Migration to add CASCADE to user_group.group_id

Add a DB migration to add `ON DELETE CASCADE` to the
`user_group.group_id` column's foreign key to `group.id`.

h/migrations/versions/857c71c8f5f3_add_on_delete_cascade_to_user_group_.py

@@ -0,0 +1,29 @@
+"""Add ON DELETE CASCADE to user_group.group_id."""
+
+from alembic import op
+
+revision = "857c71c8f5f3"
+down_revision = "22a9ef7407ee"
+
+
+def upgrade():
+    op.drop_constraint(
+        "fk__user_group__group_id__group", "user_group", type_="foreignkey"
+    )
+    op.create_foreign_key(
+        "fk__user_group__group_id__group",
+        "user_group",
+        "group",
+        ["group_id"],
+        ["id"],
+        ondelete="cascade",
+    )
+
+
+def downgrade():
+    op.drop_constraint(
+        "fk__user_group__group_id__group", "user_group", type_="foreignkey"
+    )
+    op.create_foreign_key(
+        "fk__user_group__group_id__group", "user_group", "group", ["group_id"], ["id"]
+    )