diff --git a/src/monitor_utils/database.py b/src/monitor_utils/database.py
index 82a6525..b89c948 100644
--- a/src/monitor_utils/database.py
+++ b/src/monitor_utils/database.py
@@ -1,3 +1,10 @@
+"""Database helpers for upserting monitored products and tracking price history.
+
+Provides the shared Motor client, collection access, and the upsert path that
+rewrites current product fields while preserving prior prices in a pastPrices
+map and stamping lastUpdated on every write.
+"""
+
 import logging
 from dataclasses import asdict
 from datetime import datetime, timezone
@@ -78,20 +85,16 @@ async def add_products_to_database(
     if not products:
         return
 
-    doc_ids = {
-        id(p): build_doc_id(p.product_id, category_identifier, use_composite)
-        for p in products
-    }
-    cursor = collection.find(
-        {"_id": {"$in": list(doc_ids.values())}}, {"currentPrice": 1}
-    )
+    doc_ids = [
+        build_doc_id(p.product_id, category_identifier, use_composite) for p in products
+    ]
+    cursor = collection.find({"_id": {"$in": doc_ids}}, {"currentPrice": 1})
     existing_prices = {str(doc["_id"]): doc.get("currentPrice") async for doc in cursor}
 
     ops = []
     now = datetime.now(timezone.utc)
     price_key = str(int(now.timestamp() * 1000))
-    for product in products:
-        doc_id = doc_ids[id(product)]
+    for product, doc_id in zip(products, doc_ids):
         document = convert_product_to_mongo(product, category_identifier)
         document["_id"] = doc_id
         old_price = existing_prices.get(doc_id)
@@ -144,11 +147,11 @@ def convert_product_to_mongo(product: Product, category_identifier: str) -> dict
     """Flatten a Product into its MongoDB document representation.
 
     Coerces string prices to floats and stamps lastUpdated with the current
-    UTC time.
+    UTC time. The _id is assigned by the caller via build_doc_id.
 
     Args:
         product: Product to convert.
-        category_identifier: Category suffix appended to the composite _id.
+        category_identifier: Category suffix retained for signature parity.
 
     Returns:
         The MongoDB document dict for the product.
@@ -166,7 +169,6 @@ def convert_product_to_mongo(product: Product, category_identifier: str) -> dict
         original_price = float(original_price.replace(",", ""))
 
     return {
-        "_id": f"{data['product_id']}_{category_identifier}",
         "title": data["title"],
         "url": data["url"],
         "currentPrice": current_price,