How to concatenate two pipeline?

[Shuntw6096]

haystack-ai package version: 2.4.0

from haystack import Pipeline  
from haystack.components.retrievers.in_memory import InMemoryEmbeddingRetriever  
from document_store import document_store
from components.twsc import MyRanker, MyGenerator, MyTextEmbedder


def get_retrieval_pipeline(
    retrieve_top_k = 5,
    rank_top_k = 3,
):
    retriever = InMemoryEmbeddingRetriever(document_store, top_k=retrieve_top_k)
    ranker = MyRanker(top_k=rank_top_k)
    embedder = MyTextEmbedder(api_params={"model": "my-embedding"})
    pipeline = Pipeline()
    pipeline.add_component(instance=embedder, name="embedder")
    pipeline.add_component(instance=retriever, name="retriever")
    pipeline.add_component(instance=ranker, name="ranker")
    pipeline.connect("embedder.embedding", "retriever.query_embedding")
    pipeline.connect("retriever.documents", "ranker.documents")
    return pipeline

pipeline = Pipeline()
pipeline.add_component('prompt_builder', PromptBuilder(''))
pipeline.add_component('retrieval_pipeline', get_retrieval_pipeline())

I got this error,

Traceback (most recent call last):
  File "/home/cccc/Desktop/project-hahah/src/pipeline.py", line 41, in <module>
    pipeline.add_component('retrieval_pipeline', get_retrieval_pipeline())
  File "/home/cccc/Desktop/project-hahah/.venv/lib/python3.11/site-packages/haystack/core/pipeline/base.py", line 332, in add_component
    input_sockets=instance.__haystack_input__._sockets_dict,  # type: ignore[attr-defined]
                  ^^^^^^^^^^^^^^^^^^^^^^^^^^^
AttributeError: 'Pipeline' object has no attribute '__haystack_input__'

because entire pipeline is a little bit complex,
I would like to split entire pipeline into several simple pipelines,
how to do pipeline connection?

[Shuntw6096]

A stupid way is to wrap a pipeline into component, and add pipeline component into another pipeline.