On 25 March 2011 10:41, msappler <msapp...@web.de> wrote:
> Never used metadata until now.
>
> I had an insight yesterday that I want to share with you:
>
> http://resatori.com/metadata-insight
In my view, it depends whether your :id key is a surrogate key or a natural key.
Object metadata should not affect object equality, so if your :id key
is just an internal database identifier, you could include it as
metadata.
But if your :id key is exposed to the outside world, then it should
not be metadata, because changing the :id would change the external
representation of the object, and therefore affect equality.
I should also point out that if you want information hiding, then
object composition might be a better solution. For example:
(defprotocol Identifiable
(id [self] "Return an object's unique id"))
(deftype DBRef [db-id data]
Identifiable
(id [_] db-id)
clojure.lang.IDeref
(deref [_] data))
(defn db-ref [id data]
(DBRef. id data))
Then we can get information about the reference, and deref it to get
its contents:
(def user
(db-ref 193 {:login "fred"}))
=> (id user)
193
=> @user
{:login fred}
- James
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