I'm using datomic cloud and i get the exception "http://grow.de I dont understand what the that means? why isn't it allowed? i assume their is something wrong with the function. Backing up, the goal is to enforce that every floor has a building. This is just a learning exercise, so i'm open to any interperation of how it would be best to solve this. code:

(defn floor-has-one-building-v4?
  [db eid]
  (let [building (d/pull db '[{:building/_floor [:building/name]}] eid)]
    (= (count building) 1)))

(def predicate
  {:db/ident        :floor/guard4
   :db.entity/attrs [:floor/name]
   :db.entity/preds 'grow.dev/floor-has-one-building-v4?})

(comment
  (d/transact conn {:tx-data [predicate]}))

(d/transact conn {:tx-data [{:building/name  "Big Building"
                             :building/floor [{:floor/name "bottom"
                                               :db/ensure  :floor/guard4}]}]})

https://docs.datomic.com/cloud/schema/schema-reference.html#entity-specs

@drewverlee entity predicates need to be on the classpath of the primary compute group

you need to deploy them to the transactor using https://docs.datomic.com/cloud/ions/ions-reference.html#ion-config

and mark them under the :allow section

entity preds do not run on the client submitting the transaction, they run on the server (the primary/transactor)

Below is from the datomic documentation -

The V-leading index VAET supports efficient queries for references between entities, analogous to a graph database.

The combination of EAVT and VAET supports arbitrary nested navigation among entities. This is like a document database but vastly more flexible. You can navigate in any direction at any time, instead of being limited to the containment hierarchy you selected when storing the document.

@murtaza52 There’s a lot of overlap there. For a document store, you just need an entity identifier and some attributes on that entity. For graphical data, you just need to reference other entities (via :db.type/ref).