community: FAISS vectorstore - consistent Document id field (#28728)

make sure id field of Documents in `FAISS` docstore have the same id as
values in `index_to_docstore_id`, implement `get_by_ids` method

libs/community/langchain_community/vectorstores/faiss.py

@@ -14,6 +14,7 @@
     Iterable,
     List,
     Optional,
+    Sequence,
     Sized,
     Tuple,
     Union,
@@ -284,33 +285,34 @@ def __add(
         ids: Optional[List[str]] = None,
     ) -> List[str]:
         faiss = dependable_faiss_import()
-
         if not isinstance(self.docstore, AddableMixin):
             raise ValueError(
                 "If trying to add texts, the underlying docstore should support "
                 f"adding items, which {self.docstore} does not"
             )
 
         _len_check_if_sized(texts, metadatas, "texts", "metadatas")
+
+        ids = ids or [str(uuid.uuid4()) for _ in texts]
+        _len_check_if_sized(texts, ids, "texts", "ids")
+
         _metadatas = metadatas or ({} for _ in texts)
         documents = [
-            Document(page_content=t, metadata=m) for t, m in zip(texts, _metadatas)
+            Document(id=id_, page_content=t, metadata=m)
+            for id_, t, m in zip(ids, texts, _metadatas)
         ]
 
         _len_check_if_sized(documents, embeddings, "documents", "embeddings")
-        _len_check_if_sized(documents, ids, "documents", "ids")
 
         if ids and len(ids) != len(set(ids)):
             raise ValueError("Duplicate ids found in the ids list.")
-
         # Add to the index.
         vector = np.array(embeddings, dtype=np.float32)
         if self._normalize_L2:
             faiss.normalize_L2(vector)
         self.index.add(vector)
 
         # Add information to docstore and index.
-        ids = ids or [str(uuid.uuid4()) for _ in texts]
         self.docstore.add({id_: doc for id_, doc in zip(ids, documents)})
         starting_len = len(self.index_to_docstore_id)
         index_to_id = {starting_len + j: id_ for j, id_ in enumerate(ids)}
@@ -1475,3 +1477,7 @@ def filter_func(filter: Dict[str, Any]) -> Callable[[Dict[str, Any]], bool]:
             return lambda doc: all(condition(doc) for condition in conditions)
 
         return filter_func(filter)
+
+    def get_by_ids(self, ids: Sequence[str], /) -> list[Document]:
+        docs = [self.docstore.search(id_) for id_ in ids]
+        return [doc for doc in docs if isinstance(doc, Document)]