Accumulate

Our stakeholders have a new request. Now it isn't just an inventory database, it also needs to track orders:

• An order is a collection of line items
• Each line item has a count and references an item in the inventory

We can model this directly in Datomic schema without translation:

noslide

(def order-schema
  [{:db/ident :order/items
    :db/valueType :db.type/ref
    :db/cardinality :db.cardinality/many
    :db/isComponent true}
   {:db/ident :item/id
    :db/valueType :db.type/ref
    :db/cardinality :db.cardinality/one}
   {:db/ident :item/count
    :db/valueType :db.type/long
    :db/cardinality :db.cardinality/one}])

(d/transact conn {:tx-data order-schema})

=> ;; transaction result map

Note that:

• :db.cardinality/many allows a single order to have multiple :order/items
• :db/isComponent true tells Datomic that order items belong to an order

Now let's add a sample order:

noslide

(def add-order
  {:order/items
   [{:item/id [:inv/sku "SKU-25"]
     :item/count 10}
    {:item/id [:inv/sku "SKU-26"]
     :item/count 20}]})

(d/transact conn {:tx-data [add-order]})

result ;; tx result map

In this example a nested entity map is displayed. The top level is the order, which has multiple :order/items nested within.

With this data in hand, let's explore some more features of query.