Implement DiffSyncModelFlags.NATURAL_DELETION_ORDER.

diffsync/enum.py

@@ -47,6 +47,13 @@ class DiffSyncModelFlags(enum.Flag):
     If this flag is set, the model will not be deleted from the target/"to" DiffSync.
     """
 
+    NATURAL_DELETION_ORDER = 0b10000
+    """When deleting, delete children before instances of this this element.
+
+    If this flag is set, the models children will be deleted from the target/"to" DiffSync before the models instances
+    themselves.
+    """
+
     SKIP_UNMATCHED_BOTH = SKIP_UNMATCHED_SRC | SKIP_UNMATCHED_DST
 
 

diffsync/helpers.py

@@ -350,11 +350,7 @@ def sync_diff_element(self, element: DiffElement, parent_model: Optional["DiffSy
         attrs = diffs.get("+", {})
 
         # Retrieve Source Object to get its flags
-        src_model: Optional["DiffSyncModel"]
-        try:
-            src_model = self.src_diffsync.get(self.model_class, ids)
-        except ObjectNotFound:
-            src_model = None
+        src_model = self.src_diffsync.get_or_none(self.model_class, ids)
 
         # Retrieve Dest (and primary) Object
         dst_model: Optional["DiffSyncModel"]
@@ -364,31 +360,43 @@ def sync_diff_element(self, element: DiffElement, parent_model: Optional["DiffSy
         except ObjectNotFound:
             dst_model = None
 
+        natural_deletion_order = False
+        skip_children = False
+        # Set up flag booleans
+        if dst_model:
+            natural_deletion_order = bool(dst_model.model_flags & DiffSyncModelFlags.NATURAL_DELETION_ORDER)
+            skip_children = bool(dst_model.model_flags & DiffSyncModelFlags.SKIP_CHILDREN_ON_DELETE)
+
+        changed = False
+        if natural_deletion_order and self.action == DiffSyncActions.DELETE and not skip_children:
+            for child in element.get_children():
+                changed |= self.sync_diff_element(child, parent_model=dst_model)
+
         changed, modified_model = self.sync_model(src_model=src_model, dst_model=dst_model, ids=ids, attrs=attrs)
         dst_model = modified_model or dst_model
 
         if not modified_model or not dst_model:
             self.logger.warning("No object resulted from sync, will not process child objects.")
             return changed
 
-        if self.action == DiffSyncActions.CREATE:  # type: ignore
+        if self.action == DiffSyncActions.CREATE:
             if parent_model:
                 parent_model.add_child(dst_model)
             self.dst_diffsync.add(dst_model)
         elif self.action == DiffSyncActions.DELETE:
             if parent_model:
                 parent_model.remove_child(dst_model)
 
-            skip_children = bool(dst_model.model_flags & DiffSyncModelFlags.SKIP_CHILDREN_ON_DELETE)
             self.dst_diffsync.remove(dst_model, remove_children=skip_children)
 
             if skip_children:
                 return changed
 
         self.incr_elements_processed()
 
-        for child in element.get_children():
-            changed |= self.sync_diff_element(child, parent_model=dst_model)
+        if not natural_deletion_order:
+            for child in element.get_children():
+                changed |= self.sync_diff_element(child, parent_model=dst_model)
 
         return changed
 

docs/source/core_engine/01-flags.md

@@ -53,13 +53,14 @@ class MyAdapter(DiffSync):
 
 ### Supported Model Flags
 
-| Name | Description | Binary Value |
-|---|---|---|
+| Name | Description                                                                                                                                                                                       | Binary Value |
+|---|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|---|
 | IGNORE | Do not render diffs containing this model; do not make any changes to this model when synchronizing.  Can be used to indicate a model instance that exists but should not be changed by DiffSync. | 0b1 |
-| SKIP_CHILDREN_ON_DELETE | When deleting this model, do not recursively delete its children. Can be used for the case where deletion of a model results in the automatic deletion of all its children. | 0b10 |
-| SKIP_UNMATCHED_SRC | Ignore the model if it only exists in the source/"from" DiffSync when determining diffs and syncing. If this flag is set, no new model will be created in the target/"to" DiffSync. | 0b100 |
-| SKIP_UNMATCHED_DST | Ignore the model if it only exists in the target/"to" DiffSync when determining diffs and syncing. If this flag is set, the model will not be deleted from the target/"to" DiffSync. | 0b1000 |
-| SKIP_UNMATCHED_BOTH | Convenience value combining both SKIP_UNMATCHED_SRC and SKIP_UNMATCHED_DST into a single flag | 0b1100 |
+| SKIP_CHILDREN_ON_DELETE | When deleting this model, do not recursively delete its children. Can be used for the case where deletion of a model results in the automatic deletion of all its children.                       | 0b10 |
+| SKIP_UNMATCHED_SRC | Ignore the model if it only exists in the source/"from" DiffSync when determining diffs and syncing. If this flag is set, no new model will be created in the target/"to" DiffSync.               | 0b100 |
+| SKIP_UNMATCHED_DST | Ignore the model if it only exists in the target/"to" DiffSync when determining diffs and syncing. If this flag is set, the model will not be deleted from the target/"to" DiffSync.              | 0b1000 |
+| SKIP_UNMATCHED_BOTH | Convenience value combining both SKIP_UNMATCHED_SRC and SKIP_UNMATCHED_DST into a single flag                                                                                                     | 0b1100 |
+| NATURAL_DELETION_ORDER | When deleting, delete children before instances of this model.                                                                                                                                    | 0b10000 |
 
 ## Working with flags
 

tests/unit/test_diffsync.py

@@ -973,6 +973,7 @@ class NoDeleteInterfaceDiffSync(BackendA):
     extra_models.load()
     extra_device = extra_models.device(name="nyc-spine3", site_name="nyc", role="spine")
     extra_device.model_flags |= DiffSyncModelFlags.SKIP_CHILDREN_ON_DELETE
+    extra_device.model_flags |= DiffSyncModelFlags.NATURAL_DELETION_ORDER
     extra_models.get(extra_models.site, "nyc").add_child(extra_device)
     extra_models.add(extra_device)
     extra_interface = extra_models.interface(name="eth0", device_name="nyc-spine3")

tests/unit/test_diffsync_model_flags.py

@@ -14,9 +14,11 @@
 See the License for the specific language governing permissions and
 limitations under the License.
 """
+from typing import List
 
 import pytest
 
+from diffsync import DiffSync, DiffSyncModel
 from diffsync.enum import DiffSyncModelFlags
 from diffsync.exceptions import ObjectNotFound
 
@@ -111,3 +113,52 @@ def test_diffsync_diff_with_ignore_flag_on_target_models(backend_a, backend_a_mi
     diff = backend_a.diff_from(backend_a_minus_some_models)
     print(diff.str())  # for debugging of any failure
     assert not diff.has_diffs()
+
+
+def test_diffsync_diff_with_natural_deletion_order():
+    # This list will contain the order in which the delete methods were called
+    call_order = []
+
+    class TestModelChild(DiffSyncModel):  # pylint: disable=missing-class-docstring
+        _modelname = "child"
+        _identifiers = ("name",)
+
+        name: str
+
+        def delete(self):
+            call_order.append(self.name)
+            return super().delete()
+
+    class TestModelParent(DiffSyncModel):  # pylint: disable=missing-class-docstring
+        _modelname = "parent"
+        _identifiers = ("name",)
+        _children = {"child": "children"}
+
+        name: str
+        children: List[TestModelChild] = []
+
+        def delete(self):
+            call_order.append(self.name)
+            return super().delete()
+
+    class TestBackend(DiffSync):  # pylint: disable=missing-class-docstring
+        top_level = ["parent"]
+
+        parent = TestModelParent
+        child = TestModelChild
+
+        def load(self):
+            parent = self.parent(name="Test-Parent")
+            parent.model_flags |= DiffSyncModelFlags.NATURAL_DELETION_ORDER
+            self.add(parent)
+            child = self.child(name="Test-Child")
+            parent.add_child(child)
+            self.add(child)
+
+    source = TestBackend()
+    source.load()
+    destination = TestBackend()
+    destination.load()
+    source.remove(source.get("parent", {"name": "Test-Parent"}), remove_children=True)
+    source.sync_to(destination)
+    assert call_order == ["Test-Child", "Test-Parent"]