fix: preserve metadata key named "meta" in Document.from_dict() (#11952)

Co-authored-by: Sebastian Husch Lee <10526848+sjrl@users.noreply.github.com>

Author: otiscuilei

haystack/dataclasses/document.py

@@ -155,6 +155,12 @@ def from_dict(cls, data: dict[str, Any]) -> "Document":
         # a document field is a metadata key. We treat legacy fields as document fields, so that
         # `_RemoveLegacyFields` drops them instead of them ending up in `meta`.
         flatten_meta = {}
+        # A non-mapping value under the "meta" key can't be the `meta` parameter (which must be a
+        # dictionary). It can only be a flattened metadata key literally named "meta", produced by
+        # to_dict(flatten=True) for a document whose metadata contains a "meta" key.
+        if not isinstance(meta, dict):
+            flatten_meta["meta"] = meta
+            meta = {}
         document_fields = _LEGACY_FIELDS + [f.name for f in fields(cls)]
         for key in list(data.keys()):
             if key not in document_fields:

releasenotes/notes/document-meta-key-collision-1330a997893d9f34.yaml

@@ -0,0 +1,8 @@
+---
+fixes:
+  - |
+    Fixed ``Document.from_dict()`` raising ``TypeError: 'str' object is not a mapping`` when
+    deserializing a document whose metadata contained a non-mapping key named ``meta``
+    (for example ``Document(content="hi", meta={"meta": "value"})``). Such a document is now
+    correctly reconstructed from the default flattened ``to_dict()`` output, so the
+    ``to_dict()``/``from_dict()`` round-trip no longer crashes.

test/dataclasses/test_document.py

@@ -346,6 +346,17 @@ def test_from_dict_with_flat_meta():
     )
 
 
+def test_to_dict_from_dict_roundtrip_with_meta_key_named_meta():
+    """
+    A metadata key literally named "meta" must survive the default
+    to_dict(flatten=True)/from_dict() round-trip.
+    """
+    doc = Document(content="hi", meta={"meta": "value"})
+
+    assert Document.from_dict(doc.to_dict()) == doc
+    assert Document.from_dict(doc.to_dict()).meta == {"meta": "value"}
+
+
 def test_from_dict_with_flat_and_non_flat_meta():
     with pytest.raises(ValueError, match="Pass either the 'meta' parameter or flattened metadata keys"):
         Document.from_dict(