add xor support for Q objects

.github/workflows/test-python.yml

@@ -99,6 +99,7 @@ jobs:
           sessions_tests
           timezones
           update
+          xor_lookups
 
   docs:
     name: Docs Checks

django_mongodb/query.py

@@ -1,11 +1,14 @@
-from functools import wraps
+from functools import reduce, wraps
+from operator import add as add_operator
 
 from django.core.exceptions import EmptyResultSet, FullResultSet
 from django.db import DatabaseError, IntegrityError
-from django.db.models import Value
+from django.db.models.expressions import Case, Value, When
+from django.db.models.functions import Mod
+from django.db.models.lookups import Exact
 from django.db.models.sql.constants import INNER
 from django.db.models.sql.datastructures import Join
-from django.db.models.sql.where import AND, XOR, WhereNode
+from django.db.models.sql.where import AND, OR, XOR, WhereNode
 from pymongo import ASCENDING, DESCENDING
 from pymongo.errors import DuplicateKeyError, PyMongoError
 
@@ -219,8 +222,21 @@ def where_node(self, compiler, connection):
     if self.connector == AND:
         operator = "$and"
     elif self.connector == XOR:
-        # https://github.com/mongodb-labs/django-mongodb/issues/27
-        raise NotImplementedError("XOR is not yet supported.")
+        # MongoDB doesn't support $xor, so convert:
+        #   a XOR b XOR c XOR ...
+        # to:
+        #   (a OR b OR c OR ...) AND MOD(a + b + c + ..., 2) == 1
+        # The result of an n-ary XOR is true when an odd number of operands
+        # are true.
+        lhs = self.__class__(self.children, OR)
+        rhs_sum = reduce(
+            add_operator,
+            (Case(When(c, then=1), default=0) for c in self.children),
+        )
+        if len(self.children) > 2:
+            rhs_sum = Mod(rhs_sum, 2)
+        rhs = Exact(1, rhs_sum)
+        return self.__class__([lhs, rhs], AND, self.negated).as_mql(compiler, connection)
     else:
         operator = "$or"