Add mmr to neo4j vector (#25765)

libs/community/tests/integration_tests/vectorstores/test_neo4jvector.py

@@ -14,7 +14,10 @@ from langchain_community.vectorstores.neo4j_vector import (
     _get_search_index_query,
 )
 from langchain_community.vectorstores.utils import DistanceStrategy
-from tests.integration_tests.vectorstores.fake_embeddings import FakeEmbeddings
+from tests.integration_tests.vectorstores.fake_embeddings import (
+    AngularTwoDimensionalEmbeddings,
+    FakeEmbeddings,
+)
 from tests.integration_tests.vectorstores.fixtures.filtering_test_cases import (
     DOCUMENTS,
     TYPE_1_FILTERING_TEST_CASES,
@@ -928,6 +931,45 @@ OPTIONS {indexConfig: {
     drop_vector_indexes(docsearch)
 
 
+def test_neo4j_max_marginal_relevance_search() -> None:
+    """
+    Test end to end construction and MMR search.
+    The embedding function used here ensures `texts` become
+    the following vectors on a circle (numbered v0 through v3):
+
+           ______ v2
+          /      \
+         /        |  v1
+    v3  |     .    | query
+         |        /  v0
+          |______/                 (N.B. very crude drawing)
+
+    With fetch_k==3 and k==2, when query is at (1, ),
+    one expects that v2 and v0 are returned (in some order).
+    """
+    texts = ["-0.124", "+0.127", "+0.25", "+1.0"]
+    metadatas = [{"page": i} for i in range(len(texts))]
+    docsearch = Neo4jVector.from_texts(
+        texts,
+        metadatas=metadatas,
+        embedding=AngularTwoDimensionalEmbeddings(),
+        pre_delete_collection=True,
+    )
+
+    expected_set = {
+        ("+0.25", 2),
+        ("-0.124", 0),
+    }
+
+    output = docsearch.max_marginal_relevance_search("0.0", k=2, fetch_k=3)
+    output_set = {
+        (mmr_doc.page_content, mmr_doc.metadata["page"]) for mmr_doc in output
+    }
+    assert output_set == expected_set
+
+    drop_vector_indexes(docsearch)
+
+
 def test_neo4jvector_passing_graph_object() -> None:
     """Test end to end construction and search with passing graph object."""
     graph = Neo4jGraph()