langchain[patch]: Adds MMR to memory vector store (#6481)

* Adds MMR to memory vector store

* Adds memory MMR docs

docs/core_docs/docs/integrations/vectorstores/memory.ipynb

@@ -164,7 +164,7 @@
   },
   {
    "cell_type": "code",
-   "execution_count": 4,
+   "execution_count": 3,
    "id": "aa0a16fa",
    "metadata": {},
    "outputs": [
@@ -199,7 +199,7 @@
   },
   {
    "cell_type": "code",
-   "execution_count": 5,
+   "execution_count": 4,
    "id": "5efd2eaa",
    "metadata": {},
    "outputs": [
@@ -232,7 +232,7 @@
   },
   {
    "cell_type": "code",
-   "execution_count": 6,
+   "execution_count": 5,
    "id": "f3460093",
    "metadata": {},
    "outputs": [
@@ -265,6 +265,55 @@
     "await retriever.invoke(\"biology\");"
    ]
   },
+  {
+   "cell_type": "markdown",
+   "id": "423d779a",
+   "metadata": {},
+   "source": [
+    "### Maximal marginal relevance\n",
+    "\n",
+    "This vector store also supports maximal marginal relevance (MMR), a technique that first fetches a larger number of results (given by `searchKwargs.fetchK`), with classic similarity search, then reranks for diversity and returns the top `k` results. This helps guard against redundant information:"
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": 6,
+   "id": "56817a1c",
+   "metadata": {},
+   "outputs": [
+    {
+     "name": "stdout",
+     "output_type": "stream",
+     "text": [
+      "[\n",
+      "  Document {\n",
+      "    pageContent: 'The powerhouse of the cell is the mitochondria',\n",
+      "    metadata: { source: 'https://example.com' },\n",
+      "    id: undefined\n",
+      "  },\n",
+      "  Document {\n",
+      "    pageContent: 'Buildings are made out of brick',\n",
+      "    metadata: { source: 'https://example.com' },\n",
+      "    id: undefined\n",
+      "  }\n",
+      "]\n"
+     ]
+    }
+   ],
+   "source": [
+    "const mmrRetriever = vectorStore.asRetriever({\n",
+    "  searchType: \"mmr\",\n",
+    "  searchKwargs: {\n",
+    "    fetchK: 10,\n",
+    "  },\n",
+    "  // Optional filter\n",
+    "  filter: filter,\n",
+    "  k: 2,\n",
+    "});\n",
+    "\n",
+    "await mmrRetriever.invoke(\"biology\");"
+   ]
+  },
   {
    "cell_type": "markdown",
    "id": "e2e0a211",

langchain/src/vectorstores/memory.ts

@@ -1,7 +1,11 @@
-import { VectorStore } from "@langchain/core/vectorstores";
+import {
+  MaxMarginalRelevanceSearchOptions,
+  VectorStore,
+} from "@langchain/core/vectorstores";
 import type { EmbeddingsInterface } from "@langchain/core/embeddings";
-import { Document } from "@langchain/core/documents";
+import { Document, DocumentInterface } from "@langchain/core/documents";
 import { cosine } from "../util/ml-distance/similarities.js";
+import { maximalMarginalRelevance } from "../util/math.js";
 
 /**
  * Interface representing a vector in memory. It includes the content
@@ -82,21 +86,11 @@ export class MemoryVectorStore extends VectorStore {
     this.memoryVectors = this.memoryVectors.concat(memoryVectors);
   }
 
-  /**
-   * Method to perform a similarity search in the memory vector store. It
-   * calculates the similarity between the query vector and each vector in
-   * the store, sorts the results by similarity, and returns the top `k`
-   * results along with their scores.
-   * @param query Query vector to compare against the vectors in the store.
-   * @param k Number of top results to return.
-   * @param filter Optional filter function to apply to the vectors before performing the search.
-   * @returns Promise that resolves with an array of tuples, each containing a `Document` and its similarity score.
-   */
-  async similaritySearchVectorWithScore(
+  protected async _queryVectors(
     query: number[],
     k: number,
     filter?: this["FilterType"]
-  ): Promise<[Document, number][]> {
+  ) {
     const filterFunction = (memoryVector: MemoryVector) => {
       if (!filter) {
         return true;
@@ -109,25 +103,75 @@ export class MemoryVectorStore extends VectorStore {
       return filter(doc);
     };
     const filteredMemoryVectors = this.memoryVectors.filter(filterFunction);
-    const searches = filteredMemoryVectors
+    return filteredMemoryVectors
       .map((vector, index) => ({
         similarity: this.similarity(query, vector.embedding),
         index,
+        metadata: vector.metadata,
+        content: vector.content,
+        embedding: vector.embedding,
       }))
       .sort((a, b) => (a.similarity > b.similarity ? -1 : 0))
       .slice(0, k);
+  }
 
+  /**
+   * Method to perform a similarity search in the memory vector store. It
+   * calculates the similarity between the query vector and each vector in
+   * the store, sorts the results by similarity, and returns the top `k`
+   * results along with their scores.
+   * @param query Query vector to compare against the vectors in the store.
+   * @param k Number of top results to return.
+   * @param filter Optional filter function to apply to the vectors before performing the search.
+   * @returns Promise that resolves with an array of tuples, each containing a `Document` and its similarity score.
+   */
+  async similaritySearchVectorWithScore(
+    query: number[],
+    k: number,
+    filter?: this["FilterType"]
+  ): Promise<[Document, number][]> {
+    const searches = await this._queryVectors(query, k, filter);
     const result: [Document, number][] = searches.map((search) => [
       new Document({
-        metadata: filteredMemoryVectors[search.index].metadata,
-        pageContent: filteredMemoryVectors[search.index].content,
+        metadata: search.metadata,
+        pageContent: search.content,
       }),
       search.similarity,
     ]);
 
     return result;
   }
 
+  async maxMarginalRelevanceSearch(
+    query: string,
+    options: MaxMarginalRelevanceSearchOptions<this["FilterType"]>
+  ): Promise<DocumentInterface[]> {
+    const queryEmbedding = await this.embeddings.embedQuery(query);
+
+    const searches = await this._queryVectors(
+      queryEmbedding,
+      options.fetchK ?? 20,
+      options.filter
+    );
+
+    const embeddingList = searches.map((searchResp) => searchResp.embedding);
+
+    const mmrIndexes = maximalMarginalRelevance(
+      queryEmbedding,
+      embeddingList,
+      options.lambda,
+      options.k
+    );
+
+    return mmrIndexes.map(
+      (idx) =>
+        new Document({
+          metadata: searches[idx].metadata,
+          pageContent: searches[idx].content,
+        })
+    );
+  }
+
   /**
    * Static method to create a `MemoryVectorStore` instance from an array of
    * texts. It creates a `Document` for each text and metadata pair, and

langchain/src/vectorstores/tests/memory.test.ts

@@ -97,3 +97,33 @@ test("MemoryVectorStore with custom similarity", async () => {
   expect(similarityCalledCount).toBe(4);
   expect(results).toHaveLength(3);
 });
+
+test("MemoryVectorStore with max marginal relevance", async () => {
+  const embeddings = new SyntheticEmbeddings({
+    vectorSize: 1536,
+  });
+  let similarityCalled = false;
+  let similarityCalledCount = 0;
+  const store = new MemoryVectorStore(embeddings, {
+    similarity: (a: number[], b: number[]) => {
+      similarityCalledCount += 1;
+      similarityCalled = true;
+      return cosine(a, b);
+    },
+  });
+
+  expect(store).toBeDefined();
+
+  await store.addDocuments([
+    { pageContent: "hello", metadata: { a: 1 } },
+    { pageContent: "hi", metadata: { a: 1 } },
+    { pageContent: "bye", metadata: { a: 1 } },
+    { pageContent: "what's this", metadata: { a: 1 } },
+  ]);
+
+  const results = await store.maxMarginalRelevanceSearch("hello", { k: 3 });
+
+  expect(similarityCalled).toBe(true);
+  expect(similarityCalledCount).toBe(4);
+  expect(results).toHaveLength(3);
+});