Fixed #21554 -- Incorrect SQL generated when using multiple inheritance.

tests/model_inheritance_regress/models.py

@@ -233,3 +233,17 @@ class User(models.Model):
 class Profile(User):
     profile_id = models.AutoField(primary_key=True)
     extra = models.CharField(max_length=30, blank=True)
+
+
+# Check concrete + concrete -> concrete -> concrete
+class Politician(models.Model):
+    politician_id = models.AutoField(primary_key=True)
+    title = models.CharField(max_length=50)
+
+
+class Congressman(Person, Politician):
+    state = models.CharField(max_length=2)
+
+
+class Senator(Congressman):
+    pass