fix: fix un-flattening of metadata (#6318)

* fix un-flattening of metadata

* test should pass

* add relnote

* change policy: raise an error if both meta and keys are passed

* Update document.py

* support python 3.8

* adjust wording in the error message

haystack/preview/dataclasses/document.py

@@ -137,15 +137,29 @@ class Document(metaclass=_BackwardCompatible):
             data["dataframe"] = pandas.read_json(io.StringIO(dataframe))
         if blob := data.get("blob"):
             data["blob"] = ByteStream(data=bytes(blob["data"]), mime_type=blob["mime_type"])
-        # Unflatten metadata if it was flattened
-        meta = {}
+        # Store metadata for a moment while we try un-flattening allegedly flatten metadata.
+        # We don't expect both a `meta=` keyword and flatten metadata keys so we'll raise a
+        # ValueError later if this is the case.
+        meta = data.pop("meta", {})
+        # Unflatten metadata if it was flattened. We assume any keyword argument that's not
+        # a document field is a metadata key. We treat legacy fields as document fields
+        # for backward compatibility.
+        flatten_meta = {}
         legacy_fields = ["content_type", "id_hash_keys"]
-        field_names = legacy_fields + [f.name for f in fields(cls)]
+        document_fields = legacy_fields + [f.name for f in fields(cls)]
         for key in list(data.keys()):
-            if key not in field_names:
-                meta[key] = data.pop(key)
+            if key not in document_fields:
+                flatten_meta[key] = data.pop(key)
 
-        return cls(**data, meta=meta)
+        # We don't support passing both flatten keys and the `meta` keyword parameter
+        if meta and flatten_meta:
+            raise ValueError(
+                "You can pass either the 'meta' parameter or flattened metadata keys as keyword arguments, "
+                "but currently you're passing both. Pass either the 'meta' parameter or flattened metadata keys."
+            )
+
+        # Finally put back all the metadata
+        return cls(**data, meta={**meta, **flatten_meta})
 
     @property
     def content_type(self):

releasenotes/notes/fix-document-flatten-metadata-ef61dbbae08b1db7.yaml

@@ -0,0 +1,8 @@
+---
+
+preview:
+  - |
+    Fix a failure that occurred when creating a document passing the 'meta' keyword
+    to the constructor. Raise a specific ValueError if the 'meta' keyword is passed
+    along with metadata as keyword arguments, the two options are mutually exclusive
+    now.

test/preview/dataclasses/test_document.py

@@ -282,7 +282,7 @@ def test_from_dict_with_flat_meta():
 
 @pytest.mark.unit
 def test_from_dict_with_flat_and_non_flat_meta():
-    with pytest.raises(TypeError):
+    with pytest.raises(ValueError, match="Pass either the 'meta' parameter or flattened metadata keys"):
         Document.from_dict(
             {
                 "content": "test text",