Snapshot inherited component relation inputs before replacement

CHANGELOG.md

@@ -9,6 +9,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ### Fixed
 
+- Replacing component relations from an inherited view now preserves values which depend on the destination.
 - Removing a relation tag or component now preserves query membership when the other relation still exists.
 - Queries collecting component rows now resolve inherited components instead of raising `KeyError` for prefab instances.
 - Relation components can now be copied between distinct keys in the same entity, instead of the assignment being silently ignored.

tcod/ecs/entity.py

@@ -1071,9 +1071,10 @@ def __setitem__(self, __key: ComponentKey[T], __values: Mapping[Entity, T], /) -
             and __values.key == __key
         ):
             return
+        items = tuple(__values.items())  # The input may inherit values from the mapping being replaced.
         mapping: EntityComponentRelationMapping[T] = self[__key]
         mapping.clear()
-        for target, component in __values.items():
+        for target, component in items:
             mapping[target] = component
 
     def __delitem__(self, key: ComponentKey[object]) -> None:

tests/test_relations.py

@@ -359,3 +359,30 @@ def test_overlapping_relation_queries(key: ComponentKey[int], remove_first: str)
     else:
         actor.relation_tags_many[key].remove(faction)
     assert not any(queries)
+
+
+@pytest.mark.parametrize("key", [int, ("affinity", int)])
+@pytest.mark.parametrize("depth", [1, 3])
+@pytest.mark.parametrize("override", [False, True])
+def test_component_relation_assignment_from_inherited_view(
+    key: ComponentKey[int], depth: int, *, override: bool
+) -> None:
+    world = tcod.ecs.Registry()
+    prefab = world["prefab"]
+    first, second = world["first"], world["second"]
+    prefab.relation_components[key] = {first: 10, second: 20}
+    instance = prefab
+    for _ in range(depth):
+        instance = instance.instantiate()
+    if override:
+        instance.relation_components[key][first] = 30
+    expected = {first: 30 if override else 10, second: 20}
+    query = world.Q.all_of(relations=[(key, second)])
+    assert prefab in query
+
+    prefab.relation_components[key] = instance.relation_components[key]
+
+    assert dict(prefab.relation_components[key]) == expected
+    assert dict(instance.relation_components[key]) == expected
+    assert prefab in query
+    assert instance in query