diff --git a/django_select2/forms.py b/django_select2/forms.py
index 88f04db..dbac21b 100644
--- a/django_select2/forms.py
+++ b/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 = (
-            (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)
diff --git a/tests/test_forms.py b/tests/test_forms.py
index 15a48db..9010862 100644
--- a/tests/test_forms.py
+++ b/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):
 
diff --git a/tests/testapp/forms.py b/tests/testapp/forms.py
index bffb301..cacfb75 100644
--- a/tests/testapp/forms.py
+++ b/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
+        }
diff --git a/tests/testapp/models.py b/tests/testapp/models.py
index 6687550..c324852 100644
--- a/tests/testapp/models.py
+++ b/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)