fix: rebuild FAISS index on vector deletion (#4178)

mem0/vector_stores/faiss.py

@@ -309,8 +309,23 @@ class FAISS(VectorStoreBase):
                 break
 
         if index_to_delete is not None:
+            # Reconstruct remaining vectors and rebuild the FAISS index
+            remaining_vectors = []
+            new_index_to_id = {}
+            new_idx = 0
+            for old_idx in sorted(self.index_to_id.keys()):
+                if old_idx == index_to_delete:
+                    continue
+                remaining_vectors.append(self.index.reconstruct(int(old_idx)))
+                new_index_to_id[new_idx] = self.index_to_id[old_idx]
+                new_idx += 1
+
+            self.index.reset()
+            if remaining_vectors:
+                self.index.add(np.array(remaining_vectors, dtype=np.float32))
+
             self.docstore.pop(vector_id, None)
-            self.index_to_id.pop(index_to_delete, None)
+            self.index_to_id = new_index_to_id
 
             self._save()
 

tests/vector_stores/test_faiss.py

@@ -165,19 +165,29 @@ def test_search_with_filters(faiss_instance, mock_faiss_index):
                 assert results[0].payload == {"name": "vector1", "category": "A"}
 
 
-def test_delete(faiss_instance):
+def test_delete(faiss_instance, mock_faiss_index):
     # Setup the docstore and index_to_id mapping
     faiss_instance.docstore = {"id1": {"name": "vector1"}, "id2": {"name": "vector2"}}
     faiss_instance.index_to_id = {0: "id1", 1: "id2"}
 
+    # Mock reconstruct to return vectors for remaining entries
+    mock_faiss_index.reconstruct.side_effect = lambda idx: np.array(
+        [0.1, 0.2, 0.3] if idx == 0 else [0.4, 0.5, 0.6], dtype=np.float32
+    )
+
     # Call delete
     faiss_instance.delete(vector_id="id1")
 
-    # Verify the vector was removed from docstore and index_to_id
+    # Verify the vector was removed from docstore
     assert "id1" not in faiss_instance.docstore
-    assert 0 not in faiss_instance.index_to_id
     assert "id2" in faiss_instance.docstore
-    assert 1 in faiss_instance.index_to_id
+
+    # Verify the FAISS index was rebuilt
+    mock_faiss_index.reset.assert_called_once()
+    mock_faiss_index.add.assert_called_once()
+
+    # Verify index_to_id was remapped contiguously
+    assert faiss_instance.index_to_id == {0: "id2"}
 
 
 def test_update(faiss_instance, mock_faiss_index):