[departments] Cascade-delete salary scale entries on department deletion

tests/models/test_salary_scale.py

@@ -1,5 +1,6 @@
 from tests.base import ApiDBTestCase
 
+from zou.app.models.department import Department
 from zou.app.models.salary_scale import SalaryScale
 
 from zou.app.utils import fields
@@ -15,6 +16,37 @@ def test_get_salary_scales(self):
         # salary_scales = self.get("data/salary-scales")
         # self.assertEqual(len(salary_scales), 3)
 
+    def test_delete_department_cascades_salary_scales(self):
+        """
+        A department must stay deletable even though salary scale entries
+        reference it: the entries are auto-generated and meaningless without
+        the department, so they are removed by the ON DELETE CASCADE foreign
+        key instead of blocking the deletion.
+        """
+        department = Department.create(name="Modeling", color="#FFFFFF")
+        SalaryScale.create(
+            department_id=department.id,
+            position="artist",
+            seniority="junior",
+            salary=500,
+        )
+        SalaryScale.create(
+            department_id=department.id,
+            position="lead",
+            seniority="senior",
+            salary=900,
+        )
+        self.assertEqual(
+            SalaryScale.query.filter_by(department_id=department.id).count(), 2
+        )
+
+        department.delete()
+
+        self.assertIsNone(Department.get(department.id))
+        self.assertEqual(
+            SalaryScale.query.filter_by(department_id=department.id).count(), 0
+        )
+
     """
     def test_get_salary_scale(self):
         salary_scale = self.get_first("data/salary-scales")

zou/app/models/salary_scale.py

@@ -15,7 +15,7 @@ class SalaryScale(db.Model, BaseMixin, SerializerMixin):
 
     department_id = db.Column(
         UUIDType(binary=False),
-        db.ForeignKey("department.id"),
+        db.ForeignKey("department.id", ondelete="CASCADE"),
         index=True,
         nullable=False,
     )

zou/migrations/versions/f1a2b3c4d5e6_cascade_delete_salary_scale_on_department.py

@@ -0,0 +1,52 @@
+"""cascade delete salary scale entries when a department is deleted
+
+Salary scale rows are auto-generated for every department/position/seniority
+combination and have no meaning without their department, so they should be
+removed together with it. The foreign key is recreated with ON DELETE CASCADE
+to replace the default RESTRICT behaviour that blocked department deletion.
+
+Revision ID: f1a2b3c4d5e6
+Revises: c5e8b2a4f1d3
+Create Date: 2026-06-21 10:00:00.000000
+
+"""
+
+from alembic import op
+
+
+# revision identifiers, used by Alembic.
+revision = "f1a2b3c4d5e6"
+down_revision = "c5e8b2a4f1d3"
+branch_labels = None
+depends_on = None
+
+
+def upgrade():
+    op.drop_constraint(
+        "salary_scale_department_id_fkey",
+        "salary_scale",
+        type_="foreignkey",
+    )
+    op.create_foreign_key(
+        "salary_scale_department_id_fkey",
+        "salary_scale",
+        "department",
+        ["department_id"],
+        ["id"],
+        ondelete="CASCADE",
+    )
+
+
+def downgrade():
+    op.drop_constraint(
+        "salary_scale_department_id_fkey",
+        "salary_scale",
+        type_="foreignkey",
+    )
+    op.create_foreign_key(
+        "salary_scale_department_id_fkey",
+        "salary_scale",
+        "department",
+        ["department_id"],
+        ["id"],
+    )