OpenSearch: Add Similarity Search with Score (#4089)

### Description
Add `similarity_search_with_score` method for OpenSearch to return
scores along with documents in the search results

Signed-off-by: Naveen Tatikonda <navtat@amazon.com>

langchain/vectorstores/opensearch_vector_search.py

@@ -2,7 +2,7 @@
 from __future__ import annotations
 
 import uuid
-from typing import Any, Dict, Iterable, List, Optional
+from typing import Any, Dict, Iterable, List, Optional, Tuple
 
 from langchain.docstore.document import Document
 from langchain.embeddings.base import Embeddings
@@ -410,6 +410,27 @@ class OpenSearchVectorSearch(VectorStore):
             pre_filter: script_score query to pre-filter documents before identifying
             nearest neighbors; default: {"match_all": {}}
         """
+        docs_with_scores = self.similarity_search_with_score(query, k, **kwargs)
+        return [doc[0] for doc in docs_with_scores]
+
+    def similarity_search_with_score(
+        self, query: str, k: int = 4, **kwargs: Any
+    ) -> List[Tuple[Document, float]]:
+        """Return docs and it's scores most similar to query.
+
+        By default supports Approximate Search.
+        Also supports Script Scoring and Painless Scripting.
+
+        Args:
+            query: Text to look up documents similar to.
+            k: Number of Documents to return. Defaults to 4.
+
+        Returns:
+            List of Documents along with its scores most similar to the query.
+
+        Optional Args:
+            same as `similarity_search`
+        """
         embedding = self.embedding_function.embed_query(query)
         search_type = _get_kwargs_value(kwargs, "search_type", "approximate_search")
         text_field = _get_kwargs_value(kwargs, "text_field", "text")
@@ -454,17 +475,20 @@ class OpenSearchVectorSearch(VectorStore):
             raise ValueError("Invalid `search_type` provided as an argument")
 
         response = self.client.search(index=self.index_name, body=search_query)
-        hits = [hit["_source"] for hit in response["hits"]["hits"][:k]]
-        documents = [
-            Document(
-                page_content=hit[text_field],
-                metadata=hit
-                if metadata_field == "*" or metadata_field not in hit
-                else hit[metadata_field],
+        hits = [hit for hit in response["hits"]["hits"][:k]]
+        documents_with_scores = [
+            (
+                Document(
+                    page_content=hit["_source"][text_field],
+                    metadata=hit["_source"]
+                    if metadata_field == "*" or metadata_field not in hit["_source"]
+                    else hit["_source"][metadata_field],
+                ),
+                hit["_score"],
             )
             for hit in hits
         ]
-        return documents
+        return documents_with_scores
 
     @classmethod
     def from_texts(

tests/integration_tests/vectorstores/test_opensearch.py

@@ -23,6 +23,22 @@ def test_opensearch() -> None:
     assert output == [Document(page_content="foo")]
 
 
+def test_similarity_search_with_score() -> None:
+    """Test similarity search with score using Approximate Search."""
+    metadatas = [{"page": i} for i in range(len(texts))]
+    docsearch = OpenSearchVectorSearch.from_texts(
+        texts,
+        FakeEmbeddings(),
+        metadatas=metadatas,
+        opensearch_url=DEFAULT_OPENSEARCH_URL,
+    )
+    output = docsearch.similarity_search_with_score("foo", k=2)
+    assert output == [
+        (Document(page_content="foo", metadata={"page": 0}), 1.0),
+        (Document(page_content="bar", metadata={"page": 1}), 0.5),
+    ]
+
+
 def test_opensearch_with_custom_field_name() -> None:
     """Test indexing and search using custom vector field and text field name."""
     docsearch = OpenSearchVectorSearch.from_texts(
@@ -178,7 +194,7 @@ def test_appx_search_with_lucene_filter() -> None:
 
 def test_opensearch_with_custom_field_name_appx_true() -> None:
     """Test Approximate Search with custom field name appx true."""
-    text_input = ["test", "add", "text", "method"]
+    text_input = ["add", "test", "text", "method"]
     docsearch = OpenSearchVectorSearch.from_texts(
         text_input,
         FakeEmbeddings(),
@@ -191,7 +207,7 @@ def test_opensearch_with_custom_field_name_appx_true() -> None:
 
 def test_opensearch_with_custom_field_name_appx_false() -> None:
     """Test Approximate Search with custom field name appx true."""
-    text_input = ["test", "add", "text", "method"]
+    text_input = ["add", "test", "text", "method"]
     docsearch = OpenSearchVectorSearch.from_texts(
         text_input, FakeEmbeddings(), opensearch_url=DEFAULT_OPENSEARCH_URL
     )