fix remove recursive to follow mixed links

CHANGELOG.md

@@ -7,6 +7,7 @@
 ### Fixed
 * <How do the end-user experience this issue? what was the impact?> ([#????](https://github.com/realm/realm-core/issues/????), since v?.?.?)
 * Fixed a change of mode from Strong to All when removing links from an embedded object that links to a tombstone. This affects sync apps that use embedded objects which have a `Lst<Mixed>` that contains a link to another top level object which has been deleted by another sync client (creating a tombstone locally). In this particular case, the switch would cause any remaining link removals to recursively delete the destination object if there were no other links to it. ([#7828](https://github.com/realm/realm-core/issues/7828), since 14.0.0-beta.0)
+* Fixed `Table::remove_object_recursive` which wouldn't recursively follow links through a single `Mixed` property. ([#7829](https://github.com/realm/realm-core/issues/7829), likely since the introduction of Mixed)
 
 ### Breaking changes
 * None.

src/realm/cluster.cpp

@@ -788,11 +788,8 @@ inline void Cluster::do_erase_mixed(size_t ndx, ColKey col_key, ObjKey key, Casc
     Mixed value = values.get(ndx);
     if (value.is_type(type_TypedLink)) {
         ObjLink link = value.get<ObjLink>();
-        auto target_obj = origin_table->get_parent_group()->get_object(link);
-
-        ColKey backlink_col_key = target_obj.get_table()->find_backlink_column(col_key, origin_table->get_key());
-        REALM_ASSERT(backlink_col_key);
-        target_obj.remove_one_backlink(backlink_col_key, get_real_key(ndx)); // Throws
+        Obj obj(origin_table->m_own_ref, get_mem(), key, ndx);
+        obj.remove_backlink(col_key, link, state);
     }
     if (value.is_type(type_List)) {
         Obj obj(origin_table->m_own_ref, get_mem(), key, ndx);
@@ -1541,7 +1538,7 @@ void Cluster::remove_backlinks(const Table* origin_table, ObjKey origin_key, Col
             bool last_removed = target_obj.remove_one_backlink(backlink_col_key, origin_key); // Throws
             if (is_unres) {
                 if (last_removed) {
-                    // Check is there are more backlinks
+                    // Check if there are more backlinks
                     if (!target_obj.has_backlinks(false)) {
                         // Tombstones can be erased right away - there is no cascading effect
                         target_table->m_tombstones->erase(link.get_obj_key(), state);

test/test_mixed_null_assertions.cpp

@@ -549,3 +549,24 @@ TEST(Mixed_EmbeddedLstMixedRecursiveDelete)
     CHECK_EQUAL(top3_obj4.get_backlink_count(), 0);
 }
 
+TEST(Mixed_SingleLinkRecursiveDelete)
+{
+    Group g;
+    auto top1 = g.add_table_with_primary_key("source", type_String, "_id");
+    auto top2 = g.add_table_with_primary_key("top2", type_String, "_id");
+
+    ColKey top1_mixed_col = top1->add_column(type_Mixed, "mixed");
+    auto top1_obj1 = top1->create_object_with_primary_key("top1_obj1");
+    auto top2_obj1 = top2->create_object_with_primary_key("top2_obj1");
+
+    top1_obj1.set<Mixed>(top1_mixed_col, ObjLink{top2->get_key(), top2_obj1.get_key()});
+
+    CHECK_EQUAL(top2_obj1.get_backlink_count(), 1);
+
+    top1->remove_object_recursive(top1_obj1.get_key());
+
+    CHECK_NOT(top1_obj1.is_valid());
+    CHECK_EQUAL(top1->size(), 0);
+    CHECK_NOT(top2_obj1.is_valid());
+    CHECK_EQUAL(top2->size(), 0);
+}