chore: handle empty distance list in brute force search [cog-1424] (#654)

<!-- .github/pull_request_template.md -->

## Description
- handle empty distance list in brute force search
- unit tests

## DCO Affirmation
I affirm that all code in every commit of this pull request conforms to
the terms of the Topoteretes Developer Certificate of Origin

---------

Co-authored-by: hajdul88 <52442977+hajdul88@users.noreply.github.com>

cognee/modules/retrieval/exceptions/__init__.py

@@ -4,4 +4,4 @@ Custom exceptions for the Cognee API.
 This module defines a set of exceptions for handling various data errors
 """
 
-from .exceptions import SearchTypeNotSupported, CypherSearchError
+from .exceptions import SearchTypeNotSupported, CypherSearchError, CollectionDistancesNotFoundError

cognee/modules/retrieval/exceptions/exceptions.py

@@ -2,6 +2,16 @@ from cognee.exceptions import CogneeApiError
 from fastapi import status
 
 
+class CollectionDistancesNotFoundError(CogneeApiError):
+    def __init__(
+        self,
+        message: str = "No distances found between the query and collections. It is possible that the given collection names don't exist.",
+        name: str = "CollectionDistancesNotFoundError",
+        status_code: int = status.HTTP_404_NOT_FOUND,
+    ):
+        super().__init__(message, name, status_code)
+
+
 class SearchTypeNotSupported(CogneeApiError):
     def __init__(
         self,

cognee/modules/retrieval/utils/brute_force_triplet_search.py

@@ -8,6 +8,7 @@ from cognee.modules.graph.cognee_graph.CogneeGraph import CogneeGraph
 from cognee.modules.users.methods import get_default_user
 from cognee.modules.users.models import User
 from cognee.shared.utils import send_telemetry
+from cognee.modules.retrieval.exceptions import CollectionDistancesNotFoundError
 
 logger = get_logger(level=ERROR)
 
@@ -149,6 +150,9 @@ async def brute_force_search(
             ]
         )
 
+        if all(not item for item in results):
+            raise CollectionDistancesNotFoundError()
+
         node_distances = {collection: result for collection, result in zip(collections, results)}
 
         await memory_fragment.map_vector_distances_to_graph_nodes(node_distances=node_distances)

cognee/tests/unit/modules/retrieval/utils/brute_force_triplet_search_test.py

@@ -0,0 +1,48 @@
+import pytest
+from cognee.modules.retrieval.exceptions import CollectionDistancesNotFoundError
+from cognee.modules.users.models import User
+from cognee.modules.retrieval.utils.brute_force_triplet_search import (
+    brute_force_search,
+    brute_force_triplet_search,
+)
+from unittest.mock import AsyncMock, patch
+
+
+@pytest.mark.asyncio
+@patch("cognee.modules.retrieval.utils.brute_force_triplet_search.get_vector_engine")
+async def test_brute_force_search_collection_not_found(mock_get_vector_engine):
+    user = User(id="test_user")
+    query = "test query"
+    collections = ["nonexistent_collection"]
+    top_k = 5
+    mock_memory_fragment = AsyncMock()
+    mock_vector_engine = AsyncMock()
+    mock_vector_engine.get_distance_from_collection_elements.return_value = []
+    mock_get_vector_engine.return_value = mock_vector_engine
+
+    with pytest.raises(Exception) as exc_info:
+        await brute_force_search(
+            query, user, top_k, collections=collections, memory_fragment=mock_memory_fragment
+        )
+
+    assert isinstance(exc_info.value.__cause__, CollectionDistancesNotFoundError)
+
+
+@pytest.mark.asyncio
+@patch("cognee.modules.retrieval.utils.brute_force_triplet_search.get_vector_engine")
+async def test_brute_force_triplet_search_collection_not_found(mock_get_vector_engine):
+    user = User(id="test_user")
+    query = "test query"
+    collections = ["nonexistent_collection"]
+    top_k = 5
+    mock_memory_fragment = AsyncMock()
+    mock_vector_engine = AsyncMock()
+    mock_vector_engine.get_distance_from_collection_elements.return_value = []
+    mock_get_vector_engine.return_value = mock_vector_engine
+
+    with pytest.raises(Exception) as exc_info:
+        await brute_force_triplet_search(
+            query, user, top_k, collections=collections, memory_fragment=mock_memory_fragment
+        )
+
+    assert isinstance(exc_info.value.__cause__, CollectionDistancesNotFoundError)