fix: route nested dicts through complex-replace in _create_at

Author: nathanjmcdougall

doc/specs/2026-05-13-yamltrip-design.md

@@ -115,8 +115,11 @@ All return a new `Document`.
 
 1. **Full path exists:** delegates to `replace()`.
 2. **Partial path exists:** walks down to the deepest existing key, then uses
-   yamlpatch `MergeInto` to create the remaining nested structure in a single
-   operation.
+   yamlpatch `MergeInto` for flat mapping values (scalar-only entries) or a
+   two-step `Add` placeholder + `Replace` for nested values (dicts/lists inside
+   the value). `MergeInto` uses uniform-indent serialization that only handles
+   one nesting level; the `Replace` path routes through complex-replace which
+   preserves relative indentation at arbitrary depth.
 3. **No path exists:** uses yamlpatch `Add` with a nested value to create the
    entire path in a single operation.
 

src/yamltrip/document.py

@@ -286,7 +286,19 @@ def _create_at(
         for k in reversed(child_keys[1:]):
             nested_value = {k: nested_value}
         route = _make_route(parent_keys)
-        if isinstance(nested_value, dict):
+        if isinstance(nested_value, dict) and any(
+            isinstance(v, (dict, list, tuple)) for v in nested_value.values()
+        ):
+            # Op.merge_into is scoped to flat mappings (uniform indent); for
+            # nested values, add a placeholder then replace via complex-replace
+            # which preserves relative indentation.
+            add_op = _core.Op.add(first_key, None)
+            add_patch = _core.Patch(route=route, operation=add_op)
+            replace_route = _make_route((*parent_keys, first_key))
+            replace_op = _core.Op.replace(nested_value)
+            replace_patch = _core.Patch(route=replace_route, operation=replace_op)
+            return self._apply_patches([add_patch, replace_patch])
+        elif isinstance(nested_value, dict):
             op = _core.Op.merge_into(first_key, nested_value)
         else:
             op = _core.Op.add(first_key, nested_value)

tests/test_document.py

@@ -409,6 +409,14 @@ def test_upsert_missing_with_list(self):
         doc2 = doc.upsert("items", value=["a", "b"])
         assert doc2["items"] == ["a", "b"]
 
+    def test_upsert_missing_nested_path_with_dict(self):
+        """Nested dict via intermediate keys must not flatten."""
+        doc = Document("a: 1\n")
+        doc2 = doc.upsert("parent", "child", value={"x": 1, "y": 2})
+        assert doc2["parent", "child", "x"] == 1
+        assert doc2["parent", "child", "y"] == 2
+        assert doc2["parent", "child"] == {"x": 1, "y": 2}
+
 
 class TestDocumentRemove:
     def test_remove_key(self):