courses grouped by country in form

rowers/formfields.py

@@ -0,0 +1,31 @@
+from functools import partial
+from itertools import groupby
+from operator import attrgetter
+
+from django.forms.models import ModelChoiceIterator, ModelChoiceField
+
+
+class GroupedModelChoiceIterator(ModelChoiceIterator):
+    def __init__(self, field, groupby):
+        self.groupby = groupby
+        super().__init__(field)
+
+    def __iter__(self):
+        if self.field.empty_label is not None:
+            yield ("", self.field.empty_label)
+        queryset = self.queryset
+        # Can't use iterator() when queryset uses prefetch_related()
+        if not queryset._prefetch_related_lookups:
+            queryset = queryset.iterator()
+        for group, objs in groupby(queryset, self.groupby):
+            yield (group, [self.choice(obj) for obj in objs])
+
+
+class GroupedModelChoiceField(ModelChoiceField):
+    def __init__(self, *args, choices_groupby, **kwargs):
+        if isinstance(choices_groupby, str):
+            choices_groupby = attrgetter(choices_groupby)
+        elif not callable(choices_groupby):
+            raise TypeError('choices_groupby must either be a str or a callable accepting a single argument')
+        self.iterator = partial(GroupedModelChoiceIterator, groupby=choices_groupby)
+        super().__init__(*args, **kwargs)