AnswerJoiner

Use AnswerJoiner to merge multiple lists of Answer objects from different pipeline branches into a single list.

Key Features

• Merges multiple Answer lists from different pipeline branches into one.
• Supports concatenation as the join mode.
• Optionally limits the number of answers returned with top_k.
• Optionally sorts answers by score in descending order.

Configuration

1. Drag the AnswerJoiner component onto the canvas from the Component Library.
2. Click on the component to open the configuration panel.
3. On the General tab:
   • Set the Join Mode. The available mode is concatenate, which merges all input answer lists into a single list.
   • Optionally, set Top K to limit the number of answers returned.
4. Go to the Advanced tab to enable Sort by Score if you want answers sorted in descending order by their score.

Connections

AnswerJoiner accepts lists of Answer objects from multiple components simultaneously. You typically connect it to AnswerBuilder components that produce answers from different generators. Its output is a merged List[AnswerType] that you connect to the Output component.

Source Code

To check this component's source code, open answer_joiner.py in the Haystack repository.

Usage Examples

Basic Configuration

  AnswerJoiner:
    type: haystack.components.joiners.answer_joiner.AnswerJoiner
    init_parameters:
      join_mode: concatenate
      sort_by_score: false

This example shows a pipeline that generates answers from multiple sources and joins them.

# haystack-pipeline
components:
  PromptBuilder1:
    type: haystack.components.builders.prompt_builder.PromptBuilder
    init_parameters:
      template: "Answer this question briefly: {{ query }}"
  PromptBuilder2:
    type: haystack.components.builders.prompt_builder.PromptBuilder
    init_parameters:
      template: "Provide a detailed answer to: {{ query }}"
  Generator1:
    type: haystack.components.generators.chat.openai.OpenAIChatGenerator
    init_parameters:
      api_key:
        type: env_var
        env_vars:
        - OPENAI_API_KEY
        strict: false
      model: gpt-4o-mini
  Generator2:
    type: haystack.components.generators.chat.openai.OpenAIChatGenerator
    init_parameters:
      api_key:
        type: env_var
        env_vars:
        - OPENAI_API_KEY
        strict: false
      model: gpt-4o-mini
  AnswerBuilder1:
    type: haystack.components.builders.answer_builder.AnswerBuilder
    init_parameters:
      pattern:
      reference_pattern:
      last_message_only: false
      return_only_referenced_documents: true
  AnswerBuilder2:
    type: haystack.components.builders.answer_builder.AnswerBuilder
    init_parameters:
      pattern:
      reference_pattern:
      last_message_only: false
      return_only_referenced_documents: true
  AnswerJoiner:
    type: haystack.components.joiners.answer_joiner.AnswerJoiner
    init_parameters:
      join_mode: concatenate
      sort_by_score: false

connections:
- sender: PromptBuilder1.prompt
  receiver: Generator1.messages
- sender: PromptBuilder2.prompt
  receiver: Generator2.messages
- sender: Generator1.replies
  receiver: AnswerBuilder1.replies
- sender: Generator2.replies
  receiver: AnswerBuilder2.replies
- sender: AnswerBuilder1.answers
  receiver: AnswerJoiner.answers
- sender: AnswerBuilder2.answers
  receiver: AnswerJoiner.answers

max_runs_per_component: 100

metadata: {}

inputs:
  query:
  - PromptBuilder1.query
  - PromptBuilder2.query
  - AnswerBuilder1.query
  - AnswerBuilder2.query

outputs:
  answers: AnswerJoiner.answers

Parameters

Inputs

Parameter	Type	Description
answers	Variadic[List[AnswerType]]	Nested list of answers to merge.
top_k	Optional[int]	The maximum number of answers to return. Overrides the instance's top_k if provided.

Outputs

Parameter	Type	Description
answers	List[AnswerType]	Merged list of answers.

Init Parameters

These are the parameters you can configure in Pipeline Builder:

Parameter	Type	Default	Description
join_mode	Union[str, JoinMode]	JoinMode.CONCATENATE	Specifies the join mode to use. Available modes: concatenate concatenates multiple lists of answers into a single list.
top_k	Optional[int]	None	The maximum number of answers to return.
sort_by_score	bool	False	If True, sorts the answers by score in descending order. If an answer has no score, it is handled as if its score is -infinity.

Run Method Parameters

These are the parameters you can configure for the component's run() method. This means you can pass these parameters at query time through the API, in Playground, or when running a job. For details, see Modify Pipeline Parameters at Query Time.

Parameter	Type	Default	Description
answers	Variadic[List[AnswerType]]		Nested list of answers to merge.
top_k	Optional[int]	None	The maximum number of answers to return. Overrides the instance's top_k if provided.