OGM save RelatedObjects - feature flag to keep relationships not in list of nodes

[process0]

The __db_push__ function in RelatedObjects class will delete any existing relationships for a node that are not represented by the current set of nodes on push. This creates orphan nodes when adding relations via merge to an existing node with existing relationships.

I've commented out line 340 / step 2a in this function and the relationships are preserved. Would it be possible to make this a feature flag? Also, are there any problems that could arise from doing this?

[process0]

This is possible by adding a field to the Model class.

--- venv/lib/python3.7/site-packages/py2neo/ogm/__init__.py     2021-05-10 15:52:20.237786016 -0400
+++ other/venv/lib/python3.7/site-packages/py2neo/ogm/__init__.py      2021-05-10 15:47:16.313949017 -0400
@@ -336,8 +337,10 @@
             tx.merge(related_object)
         # 2a. remove any relationships not in list of nodes
         subject_id = self.node.identity
-        tx.run("MATCH %s WHERE id(a) = $x AND NOT id(b) IN $y DELETE _" % self.__relationship_pattern,
-               x=subject_id, y=[obj.__node__.identity for obj, _ in related_objects])
+        if not self.subject.__preserve_relationships__:
+            tx.run("MATCH %s WHERE id(a) = $x AND NOT id(b) IN $y DELETE _" % self.__relationship_pattern,
+                   x=subject_id, y=[obj.__node__.identity for obj, _ in related_objects])
+
         # 2b. merge all relationships
         for related_object, properties in related_objects:
             tx.run("MATCH (a) WHERE id(a) = $x MATCH (b) WHERE id(b) = $y "
@@ -424,6 +427,7 @@
     __primarylabel__ = None
     __primarykey__ = None

+    __preserve_relationships__ = False  # False for existing behavior
     __ogm = None

     def __eq__(self, other):

[technige]

This sounds like a good feature request. I'll look into it.

[process0]

@technige any update?

[technige]

No, not had time to look into this yet.