community: [MongoDBAtlasVectorSearch] Fix KeyError 'embedding' when using MMR

[certified-dodo]

Checked other resources

• I added a very descriptive title to this issue.
• I searched the LangChain documentation with the integrated search.
• I used the GitHub search to find a similar question and didn't find it.
• I am sure that this is a bug in LangChain rather than my code.

Example Code

# test.py
from langchain_openai import OpenAIEmbeddings
from langchain_community.vectorstores.mongodb_atlas import MongoDBAtlasVectorSearch

embeddings = OpenAIEmbeddings()

MONGO_CONNECTION_URI = "mongo uri"
VECTORS_DB_NAME = "db name"
VECTOR_SEARCH_INDEX_NAME = "index name"

db = MongoDBAtlasVectorSearch.from_connection_string(
    MONGO_CONNECTION_URI,
    VECTORS_DB_NAME,
    embeddings,
    index_name=VECTOR_SEARCH_INDEX_NAME,
)

db_query = "Find info related to ..."
num_of_db_results = 10

raw_contexts = db.max_marginal_relevance_search(query=db_query, k=num_of_db_results, lambda_mult=0)

Error Message and Stack Trace (if applicable)

Traceback (most recent call last):
  File ".../test.py", line 39, in <module>
    raw_contexts = db.max_marginal_relevance_search(query=db_query, k=num_of_db_results, lambda_mult=0)
                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File ".../lib/python3.12/site-packages/langchain_community/vectorstores/mongodb_atlas.py", line 325, in max_marginal_relevance_search
    [doc.metadata[self._embedding_key] for doc, _ in docs],
     ~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^
KeyError: 'embedding'

Description

• Error when using MMR (max marginal relevance) search. This includes
  • calling. max_marginal_relevance_search directly
  • using db.as_retriever(search_type="mmr")
• self._embedding_key is deleted from docs in _similarity_search_with_score
  

  langchain/libs/community/langchain_community/vectorstores/mongodb_atlas.py

  Line 212 in f6e3aa9

  del res[self._embedding_key]

  
  the following method maximal_marginal_relevance tries to access the deleted field resulting in an error
  

  langchain/libs/community/langchain_community/vectorstores/mongodb_atlas.py

  Line 325 in f6e3aa9

  [doc.metadata[self._embedding_key] for doc, _ in docs],

Expectation

• MMR option working with no error

System Info

Python
v3.12.0

requirements.txt

langchain==0.1.8
langchain-community==0.0.21
langchain-core==0.1.25
langchain-openai==0.0.5
langsmith==0.1.5
openai==1.11.1

[certified-dodo]

#17971