fix(vector_stores): return None from ChromaDB.get() for missing IDs (#5561)

Co-authored-by: ly-wang19 <ly-wang19@users.noreply.github.com>

mem0/vector_stores/chroma.py

@@ -187,7 +187,7 @@ class ChromaDB(VectorStoreBase):
         """
         self.collection.update(ids=vector_id, embeddings=vector, metadatas=payload)
 
-    def get(self, vector_id: str) -> OutputData:
+    def get(self, vector_id: str) -> Optional[OutputData]:
         """
         Retrieve a vector by ID.
 
@@ -195,10 +195,11 @@ class ChromaDB(VectorStoreBase):
             vector_id (str): ID of the vector to retrieve.
 
         Returns:
-            OutputData: Retrieved vector.
+            Optional[OutputData]: Retrieved vector, or None if the ID is not found.
         """
         result = self.collection.get(ids=[vector_id])
-        return self._parse_output(result)[0]
+        parsed = self._parse_output(result)
+        return parsed[0] if parsed else None
 
     def list_cols(self) -> List[chromadb.Collection]:
         """

tests/vector_stores/test_chroma.py

@@ -154,6 +154,16 @@ def test_get_vector(chromadb_instance):
     assert result.payload == {"name": "vector1"}
 
 
+def test_get_missing_vector_returns_none(chromadb_instance):
+    # Chroma returns empty lists for an unknown id; get() must return None
+    # rather than raising IndexError (parity with qdrant/pgvector/faiss).
+    chromadb_instance.collection.get.return_value = {"ids": [], "metadatas": []}
+
+    result = chromadb_instance.get(vector_id="does-not-exist")
+
+    assert result is None
+
+
 def test_list_vectors(chromadb_instance):
     mock_result = {
         "ids": [["id1", "id2"]],