---
title: "MetaFieldGroupingRanker"
id: metafieldgroupingranker
slug: "/metafieldgroupingranker"
description: "Reorder the documents by grouping them based on metadata keys."
---

# MetaFieldGroupingRanker

Reorder the documents by grouping them based on metadata keys.

<div className="key-value-table">

|  |  |
| --- | --- |
| **Most common position in a pipeline** | In a query pipeline, after a component that returns a list of documents, such as a [Retriever](../retrievers.mdx) |
| **Mandatory init variables**           | `group_by`: The name of the meta field to group by                                                               |
| **Mandatory run variables**            | `documents`: A list of documents to group                                                                        |
| **Output variables**                   | `documents`: A grouped list of documents                                                                         |
| **API reference**                      | [Rankers](/reference/rankers-api)                                                                                       |
| **GitHub link**                        | https://github.com/deepset-ai/haystack/blob/main/haystack/components/rankers/meta_field_grouping_ranker.py     |

</div>

## Overview

The `MetaFieldGroupingRanker` component groups documents by a primary metadata key `group_by`, and subgroups them with an optional secondary key, `subgroup_by`.
Within each group or subgroup, the component can also sort documents by a metadata key `sort_docs_by`.

The output is a flat list of documents ordered by `group_by` and `subgroup_by` values. Any documents without a group are placed at the end of the list.

The component helps improve the efficiency and performance of subsequent processing by an LLM.

## Usage

### On its own

```python
from haystack.components.rankers import MetaFieldGroupingRanker
from haystack import Document

docs = [
    Document(content="JavaScript is popular", meta={"group": "42", "split_id": 7, "subgroup": "subB"}),
    Document(content="Python is popular", meta={"group": "42", "split_id": 4, "subgroup": "subB"}),
    Document(content="A chromosome is DNA", meta={"group": "314", "split_id": 2, "subgroup": "subC"}),
    Document(content="An octopus has three hearts", meta={"group": "11", "split_id": 2, "subgroup": "subD"}),
    Document(content="Java is popular", meta={"group": "42", "split_id": 3, "subgroup": "subB"}),
]

ranker = MetaFieldGroupingRanker(group_by="group", subgroup_by="subgroup", sort_docs_by="split_id")
result = ranker.run(documents=docs)
print(result["documents"])

```

### In a pipeline

The following pipeline uses the `MetaFieldGroupingRanker` to organize documents by certain meta fields while sorting by page number, then formats these organized documents into a chat message which is passed to the `OpenAIChatGenerator` to create a structured explanation of the content.

```python
from haystack import Pipeline
from haystack.components.generators.chat import OpenAIChatGenerator
from haystack.components.rankers import MetaFieldGroupingRanker
from haystack.dataclasses import Document, ChatMessage

docs = [
    Document(
        content="Chapter 1: Introduction to Python",
        meta={"chapter": "1", "section": "intro", "page": 1}
    ),
    Document(
        content="Chapter 2: Basic Data Types",
        meta={"chapter": "2", "section": "basics", "page": 15}
    ),
    Document(
        content="Chapter 1: Python Installation",
        meta={"chapter": "1", "section": "setup", "page": 5}
    ),
]

ranker = MetaFieldGroupingRanker(
    group_by="chapter",
    subgroup_by="section",
    sort_docs_by="page"
)

chat_generator = OpenAIChatGenerator(
    generation_kwargs={
        "temperature": 0.7,
        "max_tokens": 500
    }
)

## First run the ranker
ranked_result = ranker.run(documents=docs)
ranked_docs = ranked_result["documents"]

## Create chat messages with the ranked documents
messages = [
    ChatMessage.from_system("You are a helpful programming tutor."),
    ChatMessage.from_user(
        f"Here are the course documents in order:\n" +
        "\n".join([f"- {doc.content}" for doc in ranked_docs]) +
        "\n\nBased on these documents, explain the structure of this Python course."
    )
]

## Create and run pipeline for just the chat generator
pipeline = Pipeline()
pipeline.add_component("chat_generator", chat_generator)

result = pipeline.run(
    data={
        "chat_generator": {
            "messages": messages
        }
    }
)

print(result["chat_generator"]["replies"][0])
```
