When should you use upsert() instead of add() in ChromaDB, and what are common patterns?

import chromadb
from datetime import datetime

client = chromadb.PersistentClient(path="./upsert_demo")
col = client.get_or_create_collection("products")

# Pattern 1: Safe initial load
# Can re-run the script without duplicate ID errors
def sync_products(products: list[dict]):
    col.upsert(
        documents=[p["description"] for p in products],
        ids=       [str(p["id"])   for p in products],
        metadatas= [{"name": p["name"], "price": p["price"], "updated": int(datetime.now().timestamp())}
                    for p in products],
    )

products_v1 = [
    {"id": 1, "name": "Widget", "description": "A blue widget", "price": 9.99},
    {"id": 2, "name": "Gadget", "description": "A red gadget",  "price": 14.99},
]
sync_products(products_v1)  # inserts both
print(col.count())  # 2

# Product 1 description changed — upsert handles it cleanly
products_v2 = [
    {"id": 1, "name": "Widget", "description": "An improved blue widget v2", "price": 11.99},
    {"id": 3, "name": "Doohickey", "description": "A green doohickey", "price": 4.99},
]
sync_products(products_v2)  # updates id=1, inserts id=3
print(col.count())          # 3

# Verify the update
result = col.get(ids=["1"])
print(result["documents"][0])   # "An improved blue widget v2"
print(result["metadatas"][0]["price"])  # 11.99

# Pattern 2: Incremental indexing — only upsert changed documents
def incremental_sync(items, last_sync_ts: int):
    changed = [i for i in items if i["updated_at"] > last_sync_ts]
    if changed:
        col.upsert(
            documents=[i["body"] for i in changed],
            ids=       [i["id"]  for i in changed],
            metadatas= [{"updated_at": i["updated_at"]} for i in changed],
        )