On removing stale relationships - neo4j

Question

A shopping cart is being logged to Neo4j.
NOTE: Each cart is unique to a visitor and defined by the cart:path (which is actually a cart ID cookie). Each item is a lineitem in the cart. Its unique to the cart and product in the cart (unique key is item.key). Finally, product_id refers to a product.
The cart contains lines which need to be updated even when someone deletes a line or changes quantity.
The set updates values for existing lines, but it wouldn't remove lines that are deleted from the cart when the updated cart json arrives.
Is there a simple way to modify this query to delete removed lines automatically?
UNWIND items as item
MATCH (p:Cart {path:px.upath})
SET p.total_price=cart.total_price
MERGE (i:Item {key:item.key})
SET
i.product_id=item.product_id,
i.quantity=item.quantity,
MERGE (i)-[:LineOf]->(p)

Answer 1

I'm a little puzzled by the query, since it seems like you're merging in items as they're added to the cart, as opposed to matching on existing products. Also, the :Item nodes you're adding seem specific for a single user's transaction (you're setting the quantity based on the input), so it seems unwise to use MERGE here...what if two users are trying to add the same type of item at the same time, but different quantities? One of those transactions will overrule the other...user 1 adds 2 of :Item a, but user 2 adds 10 of item a, which changes user 1's :Cart to now read 10 of :Item a selected (though the :Cart's total hasn't been updated to that...) It seems like the model could use some improvements.
One way you could do this is to set the quantity on the relationship to the cart, instead of on the item.
Something like this:
UNWIND items as item
MATCH (p:Cart {path:px.upath})
SET p.total_price=cart.total_price
MERGE (i:Item {key:item.key})
// doesn't seem the place to set product_id, but leaving it in
SET i.product_id=item.product_id
MERGE (i)-[r:LineOf]->(p)
SET r.quantity=item.quantity
So it seems like this query is for adding items to the cart, and you need a query for deleting items.
This should do the trick:
// assume toDelete is your list of items to delete
WITH toDelete
MATCH (p:Cart {path:px.upath})
SET p.total_price=cart.total_price
MATCH (p)<-[r:LineOf]-(i)
WHERE i.key in toDelete
DELETE r
If you want a single query that will set your items as in your first query, and delete all other lines that are not sent over, then this combined query should work:
UNWIND items as item
MATCH (p:Cart {path:px.upath})
SET p.total_price=cart.total_price
// first delete all items from the cart that aren't listed
OPTIONAL MATCH (p)<-[r:LineOf]-(i)
WHERE NOT i.key in items
DELETE r
// now add/update the rest
MERGE (i:Item {key:item.key})
// doesn't seem the place to set product_id, but leaving it in
SET i.product_id=item.product_id
MERGE (i)-[r:LineOf]->(p)
SET r.quantity=item.quantity