Enable ElasticsearchStore to retrieve with the pure BM25 algorithm wi…

…thout vector search

libs/elasticsearch/langchain_elasticsearch/vectorstores.py

@@ -70,6 +70,7 @@ def index(
         self,
         dims_length: Union[int, None],
         vector_query_field: str,
+        text_field: str,
         similarity: Union[DistanceStrategy, None],
     ) -> Dict:
         """
@@ -80,6 +81,7 @@ def index(
                         or None if not using vector-based query.
             vector_query_field: The field containing the vector
                                 representations in the index.
+            text_field: The field containing the text data in the index.
             similarity: The similarity strategy to use,
                         or None if not using one.
 
@@ -210,6 +212,7 @@ def index(
         self,
         dims_length: Union[int, None],
         vector_query_field: str,
+        text_field: str,
         similarity: Union[DistanceStrategy, None],
     ) -> Dict:
         """Create the mapping for the Elasticsearch index."""
@@ -289,6 +292,7 @@ def index(
         self,
         dims_length: Union[int, None],
         vector_query_field: str,
+        text_field: str,
         similarity: Union[DistanceStrategy, None],
     ) -> Dict:
         """Create the mapping for the Elasticsearch index."""
@@ -372,6 +376,7 @@ def index(
         self,
         dims_length: Union[int, None],
         vector_query_field: str,
+        text_field: str,
         similarity: Union[DistanceStrategy, None],
     ) -> Dict:
         return {
@@ -389,6 +394,76 @@ def require_inference(self) -> bool:
         return False
 
 
+class BM25RetrievalStrategy(BaseRetrievalStrategy):
+    """Retrieval strategy using the native BM25 algorithm of Elasticsearch."""
+
+    def __init__(self, k1: Union[float, None] = None, b: Union[float, None] = None):
+        self.k1 = k1
+        self.b = b
+
+    def query(
+        self,
+        query_vector: Union[List[float], None],
+        query: Union[str, None],
+        k: int,
+        fetch_k: int,
+        vector_query_field: str,
+        text_field: str,
+        filter: List[dict],
+        similarity: Union[DistanceStrategy, None],
+    ) -> Dict:
+        return {
+            "query": {
+                "bool": {
+                    "must": [
+                        {
+                            "match": {
+                                text_field: {
+                                    "query": query,
+                                }
+                            },
+                        },
+                    ],
+                    "filter": filter,
+                },
+            },
+        }
+
+    def index(
+        self,
+        dims_length: Union[int, None],
+        vector_query_field: str,
+        text_field: str,
+        similarity: Union[DistanceStrategy, None],
+    ) -> Dict:
+        mappings: Dict = {
+            "properties": {
+                text_field: {
+                    "type": "text",
+                    "similarity": "custom_bm25",
+                },
+            },
+        }
+        settings: Dict = {
+            "similarity": {
+                "custom_bm25": {
+                    "type": "BM25",
+                },
+            },
+        }
+
+        if self.k1 is not None:
+            settings["similarity"]["custom_bm25"]["k1"] = self.k1
+
+        if self.b is not None:
+            settings["similarity"]["custom_bm25"]["b"] = self.b
+
+        return {"mappings": mappings, "settings": settings}
+
+    def require_inference(self) -> bool:
+        return False
+
+
 class ElasticsearchStore(VectorStore):
     """`Elasticsearch` vector store.
 
@@ -905,6 +980,7 @@ def _create_index_if_not_exists(
 
             indexSettings = self.strategy.index(
                 vector_query_field=self.vector_query_field,
+                text_field=self.query_field,
                 dims_length=dims_length,
                 similarity=self.distance_strategy,
             )
@@ -1284,3 +1360,17 @@ def SparseVectorRetrievalStrategy(
                     deployed to Elasticsearch.
         """
         return SparseRetrievalStrategy(model_id=model_id)
+
+    @staticmethod
+    def BM25RetrievalStrategy(
+        k1: Union[float, None] = None, b: Union[float, None] = None
+    ) -> "BM25RetrievalStrategy":
+        """Used to apply BM25 without vector search.
+
+        Args:
+            k1: Optional. This corresponds to the BM25 parameter, k1. Default is None,
+                which uses the default setting of Elasticsearch.
+            b: Optional. This corresponds to the BM25 parameter, b. Default is None,
+               which uses the default setting of Elasticsearch.
+        """
+        return BM25RetrievalStrategy(k1=k1, b=b)

libs/elasticsearch/tests/integration_tests/test_vectorstores.py

@@ -777,6 +777,67 @@ def test_elasticsearch_with_relevance_score(
         )
         assert output == [(Document(page_content="foo", metadata={"page": "0"}), 1.0)]
 
+    def test_similarity_search_bm25_search(
+        self, elasticsearch_connection: dict, index_name: str
+    ) -> None:
+        """Test end to end using the BM25 retrieval strategy."""
+        texts = ["foo", "bar", "baz"]
+        docsearch = ElasticsearchStore.from_texts(
+            texts,
+            None,
+            **elasticsearch_connection,
+            index_name=index_name,
+            strategy=ElasticsearchStore.BM25RetrievalStrategy(),
+        )
+
+        def assert_query(query_body: dict, query: str) -> dict:
+            assert query_body == {
+                "query": {
+                    "bool": {
+                        "must": [{"match": {"text": {"query": "foo"}}}],
+                        "filter": [],
+                    }
+                }
+            }
+            return query_body
+
+        output = docsearch.similarity_search("foo", k=1, custom_query=assert_query)
+        assert output == [Document(page_content="foo")]
+
+    def test_similarity_search_bm25_search_with_filter(
+        self, elasticsearch_connection: dict, index_name: str
+    ) -> None:
+        """Test end to using the BM25 retrieval strategy with metadata."""
+        texts = ["foo", "foo", "foo"]
+        metadatas = [{"page": i} for i in range(len(texts))]
+        docsearch = ElasticsearchStore.from_texts(
+            texts,
+            None,
+            **elasticsearch_connection,
+            index_name=index_name,
+            metadatas=metadatas,
+            strategy=ElasticsearchStore.BM25RetrievalStrategy(),
+        )
+
+        def assert_query(query_body: dict, query: str) -> dict:
+            assert query_body == {
+                "query": {
+                    "bool": {
+                        "must": [{"match": {"text": {"query": "foo"}}}],
+                        "filter": [{"term": {"metadata.page": 1}}],
+                    }
+                }
+            }
+            return query_body
+
+        output = docsearch.similarity_search(
+            "foo",
+            k=3,
+            custom_query=assert_query,
+            filter=[{"term": {"metadata.page": 1}}],
+        )
+        assert output == [Document(page_content="foo", metadata={"page": 1})]
+
     def test_elasticsearch_with_relevance_threshold(
         self, elasticsearch_connection: dict, index_name: str
     ) -> None: