
Name	WeaviateBM25Retriever
Source	https://github.com/deepset-ai/haystack-core-integrations/tree/main/integrations/weaviate
Most common position in a pipeline	1. Before a `PromptBuilder` in a RAG pipeline 2. The last component in the semantic search pipeline 3. Before an `ExtractiveReader` in an extractive QA pipeline
Mandatory input variables	“query”: A string
Output variables	"documents": A list of documents (matching the query)

Overview

WeaviateBM25Retriever is a keyword-based Retriever that fetches Documents matching a query from WeaviateDocumentStore. It determines the similarity between Documents and the query based on the BM25 algorithm, which computes a weighted word overlap between the
two strings.

Since the WeaviateBM25Retriever matches strings based on word overlap, it’s often used to find exact matches to names of persons or products, IDs, or well-defined error messages. The BM25 algorithm is very lightweight and simple. Beating it with more complex embedding-based approaches on out-of-domain data can be hard.

If you want a semantic match between a query and documents, use the WeaviateEmbeddingRetriever, which uses vectors created by embedding models to retrieve relevant information.

Parameters

In addition to the query, the WeaviateBM25Retriever accepts other optional parameters, including top_k (the maximum number of Documents to retrieve) and filters to narrow down the search space.

Usage

Installation

To start using Weaviate with Haystack, install the package with:

pip install weaviate-haystack

On its own

This Retriever needs an instance of WeaviateDocumentStore and indexed Documents to run.

from haystack_integrations.document_stores.weaviate.document_store import WeaviateDocumentStore
from haystack_integrations.components.retrievers.weaviate.bm25_retriever import WeaviateBM25Retriever

document_store = WeaviateDocumentStore(url="http://localhost:8080")

retriever = WeaviateBM25Retriever(document_store=document_store)

retriever.run(query="How to make a pizza", top_k=3)

In a Pipeline

from haystack_integrations.document_stores.weaviate.document_store import (
    WeaviateDocumentStore,
)
from haystack_integrations.components.retrievers.weaviate.bm25_retriever import (
    WeaviateBM25Retriever,
)

from haystack import Document
from haystack import Pipeline
from haystack.components.builders.answer_builder import AnswerBuilder
from haystack.components.builders.prompt_builder import PromptBuilder
from haystack.components.generators import OpenAIGenerator
from haystack.document_stores.types import DuplicatePolicy

# Create a RAG query pipeline
prompt_template = """
    Given these documents, answer the question.\nDocuments:
    {% for doc in documents %}
        {{ doc.content }}
    {% endfor %}

    \nQuestion: {{question}}
    \nAnswer:
    """

document_store = WeaviateDocumentStore(url="http://localhost:8080")

# Add Documents
documents = [
    Document(content="There are over 7,000 languages spoken around the world today."),
    Document(
        content="Elephants have been observed to behave in a way that indicates a high level of self-awareness, such as recognizing themselves in mirrors."
    ),
    Document(
        content="In certain parts of the world, like the Maldives, Puerto Rico, and San Diego, you can witness the phenomenon of bioluminescent waves."
    ),
]

# DuplicatePolicy.SKIP param is optional, but useful to run the script multiple times without throwing errors
document_store.write_documents(documents=documents, policy=DuplicatePolicy.SKIP)

rag_pipeline = Pipeline()
rag_pipeline.add_component(
    name="retriever", instance=WeaviateBM25Retriever(document_store=document_store)
)
rag_pipeline.add_component(
    instance=PromptBuilder(template=prompt_template), name="prompt_builder"
)
rag_pipeline.add_component(instance=OpenAIGenerator(), name="llm")
rag_pipeline.add_component(instance=AnswerBuilder(), name="answer_builder")
rag_pipeline.connect("retriever", "prompt_builder.documents")
rag_pipeline.connect("prompt_builder", "llm")
rag_pipeline.connect("llm.replies", "answer_builder.replies")
rag_pipeline.connect("llm.metadata", "answer_builder.metadata")
rag_pipeline.connect("retriever", "answer_builder.documents")

question = "How many languages are spoken around the world today?"
result = rag_pipeline.run(
    {
        "retriever": {"query": question},
        "prompt_builder": {"question": question},
        "answer_builder": {"query": question},
    }
)
print(result["answer_builder"]["answers"][0])