Seeing previous datum in a transaction?

Datomic Applications
1

Let’s say I need to update a property of an entity base on the summary of multiple entities.

Schema:

[{:db/ident :order/total,     :db/valueType :db.type/bigdec, ...}
 {:db/ident :item/unit-price, :db/valueType :db.type/bigdec, ...}
 {:db/ident :item/quantity,   :db/valueType :db.type/long,   ...}]

Transaction

(d/transact conn {:tx-data [[:db/cas 42 :db/ident :item/quantity 100 110]
                            '(my.ion/recalculate-total 42)]})

Preferably when tested using a dev-local db, the custom transaction function my.ion/recalculate-total cannot see the previous datoms (e.g. 110). Is there anyway we can update a value based on previous data in a transaction?

2

Maybe makes sense to pass 110 as additional parameter to recalculate-total ?