
Name	LostInTheMiddleRanker
Folder path	/rankers/
Most common position in a pipeline	In a query pipeline, after a component that returns a list of documents (such as a Retriever)
Mandatory input variables	“documents”: A list of documents
Output variables	“documents”: A list of documents

Overview

The LostInTheMiddleRanker reorders the documents based on the "Lost in the Middle" order, described in the "Lost in the Middle: How Language Models Use Long Contexts" research paper. It aims to lay out paragraphs into LLM context so that the relevant paragraphs are at the beginning or end of the input context, while the least relevant information is in the middle of the context. This reordering is helpful when very long contexts are sent to an LLM, as current models pay more attention to the start and end of long input contexts.

In contrast to other rankers, LostInTheMiddleRanker assumes that the input documents are already sorted by relevance, and it doesn’t require a query as input. It is typically used as the last component before building a prompt for an LLM to prepare the input context for the LLM.

Parameters

If you specify the word_count_threshold when running the component, the Ranker includes all documents up until the point where adding another document would exceed the given threshold. The last document that exceeds the threshold will be included in the resulting list of Documents, but all following documents will be discarded.

You can also specify the top_k parameter to set the maximum number of documents to return.

Usage

On its own

from haystack import Document
from haystack.components.rankers import LostInTheMiddleRanker

ranker = LostInTheMiddleRanker()
docs = [Document(content="Paris"), 
		Document(content="Berlin"), 
		Document(content="Madrid")]
result = ranker.run(documents=docs)

for doc in result["documents"]:
    print(doc.content)

In a pipeline

Note that this example requires an OpenAI key to run.

from haystack import Document, Pipeline
from haystack.document_stores.in_memory import InMemoryDocumentStore
from haystack.components.retrievers.in_memory import InMemoryBM25Retriever
from haystack.components.rankers import LostInTheMiddleRanker
from haystack.components.generators import OpenAIGenerator
from haystack.components.builders.prompt_builder import PromptBuilder

prompt_template = """
    Given these documents, answer the question.\nDocuments:
    {% for doc in documents %}
        {{ doc.content }}
    {% endfor %}

    \nQuestion: {{query}}
    \nAnswer:
    """

docs = [Document(content="Paris is in France..."), 
        Document(content="Berlin is in Germany..."),
        Document(content="Lyon is in France...")]
document_store = InMemoryDocumentStore()
document_store.write_documents(docs)

retriever = InMemoryBM25Retriever(document_store = document_store)
ranker = LostInTheMiddleRanker(word_count_threshold=1024)
builder = PromptBuilder(template=prompt_template)
generator = OpenAIGenerator()

p = Pipeline()
p.add_component(instance=retriever, name="retriever")
p.add_component(instance=ranker, name="ranker")
p.add_component(instance=builder, name="prompt_builder")
p.add_component(instance=generator, name="llm")

p.connect("retriever.documents", "ranker.documents")
p.connect("ranker.documents", "prompt_builder.documents")
p.connect("prompt_builder", "llm")

p.run({"retriever": {"query": "What cities are in France?", "top_k": 3},
       "prompt_builder":{"query": "What cities are in France?"}})