hey, I’m very new to datomic, and I’m trying to set up an auto incrementing field (I need a serial id for reference). I get the general idea of database functions, but how can I set up a field to be auto incrementing? Can anyone point me to a beginner friendly resource? 😄

depending on the constraints you need for this number, perhaps the db/id itself may suffice

you could keep an entity to control the sequential generation, always keeping the current value, and calling inc in a transaction fn alongside with persisted the incremented value at the entity using it (and also updating the value at the control entity).

a function I use to generate order numbers: (let [new-number (d/q ' [:find (max ?number) . :in $ :where [_ :order/number ?number]] db)] [{:db/id eid, :order/number (inc (or new-number 0))}])

my experience with using max for high insertion volumes is not so good 😕

having a control entity helped tremendously - although it’s not sharding-proof, for this case we plan on using zookeeper.

@viniciushana: shouldn't that attribute be indexed so max would be pretty instant?

@bendy, the idea would be to transact a transaction that uses this fn, rather than setting :order/number directly

even indexed, for hundreds of thousands of entities a day, it takes more than 20 seconds to run a max

we keep those sequentials unique

the other option is to remember the highest order number in an entity somewhere

yep, and I also recommend to keep it accessible by a :db/ident to ease retrieving it

that's what I would do too

@bendy so in practical terms you’ll want the transactor fn to be like: 1 - lookup the control entity 2 - call inc on the attribute you keep as a sequential control 3 - return the tx vector asserting the business entity with the incremented sequential + a db.add for asserting the incremented sequential attribute at the control entity

just to make sure, this is what I’ve come up with so far - is this what you’re suggesting?

{:db/id #db/id[:db.part/db]
  :db/ident :invoice/next
  :db/valueType :db.type/long
  :db/cardinality :db.cardinality/one
  :db/doc "Next invoice number"
  :db/noHistory true
  :db.install/_attribute :db.part/db}

 {:db/id #db/id [:db.part/user]
  :invoice/next 1}

 {:db/id #db/id [:db.part/user]
  :db/ident :number
  :db/doc "Function that returns the next id for an invoice."
  :db/fn #db/fn {:lang "clojure"
                 :params [db id]
                 :code (let [[e n] (datomic.api/q '[:find [?e ?n]
                                                                      :where [?e :invoice/next ?n]]
                                                                    db)]
                         [[:db/add id :invoice/number n]
                          [:db/add e :invoice/next (inc n)]])}}

(don’t worry this isn’t for a real billing system)

I also ask because I’m getting an error and I can’t seem to figure out why… still debugging

ah… well the issue is with my query, apparently you can’t do a :find ?a ?b ?c . 😁

anyways, I think I’m doing what your recommending, so thanks for the confirmation!

didn’t know whether this was the right track or not

you can put them in [?a ?b ?c] . (omit the . actually)

oh ok, that gave me a null pointer exception

I guess there also another bug somewhere else!

don’t mind me haha

wait one second

no ., but you do want find single tuple from here: http://docs.datomic.com/query.html#find-specifications

ah, bingo!

thanks

as soon as I add datomic as a dependency to the peer, logging in other parts of the application starts to break. I have configured leiningen as instructed here http://docs.datomic.com/configuring-logging.html

but e.g. ExceptionInfo doesn't get logged with the data printed

without datomic

(log/error (ex-info "oo" {:a 1 :b 3}) "aaa")
Jun 03, 2016 3:32:40 PM clojure.tools.logging$eval420$fn__425 invoke
SEVERE: aaa
clojure.lang.ExceptionInfo: oo {:a 1, :b 3}
	at clojure.core$ex_info.invokeStatic(core.clj:4617)
	at clojure.core$ex_info.invoke(core.clj:4617)

with datomic & logback

(log/error (ex-info "oo" {:a 1 :b 3}) "aaa")
=> nil
19576820 2016-06-03 15:39:40,202 [nREPL-worker-1] ERROR user - aaa
 - clojure.lang.ExceptionInfo: oo
	at clojure.core$ex_info.invokeStatic(core.clj:4617)
	at clojure.core$ex_info.invoke(core.clj:4617)

let's say I have zero experience with JVM style logging 🙂

It is a problem with logback (pattern:

%date{ISO8601} [%thread] %-5level %logger{36} - %msg%n %ex{full}

got everything working, updated my snippet with the working code, thanks everyone for your help!

I know that clojure.lang.ExceptionInfo includes data in its toString implementation but not its getMessage.

It looks like Logback only uses getMessage and the stack trace in its default exception printer.

@stijn: Whatever logging framework is active in your standalone REPL (maybe java.util.logging) must be printing the ExceptionInfo with toString.

@stuartsierra: thanks I'll try with another logging backend, or get logback to use toString

got it to work with logback by getting the formatted message, which displays exceptions the same way as a standard leiningen repl