fix: preserve rest rules when priority overrides chronological order

app/models.py

@@ -58,22 +58,41 @@ def has_no_shift_same_day(self, shift):
         )
 
     def has_required_rest_time(self, shift):
+        new_date = datetime.date.fromisoformat(shift.date)
+
         for assigned_shift in self.assigned_shifts:
-            existing_date = datetime.datetime.strptime(assigned_shift.date, "%Y-%m-%d")
-            new_date = datetime.datetime.strptime(shift.date, "%Y-%m-%d")
+            existing_date = datetime.date.fromisoformat(assigned_shift.date)
 
-            is_next_day = new_date == existing_date + datetime.timedelta(days=1)
-            is_evening_to_day = (
-            assigned_shift.shift_type == ShiftType.EVENING 
-            and shift.shift_type == ShiftType.DAY
+            # A night blocks the following day; an evening blocks the
+            # following morning. Check both orders because shifts are
+            # assigned by priority, not necessarily by chronological date.
+            existing_is_previous_day = (
+                new_date == existing_date + datetime.timedelta(days=1)
+            )
+            new_is_previous_day = (
+                existing_date == new_date + datetime.timedelta(days=1)
             )
-            is_night_before = assigned_shift.shift_type == ShiftType.NIGHT
 
-            if is_next_day and (is_evening_to_day or is_night_before):
+            if existing_is_previous_day and (
+                assigned_shift.shift_type == ShiftType.NIGHT
+                or (
+                    assigned_shift.shift_type == ShiftType.EVENING
+                    and shift.shift_type == ShiftType.DAY
+                )
+            ):
+                return False
+
+            if new_is_previous_day and (
+                shift.shift_type == ShiftType.NIGHT
+                or (
+                    shift.shift_type == ShiftType.EVENING
+                    and assigned_shift.shift_type == ShiftType.DAY
+                )
+            ):
                 return False
 
         return True
-    
+
     def get_risk_flags(self, workload_ratio_threshold: float, average_score_threshold: float) -> list[str]:
         flags = []
 

tests/test_employee.py

@@ -99,4 +99,41 @@ def test_employee_cannot_take_two_shifts_on_same_day():
     result = employee.can_take_shift(shift2)
 
     # Assert
-    assert result is False, "An employee may not have two shifts on the same date."
+    assert result is False, "An employee may not have two shifts on the same date."
+
+def test_employee_cannot_take_evening_before_assigned_next_day():
+    employee = Employee(name="Reverse-Evening", max_shifts=5, skills=["CT"])
+    day_shift = Shift(date="2026-05-12", shift_type=ShiftType.DAY, required_skill="CT")
+    previous_evening = Shift(
+        date="2026-05-11", shift_type=ShiftType.EVENING, required_skill="CT"
+    )
+    employee.assigned_shifts.append(day_shift)
+
+    assert employee.can_take_shift(previous_evening) is False
+    assert "rest time violation" in employee.get_shift_blockers(previous_evening)
+
+
+def test_employee_cannot_take_night_before_assigned_next_day():
+    employee = Employee(name="Reverse-Night", max_shifts=5, skills=["MR"])
+    next_evening = Shift(
+        date="2026-05-12", shift_type=ShiftType.EVENING, required_skill="MR"
+    )
+    previous_night = Shift(
+        date="2026-05-11", shift_type=ShiftType.NIGHT, required_skill="MR"
+    )
+    employee.assigned_shifts.append(next_evening)
+
+    assert employee.can_take_shift(previous_night) is False
+
+
+def test_employee_can_take_day_before_assigned_next_day_evening():
+    employee = Employee(name="Safe-Sequence", max_shifts=5, skills=["CT"])
+    next_evening = Shift(
+        date="2026-05-12", shift_type=ShiftType.EVENING, required_skill="CT"
+    )
+    previous_day = Shift(
+        date="2026-05-11", shift_type=ShiftType.DAY, required_skill="CT"
+    )
+    employee.assigned_shifts.append(next_evening)
+
+    assert employee.can_take_shift(previous_day) is True

tests/test_scheduler.py

@@ -161,4 +161,26 @@ def test_assign_shifts_still_works_without_employee_profiles():
         employee_profiles=None,
     )
 
-    assert employee in shift.assigned_employees
+    assert employee in shift.assigned_employees
+
+def test_priority_order_does_not_bypass_rest_rules():
+    employee = Employee(name="Priority-Rest", max_shifts=3, skills=["CT"])
+    # Later calendar date has the higher scheduling priority.
+    next_day = Shift(
+        date="2026-05-12",
+        shift_type=ShiftType.DAY,
+        required_skill="CT",
+        priority=1,
+    )
+    previous_night = Shift(
+        date="2026-05-11",
+        shift_type=ShiftType.NIGHT,
+        required_skill="CT",
+        priority=2,
+    )
+
+    assign_shifts([employee], [previous_night, next_day])
+
+    assert employee in next_day.assigned_employees
+    assert previous_night.assigned_employees == []
+    assert employee.assigned_shifts == [next_day]