Fixed #373 -- Added CompositePrimaryKey.

Thanks Lily Foote and Simon Charette for reviews and mentoring
this Google Summer of Code 2024 project.

Co-authored-by: Simon Charette <charette.s@gmail.com>
Co-authored-by: Lily Foote <code@lilyf.org>

tests/composite_pk/models/tenant.py

@@ -0,0 +1,50 @@
+from django.db import models
+
+
+class Tenant(models.Model):
+    name = models.CharField(max_length=10, default="", blank=True)
+
+
+class Token(models.Model):
+    pk = models.CompositePrimaryKey("tenant_id", "id")
+    tenant = models.ForeignKey(Tenant, on_delete=models.CASCADE, related_name="tokens")
+    id = models.SmallIntegerField()
+    secret = models.CharField(max_length=10, default="", blank=True)
+
+
+class BaseModel(models.Model):
+    pk = models.CompositePrimaryKey("tenant_id", "id")
+    tenant = models.ForeignKey(Tenant, on_delete=models.CASCADE)
+    id = models.SmallIntegerField(unique=True)
+
+    class Meta:
+        abstract = True
+
+
+class User(BaseModel):
+    email = models.EmailField(unique=True)
+
+
+class Comment(models.Model):
+    pk = models.CompositePrimaryKey("tenant", "id")
+    tenant = models.ForeignKey(
+        Tenant,
+        on_delete=models.CASCADE,
+        related_name="comments",
+    )
+    id = models.SmallIntegerField(unique=True, db_column="comment_id")
+    user_id = models.SmallIntegerField()
+    user = models.ForeignObject(
+        User,
+        on_delete=models.CASCADE,
+        from_fields=("tenant_id", "user_id"),
+        to_fields=("tenant_id", "id"),
+        related_name="comments",
+    )
+    text = models.TextField(default="", blank=True)
+
+
+class Post(models.Model):
+    pk = models.CompositePrimaryKey("tenant_id", "id")
+    tenant = models.ForeignKey(Tenant, on_delete=models.CASCADE)
+    id = models.UUIDField()