Fixed #14370 -- Allowed using a Select2 widget for ForeignKey and ManyToManyField in the admin.

Thanks Florian Apolloner and Tim Graham for review and
contributing to the patch.

tests/admin_widgets/models.py

@@ -27,6 +27,7 @@ class Band(models.Model):
 
 class Album(models.Model):
     band = models.ForeignKey(Band, models.CASCADE)
+    featuring = models.ManyToManyField(Band, related_name='featured')
     name = models.CharField(max_length=100)
     cover_art = models.FileField(upload_to='albums')
     backside_art = MyFileField(upload_to='albums_back', null=True)

tests/admin_widgets/test_autocomplete_widget.py

@@ -0,0 +1,133 @@
+from django import forms
+from django.contrib.admin.widgets import AutocompleteSelect
+from django.forms import ModelChoiceField
+from django.test import TestCase, override_settings
+from django.utils import translation
+
+from .models import Album, Band
+
+
+class AlbumForm(forms.ModelForm):
+    class Meta:
+        model = Album
+        fields = ['band', 'featuring']
+        widgets = {
+            'band': AutocompleteSelect(
+                Album._meta.get_field('band').remote_field,
+                attrs={'class': 'my-class'},
+            ),
+            'featuring': AutocompleteSelect(
+                Album._meta.get_field('featuring').remote_field,
+            )
+        }
+
+
+class NotRequiredBandForm(forms.Form):
+    band = ModelChoiceField(
+        queryset=Album.objects.all(),
+        widget=AutocompleteSelect(Album._meta.get_field('band').remote_field),
+        required=False,
+    )
+
+
+class RequiredBandForm(forms.Form):
+    band = ModelChoiceField(
+        queryset=Album.objects.all(),
+        widget=AutocompleteSelect(Album._meta.get_field('band').remote_field),
+        required=True,
+    )
+
+
+@override_settings(ROOT_URLCONF='admin_widgets.urls')
+class AutocompleteMixinTests(TestCase):
+    empty_option = '<option value=""></option>'
+    maxDiff = 1000
+
+    def test_build_attrs(self):
+        form = AlbumForm()
+        attrs = form['band'].field.widget.get_context(name='my_field', value=None, attrs={})['widget']['attrs']
+        self.assertEqual(attrs, {
+            'class': 'my-classadmin-autocomplete',
+            'data-ajax--cache': 'true',
+            'data-ajax--type': 'GET',
+            'data-ajax--url': '/admin_widgets/band/autocomplete/',
+            'data-theme': 'admin-autocomplete',
+            'data-allow-clear': 'false',
+            'data-placeholder': ''
+        })
+
+    def test_build_attrs_not_required_field(self):
+        form = NotRequiredBandForm()
+        attrs = form['band'].field.widget.build_attrs({})
+        self.assertJSONEqual(attrs['data-allow-clear'], True)
+
+    def test_build_attrs_required_field(self):
+        form = RequiredBandForm()
+        attrs = form['band'].field.widget.build_attrs({})
+        self.assertJSONEqual(attrs['data-allow-clear'], False)
+
+    def test_get_url(self):
+        rel = Album._meta.get_field('band').remote_field
+        w = AutocompleteSelect(rel)
+        url = w.get_url()
+        self.assertEqual(url, '/admin_widgets/band/autocomplete/')
+
+    def test_render_options(self):
+        beatles = Band.objects.create(name='The Beatles', style='rock')
+        who = Band.objects.create(name='The Who', style='rock')
+        # With 'band', a ForeignKey.
+        form = AlbumForm(initial={'band': beatles.pk})
+        output = form.as_table()
+        selected_option = '<option value="%s" selected>The Beatles</option>' % beatles.pk
+        option = '<option value="%s">The Who</option>' % who.pk
+        self.assertIn(selected_option, output)
+        self.assertNotIn(option, output)
+        # With 'featuring', a ManyToManyField.
+        form = AlbumForm(initial={'featuring': [beatles.pk, who.pk]})
+        output = form.as_table()
+        selected_option = '<option value="%s" selected>The Beatles</option>' % beatles.pk
+        option = '<option value="%s" selected>The Who</option>' % who.pk
+        self.assertIn(selected_option, output)
+        self.assertIn(option, output)
+
+    def test_render_options_required_field(self):
+        """Empty option is present if the field isn't required."""
+        form = NotRequiredBandForm()
+        output = form.as_table()
+        self.assertIn(self.empty_option, output)
+
+    def test_render_options_not_required_field(self):
+        """Empty option isn't present if the field isn't required."""
+        form = RequiredBandForm()
+        output = form.as_table()
+        self.assertNotIn(self.empty_option, output)
+
+    def test_media(self):
+        rel = Album._meta.get_field('band').remote_field
+        base_files = (
+            'admin/js/vendor/jquery/jquery.min.js',
+            'admin/js/vendor/select2/select2.full.min.js',
+            # Language file is inserted here.
+            'admin/js/jquery.init.js',
+            'admin/js/autocomplete.js',
+        )
+        languages = (
+            ('de', 'de'),
+            # Language with code 00 does not exist.
+            ('00', None),
+            # Language files are case sensitive.
+            ('sr-cyrl', 'sr-Cyrl'),
+            ('zh-cn', 'zh-CN'),
+        )
+        for lang, select_lang in languages:
+            with self.subTest(lang=lang):
+                if select_lang:
+                    expected_files = (
+                        base_files[:2] +
+                        (('admin/js/vendor/select2/i18n/%s.js' % select_lang),) +
+                        base_files[2:]
+                    )
+                else:
+                    expected_files = base_files
+                with translation.override(lang):
+                    self.assertEqual(AutocompleteSelect(rel).media._js, expected_files)