BM25

BM25 (Wikipedia) also known as the Okapi BM25, is a ranking function used in information retrieval systems to estimate the relevance of documents to a given search query.

BM25Retriever retriever uses the rank_bm25 package.

%pip install --upgrade --quiet  rank_bm25

from langchain_community.retrievers import BM25Retriever

API Reference:BM25Retriever

Create New Retriever with Texts

retriever = BM25Retriever.from_texts(["foo", "bar", "world", "hello", "foo bar"])

Create a New Retriever with Documents

You can now create a new retriever with the documents you created.

from langchain_core.documents import Document

retriever = BM25Retriever.from_documents(
    [
        Document(page_content="foo"),
        Document(page_content="bar"),
        Document(page_content="world"),
        Document(page_content="hello"),
        Document(page_content="foo bar"),
    ]
)

API Reference:Document

Use Retriever

We can now use the retriever!

result = retriever.invoke("foo")

result

[Document(page_content='foo', metadata={}),
 Document(page_content='foo bar', metadata={}),
 Document(page_content='hello', metadata={}),
 Document(page_content='world', metadata={})]

Related

• Retriever conceptual guide
• Retriever how-to guides