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.

django_select2/forms.py

@@ -444,11 +444,12 @@ class ModelSelect2Mixin(object):
             c for c in selected_choices
             if c not in self.choices.field.empty_values
         }
-        choices = (
+        field_name = self.choices.field.to_field_name or 'pk'
-            (obj.pk, self.label_from_instance(obj))
+        query = Q(**{'%s__in' % field_name: selected_choices})
-            for obj in self.choices.queryset.filter(pk__in=selected_choices)
+        for obj in self.choices.queryset.filter(query):
-        )
+            option_value = self.choices.choice(obj)[0]
-        for option_value, option_label in choices:
+            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 import forms
 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 @@ class TestModelSelect2Mixin(TestHeavySelect2Mixin):
         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 @@ class Genre(models.Model):
 
 
 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 City(models.Model):
 
     def __str__(self):
         return self.name
+
+
+class Groupie(models.Model):
+    obsession = models.ForeignKey(Artist, to_field='title', on_delete=models.CASCADE)