Harrison/faiss merge (#1843)

Co-authored-by: Ting Su <ting.su.1995@outlook.com>

docs/modules/indexes/vectorstore_examples/faiss.ipynb

@@ -62,10 +62,6 @@
      "name": "stdout",
      "output_type": "stream",
      "text": [
-      "In state after state, new laws have been passed, not only to suppress the vote, but to subvert entire elections. \n",
-      "\n",
-      "We cannot let this happen. \n",
-      "\n",
       "Tonight. I call on the Senate to: Pass the Freedom to Vote Act. Pass the John Lewis Voting Rights Act. And while you’re at it, pass the Disclose Act so Americans can know who is funding our elections. \n",
       "\n",
       "Tonight, I’d like to honor someone who has dedicated his life to serve this country: Justice Stephen Breyer—an Army veteran, Constitutional scholar, and retiring Justice of the United States Supreme Court. Justice Breyer, thank you for your service. \n",
@@ -200,10 +196,104 @@
     "docs[0]"
    ]
   },
+  {
+   "cell_type": "markdown",
+   "id": "57da60d4",
+   "metadata": {},
+   "source": [
+    "## Merging\n",
+    "You can also merge two FAISS vectorstores"
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": 6,
+   "id": "6dfd2b78",
+   "metadata": {},
+   "outputs": [],
+   "source": [
+    "db1 = FAISS.from_texts([\"foo\"], embeddings)\n",
+    "db2 = FAISS.from_texts([\"bar\"], embeddings)"
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": 8,
+   "id": "29960da7",
+   "metadata": {},
+   "outputs": [
+    {
+     "data": {
+      "text/plain": [
+       "{'e0b74348-6c93-4893-8764-943139ec1d17': Document(page_content='foo', lookup_str='', metadata={}, lookup_index=0)}"
+      ]
+     },
+     "execution_count": 8,
+     "metadata": {},
+     "output_type": "execute_result"
+    }
+   ],
+   "source": [
+    "db1.docstore._dict"
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": 9,
+   "id": "83392605",
+   "metadata": {},
+   "outputs": [
+    {
+     "data": {
+      "text/plain": [
+       "{'bdc50ae3-a1bb-4678-9260-1b0979578f40': Document(page_content='bar', lookup_str='', metadata={}, lookup_index=0)}"
+      ]
+     },
+     "execution_count": 9,
+     "metadata": {},
+     "output_type": "execute_result"
+    }
+   ],
+   "source": [
+    "db2.docstore._dict"
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": 10,
+   "id": "a3fcc1c7",
+   "metadata": {},
+   "outputs": [],
+   "source": [
+    "db1.merge_from(db2)"
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": 11,
+   "id": "41c51f89",
+   "metadata": {},
+   "outputs": [
+    {
+     "data": {
+      "text/plain": [
+       "{'e0b74348-6c93-4893-8764-943139ec1d17': Document(page_content='foo', lookup_str='', metadata={}, lookup_index=0),\n",
+       " 'd5211050-c777-493d-8825-4800e74cfdb6': Document(page_content='bar', lookup_str='', metadata={}, lookup_index=0)}"
+      ]
+     },
+     "execution_count": 11,
+     "metadata": {},
+     "output_type": "execute_result"
+    }
+   ],
+   "source": [
+    "db1.docstore._dict"
+   ]
+  },
   {
    "cell_type": "code",
    "execution_count": null,
-   "id": "bc8b71f7",
+   "id": "f80b60de",
    "metadata": {},
    "outputs": [],
    "source": []

langchain/vectorstores/faiss.py

@@ -221,6 +221,38 @@ class FAISS(VectorStore):
         docs = self.max_marginal_relevance_search_by_vector(embedding, k, fetch_k)
         return docs
 
+    def merge_from(self, target: FAISS) -> None:
+        """Merge another FAISS object with the current one.
+
+        Add the target FAISS to the current one.
+
+        Args:
+            target: FAISS object you wish to merge into the current one
+
+        Returns:
+            None.
+        """
+        if not isinstance(self.docstore, AddableMixin):
+            raise ValueError("Cannot merge with this type of docstore")
+        # Numerical index for target docs are incremental on existing ones
+        starting_len = len(self.index_to_docstore_id)
+
+        # Merge two IndexFlatL2
+        self.index.merge_from(target.index)
+
+        # Create new id for docs from target FAISS object
+        full_info = []
+        for i in target.index_to_docstore_id:
+            doc = target.docstore.search(target.index_to_docstore_id[i])
+            if not isinstance(doc, Document):
+                raise ValueError("Document should be returned")
+            full_info.append((starting_len + i, str(uuid.uuid4()), doc))
+
+        # Add information to docstore and index_to_docstore_id.
+        self.docstore.add({_id: doc for _, _id, doc in full_info})
+        index_to_id = {index: _id for index, _id, _ in full_info}
+        self.index_to_docstore_id.update(index_to_id)
+
     @classmethod
     def from_texts(
         cls,