fix: _ensure_index_exists for aliases in OpenSearch

[tstadel]

Related Issues

• fixes _ensure_index_exists for aliases in OpenSearch

Traceback (most recent call last):
  File "/home/haystackd/.local/lib/python3.12/site-packages/haystack/core/pipeline/pipeline.py", line 84, in _run_component
    component_output = instance.run(**inputs_copy)
                       ^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/haystackd/.local/lib/python3.12/site-packages/haystack_integrations/components/retrievers/opensearch/bm25_retriever.py", line 283, in run
    raise e
  File "/home/haystackd/.local/lib/python3.12/site-packages/haystack_integrations/components/retrievers/opensearch/bm25_retriever.py", line 280, in run
    docs = doc_store._bm25_retrieval(**bm25_args)  # example for BM25Retriever
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/haystackd/.local/lib/python3.12/site-packages/haystack_integrations/document_stores/opensearch/document_store.py", line 1079, in _bm25_retrieval
    self._ensure_initialized()
  File "/home/haystackd/.local/lib/python3.12/site-packages/haystack_integrations/document_stores/opensearch/document_store.py", line 308, in _ensure_initialized
    self._ensure_index_exists()
  File "/home/haystackd/.local/lib/python3.12/site-packages/haystack_integrations/document_stores/opensearch/document_store.py", line 420, in _ensure_index_exists
    properties = mapping[self._index]["mappings"].get("properties", {})
                 ~~~~~~~^^^^^^^^^^^^^
KeyError: 'ALIAS_NAME'

Proposed Changes:

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• I have read the contributors guidelines and the code of conduct
• I have updated the related issue with new insights and changes
• I added unit tests and updated the docstrings
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[github-actions]

Coverage report (opensearch)

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File	Statements	Missing	Coverage	Coverage (new stmts)	Lines missing
integrations/opensearch/src/haystack_integrations/document_stores/opensearch
document_store.py
Project Total

This report was generated by python-coverage-comment-action

[bogdankostic]

The same mapping[self._index] pattern that causes KeyError with aliases still exists in 6 other methods

count_unique_metadata_by_filter at document_store.py:1820
count_unique_metadata_by_filter_async at document_store.py:1863
get_metadata_fields_info at document_store.py:1915
get_metadata_fields_info_async at document_store.py:1948
delete_all_documents (with recreate_index=True) at document_store.py:790-791 (uses index_info[index_name] where index_name = self._index, and indices.get() also keys by the real index name)
delete_all_documents_async (with recreate_index=True) at document_store.py:834-835 (same pattern)

Let's fix those in a separate PR.