Weaviate: Add QnA with sources example (#5247)

# Add QnA with sources example 

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Fixes: see
https://stackoverflow.com/questions/76207160/langchain-doesnt-work-with-weaviate-vector-database-getting-valueerror/76210017#76210017

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@dev2049

docs/modules/indexes/vectorstores/examples/weaviate.ipynb

@@ -257,6 +257,101 @@
     "retriever = db.as_retriever(search_type=\"mmr\")\n",
     "retriever.get_relevant_documents(query)[0]"
    ]
+  },
+  {
+   "attachments": {},
+   "cell_type": "markdown",
+   "id": "fbd7a6cb",
+   "metadata": {},
+   "source": [
+    "## Question Answering with Sources"
+   ]
+  },
+  {
+   "attachments": {},
+   "cell_type": "markdown",
+   "id": "f349acb9",
+   "metadata": {},
+   "source": [
+    "This section goes over how to do question-answering with sources over an Index. It does this by using the `RetrievalQAWithSourcesChain`, which does the lookup of the documents from an Index. "
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": 12,
+   "id": "5e824f3b",
+   "metadata": {},
+   "outputs": [],
+   "source": [
+    "from langchain.chains import RetrievalQAWithSourcesChain\n",
+    "from langchain import OpenAI"
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": 13,
+   "id": "61209cc3",
+   "metadata": {},
+   "outputs": [],
+   "source": [
+    "with open(\"../../../state_of_the_union.txt\") as f:\n",
+    "    state_of_the_union = f.read()\n",
+    "text_splitter = CharacterTextSplitter(chunk_size=1000, chunk_overlap=0)\n",
+    "texts = text_splitter.split_text(state_of_the_union)"
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": 14,
+   "id": "4abc3d37",
+   "metadata": {},
+   "outputs": [],
+   "source": [
+    "docsearch = Weaviate.from_texts(\n",
+    "    texts,\n",
+    "    embeddings,\n",
+    "    weaviate_url=WEAVIATE_URL,\n",
+    "    by_text=False,\n",
+    "    metadatas=[{\"source\": f\"{i}-pl\"} for i in range(len(texts))],\n",
+    ")"
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": 15,
+   "id": "c7062393",
+   "metadata": {},
+   "outputs": [],
+   "source": [
+    "chain = RetrievalQAWithSourcesChain.from_chain_type(\n",
+    "    OpenAI(temperature=0), chain_type=\"stuff\", retriever=docsearch.as_retriever()\n",
+    ")"
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": 16,
+   "id": "7e41b773",
+   "metadata": {},
+   "outputs": [
+    {
+     "data": {
+      "text/plain": [
+       "{'answer': \" The president honored Justice Breyer for his service and mentioned his legacy of excellence. He also nominated Circuit Court of Appeals Judge Ketanji Brown Jackson to continue Justice Breyer's legacy.\\n\",\n",
+       " 'sources': '31-pl, 34-pl'}"
+      ]
+     },
+     "execution_count": 16,
+     "metadata": {},
+     "output_type": "execute_result"
+    }
+   ],
+   "source": [
+    "chain(\n",
+    "    {\"question\": \"What did the president say about Justice Breyer\"},\n",
+    "    return_only_outputs=True,\n",
+    ")"
+   ]
   }
  ],
  "metadata": {