fix!: InMemoryBM25Retriever no longer returns documents that have a score of 0.0 (#6717)

* fix!: `InMemoryBM25Retriever` no longer returns documents that have a score of 0.0

Also update tests to accommodate the new behavior.

* Remove superfluous code

haystack/document_stores/in_memory/document_store.py

@@ -212,8 +212,11 @@ class InMemoryDocumentStore:
         return_documents = []
         for i in top_docs_positions:
             doc = all_documents[i]
+            score = docs_scores[i]
+            if score <= 0.0:
+                continue
             doc_fields = doc.to_dict()
-            doc_fields["score"] = docs_scores[i]
+            doc_fields["score"] = score
             return_document = Document.from_dict(doc_fields)
             return_documents.append(return_document)
         return return_documents

releasenotes/notes/inmemorybm25retriever-zero-score-docs-67406062a76aa7f4.yaml

@@ -0,0 +1,3 @@
+---
+fixes:
+  - Prevent InMemoryBM25Retriever from returning documents with a score of 0.0.

test/components/retrievers/test_in_memory_bm25_retriever.py

@@ -113,15 +113,14 @@ class TestMemoryBM25Retriever:
             InMemoryBM25Retriever.from_dict(data)
 
     def test_retriever_valid_run(self, mock_docs):
-        top_k = 5
         ds = InMemoryDocumentStore()
         ds.write_documents(mock_docs)
 
-        retriever = InMemoryBM25Retriever(ds, top_k=top_k)
+        retriever = InMemoryBM25Retriever(ds, top_k=5)
         result = retriever.run(query="PHP")
 
         assert "documents" in result
-        assert len(result["documents"]) == top_k
+        assert len(result["documents"]) == 1
         assert result["documents"][0].content == "PHP is a popular programming language"
 
     def test_invalid_run_wrong_store_type(self):
@@ -174,5 +173,5 @@ class TestMemoryBM25Retriever:
         assert "retriever" in result
         results_docs = result["retriever"]["documents"]
         assert results_docs
-        assert len(results_docs) == top_k
+        assert len(results_docs) == 1
         assert results_docs[0].content == query_result

test/document_stores/test_in_memory.py

@@ -5,8 +5,8 @@ import pandas as pd
 import pytest
 
 from haystack import Document
-from haystack.document_stores.in_memory import InMemoryDocumentStore
 from haystack.document_stores.errors import DocumentStoreError, DuplicateDocumentError
+from haystack.document_stores.in_memory import InMemoryDocumentStore
 from haystack.testing.document_store import DocumentStoreBaseTests
 
 
@@ -17,7 +17,7 @@ class TestMemoryDocumentStore(DocumentStoreBaseTests):  # pylint: disable=R0904
 
     @pytest.fixture
     def document_store(self) -> InMemoryDocumentStore:
-        return InMemoryDocumentStore()
+        return InMemoryDocumentStore(bm25_algorithm="BM25L")
 
     def test_to_dict(self):
         store = InMemoryDocumentStore()
@@ -73,7 +73,6 @@ class TestMemoryDocumentStore(DocumentStoreBaseTests):  # pylint: disable=R0904
             document_store.write_documents(docs)
 
     def test_bm25_retrieval(self, document_store: InMemoryDocumentStore):
-        document_store = InMemoryDocumentStore()
         # Tests if the bm25_retrieval method returns the correct document based on the input query.
         docs = [Document(content="Hello world"), Document(content="Haystack supports multiple languages")]
         document_store.write_documents(docs)
@@ -106,7 +105,7 @@ class TestMemoryDocumentStore(DocumentStoreBaseTests):  # pylint: disable=R0904
         document_store.write_documents(docs)
 
         # top_k = 2
-        results = document_store.bm25_retrieval(query="languages", top_k=2)
+        results = document_store.bm25_retrieval(query="language", top_k=2)
         assert len(results) == 2
 
         # top_k = 3
@@ -141,7 +140,7 @@ class TestMemoryDocumentStore(DocumentStoreBaseTests):  # pylint: disable=R0904
         document_store.write_documents(docs)
 
         results = document_store.bm25_retrieval(query="Python", top_k=1)
-        assert len(results) == 1
+        assert len(results) == 0
 
         document_store.write_documents([Document(content="Python is a popular programming language")])
         results = document_store.bm25_retrieval(query="Python", top_k=1)
@@ -199,10 +198,10 @@ class TestMemoryDocumentStore(DocumentStoreBaseTests):  # pylint: disable=R0904
         docs = [Document(), Document(content="Gardening"), Document(content="Bird watching")]
         document_store.write_documents(docs)
         results = document_store.bm25_retrieval(query="doesn't matter, top_k is 10", top_k=10)
-        assert len(results) == 2
+        assert len(results) == 0
 
     def test_bm25_retrieval_with_filters(self, document_store: InMemoryDocumentStore):
-        selected_document = Document(content="Gardening", meta={"selected": True})
+        selected_document = Document(content="Java is, well...", meta={"selected": True})
         docs = [Document(), selected_document, Document(content="Bird watching")]
         document_store.write_documents(docs)
         results = document_store.bm25_retrieval(query="Java", top_k=10, filters={"selected": True})
@@ -224,10 +223,10 @@ class TestMemoryDocumentStore(DocumentStoreBaseTests):  # pylint: disable=R0904
         assert results[0].id == document.id
 
     def test_bm25_retrieval_with_documents_with_mixed_content(self, document_store: InMemoryDocumentStore):
-        double_document = Document(content="Gardening", embedding=[1.0, 2.0, 3.0])
+        double_document = Document(content="Gardening is a hobby", embedding=[1.0, 2.0, 3.0])
         docs = [Document(embedding=[1.0, 2.0, 3.0]), double_document, Document(content="Bird watching")]
         document_store.write_documents(docs)
-        results = document_store.bm25_retrieval(query="Java", top_k=10, filters={"embedding": {"$not": None}})
+        results = document_store.bm25_retrieval(query="Gardening", top_k=10, filters={"embedding": {"$not": None}})
         assert len(results) == 1
         assert results[0].id == double_document.id