields=['title__icontains'])
         assert isinstance(widget.get_url(), text_type)
 
+    def test_custom_to_field_name(self):
+        the_best_band_in_the_world = Artist.objects.create(title='Take That')
+        groupie = Groupie.objects.create(obsession=the_best_band_in_the_world)
+        form = forms.GroupieForm(instance=groupie)
+        assert '<option value="Take That" selected>TAKE THAT</option>' in form.as_p()
+
 
 class TestHeavySelect2TagWidget(TestHeavySelect2Mixin):
 

tests/testapp/forms.py

@@ -195,3 +195,12 @@ class AddressChainedSelect2WidgetForm(forms.Form):
             max_results=500,
         )
     )
+
+
+class GroupieForm(forms.ModelForm):
+    class Meta:
+        model = models.Groupie
+        fields = '__all__'
+        widgets = {
+            'obsession': ArtistCustomTitleWidget
+        }

tests/testapp/models.py

@@ -12,7 +12,7 @@ def __str__(self):
 
 
 class Artist(models.Model):
-    title = models.CharField(max_length=50)
+    title = models.CharField(max_length=50, unique=True)
     genres = models.ManyToManyField(Genre)
 
     class Meta:
@@ -56,3 +56,7 @@ class Meta:
 
     def __str__(self):
         return self.name
+
+
+class Groupie(models.Model):
+    obsession = models.ForeignKey(Artist, to_field='title', on_delete=models.CASCADE)