Fix: Nested Dicts Handling of Document Metadata (#9880)

## Description
When the `MultiQueryRetriever` is used to get the list of documents
relevant according to a query, inside a vector store, and at least one
of these contain metadata with nested dictionaries, a `TypeError:
unhashable type: 'dict'` exception is thrown.
This is caused by the `unique_union` function which, to guarantee the
uniqueness of the returned documents, tries, unsuccessfully, to hash the
nested dictionaries and use them as a part of key.
```python
unique_documents_dict = {
    (doc.page_content, tuple(sorted(doc.metadata.items()))): doc
    for doc in documents
}
```

## Issue
#9872 (MultiQueryRetriever (get_relevant_documents) raises TypeError:
unhashable type: 'dict' with dic metadata)

## Solution
A possible solution is to dump the metadata dict to a string and use it
as a part of hashed key.
```python
unique_documents_dict = {
    (doc.page_content, json.dumps(doc.metadata, sort_keys=True)): doc
    for doc in documents
}
```

---------

Co-authored-by: Bagatur <baskaryan@gmail.com>

libs/langchain/langchain/retrievers/multi_query.py

@@ -1,5 +1,5 @@
 import logging
-from typing import List
+from typing import List, Sequence
 
 from langchain.callbacks.manager import CallbackManagerForRetrieverRun
 from langchain.chains.llm import LLMChain
@@ -43,10 +43,14 @@ DEFAULT_QUERY_PROMPT = PromptTemplate(
 )
 
 
+def _unique_documents(documents: Sequence[Document]) -> List[Document]:
+    return [doc for i, doc in enumerate(documents) if doc not in documents[:i]]
+
+
 class MultiQueryRetriever(BaseRetriever):
     """Given a query, use an LLM to write a set of queries.
 
-    Retrieve docs for each query. Rake the unique union of all retrieved docs.
+    Retrieve docs for each query. Return the unique union of all retrieved docs.
     """
 
     retriever: BaseRetriever
@@ -85,7 +89,7 @@ class MultiQueryRetriever(BaseRetriever):
         *,
         run_manager: CallbackManagerForRetrieverRun,
     ) -> List[Document]:
-        """Get relevated documents given a user query.
+        """Get relevant documents given a user query.
 
         Args:
             question: user query
@@ -95,8 +99,7 @@ class MultiQueryRetriever(BaseRetriever):
         """
         queries = self.generate_queries(query, run_manager)
         documents = self.retrieve_documents(queries, run_manager)
-        unique_documents = self.unique_union(documents)
-        return unique_documents
+        return self.unique_union(documents)
 
     def generate_queries(
         self, question: str, run_manager: CallbackManagerForRetrieverRun
@@ -145,12 +148,4 @@ class MultiQueryRetriever(BaseRetriever):
         Returns:
             List of unique retrieved Documents
         """
-        # Create a dictionary with page_content as keys to remove duplicates
-        # TODO: Add Document ID property (e.g., UUID)
-        unique_documents_dict = {
-            (doc.page_content, tuple(sorted(doc.metadata.items()))): doc
-            for doc in documents
-        }
-
-        unique_documents = list(unique_documents_dict.values())
-        return unique_documents
+        return _unique_documents(documents)

libs/langchain/tests/unit_tests/retrievers/test_multi_query.py

@@ -0,0 +1,40 @@
+from typing import List
+
+import pytest as pytest
+
+from langchain.retrievers.multi_query import _unique_documents
+from langchain.schema import Document
+
+
+@pytest.mark.parametrize(
+    "documents,expected",
+    [
+        ([], []),
+        ([Document(page_content="foo")], [Document(page_content="foo")]),
+        ([Document(page_content="foo")] * 2, [Document(page_content="foo")]),
+        (
+            [Document(page_content="foo", metadata={"bar": "baz"})] * 2,
+            [Document(page_content="foo", metadata={"bar": "baz"})],
+        ),
+        (
+            [Document(page_content="foo", metadata={"bar": [1, 2]})] * 2,
+            [Document(page_content="foo", metadata={"bar": [1, 2]})],
+        ),
+        (
+            [Document(page_content="foo", metadata={"bar": {1, 2}})] * 2,
+            [Document(page_content="foo", metadata={"bar": {1, 2}})],
+        ),
+        (
+            [
+                Document(page_content="foo", metadata={"bar": [1, 2]}),
+                Document(page_content="foo", metadata={"bar": [2, 1]}),
+            ],
+            [
+                Document(page_content="foo", metadata={"bar": [1, 2]}),
+                Document(page_content="foo", metadata={"bar": [2, 1]}),
+            ],
+        ),
+    ],
+)
+def test__unique_documents(documents: List[Document], expected: List[Document]) -> None:
+    assert _unique_documents(documents) == expected