add NotSupportedError for delete queries that use multiple collections

Author: timgraham

README.md

@@ -112,6 +112,7 @@ Migrations for 'admin':
   - `aggregate()`
   - `dates()`
   - `datetimes()`
+  - `delete()`, if the query uses multiple collections.
   - `distinct()`
   - `extra()`
 

django_mongodb/compiler.py

@@ -265,12 +265,19 @@ def insert(self, docs, returning_fields=None):
         return inserted_ids if returning_fields else []
 
 
-class SQLDeleteCompiler(SQLCompiler):
+class SQLDeleteCompiler(compiler.SQLDeleteCompiler, SQLCompiler):
     def execute_sql(self, result_type=MULTI):
         cursor = Cursor()
         cursor.rowcount = self.build_query([self.query.get_meta().pk]).delete()
         return cursor
 
+    def check_query(self):
+        super().check_query()
+        if not self.single_alias:
+            raise NotSupportedError(
+                "Cannot use QuerySet.delete() when querying across multiple collections on MongoDB."
+            )
+
 
 class SQLUpdateCompiler(SQLCompiler):
     def execute_sql(self, result_type):