make unique indexes exclude nulls

Author: timgraham

django_mongodb/schema.py

@@ -127,13 +127,20 @@ def _get_name(field_names):
     def add_index(self, model, index, unique=False):
         if index.contains_expressions:
             return
+        kwargs = {}
+        if unique:
+            filter_expression = {}
+            for field_name, _ in index.fields_orders:
+                field = model._meta.get_field(field_name)
+                filter_expression[field_name] = {"$type": field.db_type(self.connection)}
+            kwargs = {"partialFilterExpression": filter_expression, "unique": True}
         idx = IndexModel(
             [
                 (model._meta.get_field(field_name).column, 1 if order == "" else -1)
                 for field_name, order in index.fields_orders
             ],
             name=index.name,
-            unique=unique,
+            **kwargs,
         )
         self.connection.database[model._meta.db_table].create_indexes([idx])