Hi
You can use the tempid
(http://docs.datomic.com/clojure/index.html#datomic.api/tempid) function to
generate new temporary ids.
Jonas
On Monday, March 4, 2013 8:50:56 AM UTC+2, edw...@kenworthy.info wrote:
>
> Okay, I think I understand that.
>
> Does that mean this code could never work as intended in a Clojure
> program, only at the repl (which seems a bit of a limitation) or is there a
> way to make it work as intended, generating a different id each time? Or is
> the whole approach taken in this code flawed?
>
> On Sunday, 3 March 2013 21:59:45 UTC, Michał Marczyk wrote:
>>
>> #db/id ... is a "tagged literal". It gets read in as an id object with
>> some integer inside. The way this happens is that the reader looks up
>> a "data reader" function associated with the symbol db/id in the map
>> held by the var *data-readers* and passes to it the result of parsing
>> the rest of the literal (a vector holding the keyword :db.part/db in
>> this case). Importantly, this function is not pure and the integer it
>> uses will be different on each invocation. (Given this form of the
>> #db/id literal; you can also specify a particular number yourself --
>> #db/id [:db.part/db some-number-here].)
>>
>> In any case, once the reader reads in your code including some
>> particular id, the compiler will compile the result preserving the
>> particular values it sees, so it will embed code to construct *exactly
>> this* id in the compiled output. Thus you end up with a particular id
>> hardwired into the first version of your function.
>>
>> With the second version, if you invoke it multiple times at the REPL,
>> you ask the reader for a new id at each invocation, so it works as you
>> expect. If instead you were to use it inside a function like so:
>>
>> (defn foo []
>> (make-column-schema #db/id [:db.part/db] :results/subject
>> :db.type/string)),
>>
>> then again the same id would be used for every (foo) call.
>>
>> Cheers,
>> Michał
>>
>>
>> On 3 March 2013 21:58, <edw...@kenworthy.info> wrote:
>> > So, I am studying a piece of code from the web. I've dissected most of
>> it
>> > and am in the process of re-factoring it.
>> >
>> > What I don't understand is why one version works and the other doesn't.
>> >
>> > So for both:
>> >
>> > (ns gcse-results.core (:use [datomic.api :only [q db] :as d]) (:use
>> > quil.core))
>> >
>> > This doesn't work:
>> >
>> > (defn make-column-schema [db-ident db-type]
>> > {:db/id #db/id[:db.part/db]
>> > :db/ident db-ident
>> > :db/valueType db-type
>> > :db/cardinality :db.cardinality/one
>> > :db.install/_attribute :db.part/db})
>> >
>> > Each call to:
>> >
>> > (make-column-schema :results/subject :db.type/string)
>> >
>> > The value of #db/id[:db.part/db] is the same.
>> >
>> > And this works:
>> >
>> > (defn make-column-schema [db-id db-ident db-type]
>> > {:db/id db-id
>> > :db/ident db-ident
>> > :db/valueType db-type
>> > :db/cardinality :db.cardinality/one
>> > :db.install/_attribute :db.part/db})
>> >
>> > Each call to: (make-column-schema #db/id[:db.part/db] :results/subject
>> > :db.type/string)
>> >
>> > The value of #db/id[:db.part/db] is the different, as expected.
>> >
>> > Now I thought that I understood #db/id[:db.part/db] (call to a Java
>> > constructor) but obviously my understanding is flawed as I would expect
>> both
>> > of these functions to produce the same thing, but they don't so there's
>> > obviously some gap in my understanding.
>> >
>> > Help?
>> >
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