Generate datom :db.type/ref s with fn

refsymbols.clj

(use '[datomic.api :only [db q] :as d])

;;LOOK IN THE BOTTOM FOR CORRECT USE OF DATOMIC :)

;;not supplied:
;;* connecting to a db named "conn" 
;;      (checkout awesome Jonas Edlunds gist 
;;       for inspiration: https://gist.github.com/3122363 )
;;* a schema that fit's the following person-datoms

;; Problem:
;;it's a bit quirky to generate references by functions to transact them to datoms
;;since it's an instant literal it seems to be evaluated when created by a function

(defn refsymbol
  "given a reference long, this generates a correct, non-evaluated reference"
  [ref]
  (read-string (apply str "#db/id[:db.part/user " ref "]")))

;;that is

(refsymbol 12345) => #db/id[:db.part/user 12345] ;; all the way into datomic!

;;12345 should not be a random number, it's usually found by a query like
;;(q '[:find ?id :where [?id :company/name "GoodCorp Inc."]] (db conn))
;;=>#<HashSet [[17592186045497]]> given GoodCorp Inc.-datom exists in the database
;;this is a hash nested two times (given we only have one GoodCorp Inc.-datom)

;;this first ref-number can be picked out by (first (first (q ...))) 
;;or (ffirst (q ...)) for short

;;btw, good intro to shape of datoms here: 
;;http://pelle.github.com/Datomic/2012/07/08/thinking-in-datomic/

(def goodcorp-refno (ffirst (q '[:find ?id :where [?id :company/name "GoodCorp Inc."]] (db conn)))) 

;;which can also be automated like this:

(defn company-refno [company-name]
  (ffirst (q `[:find ?ref :where [?ref :company/name ~company-name]] (db conn))))

;;=>17592186045497

;;armed with a ref-number we can make datoms with functions like this:

(defn add-new-person
  "lets say we want to connect a person with a name to a company with a reference"
  [name ref-long]
  (let [correct-ref (refsymbol ref-long)]
    `[{:db/id ~#db/id[:db.part/user] :person/name ~name :person/company ~correct-ref}]))

;;` means "don't evaluate the following form" 
;;~x means "lookup the symbol x and insert it here anyway `"

(add-new-person "Bob Smith" 17592186045497)

or

(add-new-person "Bob Smith" goodcorp-refno)

or

(add-new-person "Bob Smith" (company-refno "GoodCorp Inc.")) ;;hairball warning!

;;which creates a record

[{:person/company #db/id[:db.part/user 17592186045497], 
  :person/name "Bob Smith", 
  :db/id #db/id[:db.part/user -1000416]}]

;;that could be added to a database with:

(d/transact conn (add-new-person "Bob Smith" 17592186045497))

;;if you want to use (map add-new-person [["Bob Smith" 17592186045497] ["Ben Andersen" 17592186043525]])
;;you have to remove the [vector] holding the {map} in add-new-person like

(defn add-new-person2
  [name ref-long]
  (let [correct-ref (refsymbol ref-long)]
    `{:db/id ~#db/id[:db.part/user] ;;no vector outside the map!
      :person/name ~name 
      :person/company ~correct-ref}))

;;now:

(d/transact conn (map add-new-person2 [["Bob Smith" 17592186045497] ["Ben Andersen" 17592186043525]]))

;;(with-datomic-awesomeness (success!))

;;some things helped me much:

;;jonas edlunds getting-stared gist
;;https://gist.github.com/3122363
;;esp. check out how elegant the ids are generated

;;pelle braendgaard on datom concepts
;;http://pelle.github.com/Datomic/2012/07/08/thinking-in-datomic/

;;more on transact-stuff:
;;http://pelle.github.com/Datomic/2012/07/09/transactions-as-entities/

;;a banking app example, also from mr braendgaard
;;https://gist.github.com/2635666

;;also checkout the /samples/seattle/getting-started.clj 
;;and the .dtm files in the same folder (everything in datomic distribution)

;;and the attribute stuff at the Datomic reference:
;;http://www.datomic.com/company/resources/schema


;; OR DO IT RIGHT (thank you Stuart Halloway, Datomic team)

(1) If you need to programmatically create a tempid, there is no need to do weird reader tricks. Just use the Peer.tempid (Java) or api/tempid (Clojure) function.

(2) The entity id returned by a query is a complete id. You do not need to plug it back into an id literal to use it:

(defn add-new-person
  [name ref-long]
  [{:db/id (d/tempid :db.part/user) 
   :person/name name 
    :person/company ref-long}]))

Note also that backquote is not necessary.

(3) The return value of add-new-person above is shaped like transaction data. When you remove the vector to facilitate calling map, you make your function less general (now you require that it return only a single piece of transaction data). Better to leave the return value flexible and call mapcat instead of map.