Fixed #24951 -- Fixed AssertionError in delete queries involving a foreign/primary key.

Thanks Anssi Kääriäinen for help.

django/db/models/sql/compiler.py

@@ -995,7 +995,7 @@ class SQLDeleteCompiler(SQLCompiler):
         Creates the SQL for this query. Returns the SQL string and list of
         parameters.
         """
-        assert len(self.query.tables) == 1, \
+        assert len([t for t in self.query.tables if self.query.alias_refcount[t] > 0]) == 1, \
             "Can only delete from one table at a time."
         qn = self.quote_name_unless_alias
         result = ['DELETE FROM %s' % qn(self.query.tables[0])]