fix: prevent append with overlapping ids

src/power_grid_model_ds/_core/model/containers/base.py

@@ -212,11 +212,18 @@ def _update_id_counter(self, array, check_max_id: bool = True):
             array = self.attach_ids(array)
         elif np.any(array.id == EMPTY_ID):
             raise ValueError(f"Cannot append: array contains empty [{EMPTY_ID}] and non-empty ids.")
+        elif check_max_id and self.id_counter > 0:
+            # Only check for overlaps when array has prescribed (non-empty) IDs
+            # Check if any incoming ID might overlap with existing IDs
+            # This prevents overlaps since counter tracks the highest used ID
+            new_min_id = np.min(array.id)
+            if new_min_id <= self._id_counter:
+                raise ValueError(
+                    f"Cannot append: minimum id {new_min_id} is not greater than "
+                    f"the current id counter {self._id_counter}"
+                )
 
         new_max_id = np.max(array.id)
-        if check_max_id and new_max_id < self._id_counter:
-            raise ValueError(f"Cannot append: id {new_max_id} is lower than the id counter")
-
         # Update _id_counter
         self._id_counter = max(self._id_counter, new_max_id)
 

tests/unit/model/test_containers.py

@@ -155,6 +155,46 @@ def test_id_counter():
     assert 42 == container.id_counter
 
 
+def test_append_with_overlapping_ids():
+    """Test that appending arrays with overlapping IDs raises an error."""
+    grid = Grid.empty()
+
+    # Create first array with IDs [1, 2, 3]
+    nodes_1 = NodeArray.zeros(3)
+    nodes_1.id = [1, 2, 3]
+    grid.append(nodes_1)
+
+    # Create second array with overlapping IDs [3, 4, 5] (ID 3 overlaps)
+    nodes_2 = NodeArray.zeros(3)
+    nodes_2.id = [3, 4, 5]
+
+    # This should raise a ValueError due to overlapping ID 3
+    with pytest.raises(ValueError, match="Cannot append: minimum id 3 is not greater than the current id counter 3"):
+        grid.append(nodes_2)
+
+
+def test_append_with_non_overlapping_ids():
+    """Test that appending arrays with non-overlapping IDs works correctly."""
+    grid = Grid.empty()
+
+    # Create first array with IDs [1, 2, 3]
+    nodes_1 = NodeArray.zeros(3)
+    nodes_1.id = [1, 2, 3]
+    grid.append(nodes_1)
+
+    # Create second array with non-overlapping IDs [4, 5, 6]
+    nodes_2 = NodeArray.zeros(3)
+    nodes_2.id = [4, 5, 6]
+
+    # This should work without error
+    grid.append(nodes_2)
+
+    # Verify all nodes are in the grid
+    assert grid.node.size == 6
+    expected_ids = [1, 2, 3, 4, 5, 6]
+    assert sorted(grid.node.id.tolist()) == expected_ids
+
+
 def test_branches(grid: Grid):
     nodes = NodeArray.zeros(10)
     grid.append(nodes)