BM25 support with MongoDB is broken with Haystack v1.24.0

[tillwf]

Describe the bug Like this issue BM25 support with Weaviate got broken with Haystack v1.11.0+, it seems that MongoDB Document Store can no longer be used with the BM25 node

Error message When you try to use MongoDBAtlasDocumentStore in Haystack v1.24.0 with a BM25 node you will see the following error:

Expected behavior BM25 nodes should work with MongoDB

To Reproduce

from haystack.document_stores import MongoDBAtlasDocumentStore
from haystack.pipelines import DocumentSearchPipeline
from haystack.nodes import BM25Retriever
import os

document_store = MongoDBAtlasDocumentStore(
    mongo_connection_string=f"mongodb+srv://{os.getenv('MONGO_USER')}:{os.getenv('MONGO_PASS')}@{os.getenv('MONGO_URL')}",
    database_name=os.getenv("MONGO_DB"),
    collection_name="embeddings",
    vector_search_index="uuid",
    embedding_dim=384
)
retriever = BM25Retriever(document_store)
pipeline = DocumentSearchPipeline(retriever=retriever)

result = pipeline.run(
    query="test",
    params={
        "Retriever": {
            "top_k": 10,
        }
    }
)

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System:

• OS: Ubuntu, Python 3.10.7
• GPU/CPU:
• Haystack version (commit or version number): v1.24.0
• DocumentStore: MongoDBAtlasDocumentStore
• Reader:
• Retriever: BM25