Clojurians Log v2

Hi, I'm trying to build a first app using datalog/datascript. In the db I want to store FX exchange rates. For each pair (eg. GBP/USD, GBP/EUR), for every day, one value. I'm wandering how to design schema for such application. I'm struggling with what should be marked as an entity. Currency, currency pair, currency pair for given date, or all actually? I'd much grateful for some ideas or sample schemas that you think might make sense. Thanks

Hi @grzegorz.rynkowski, are exchange rates directional? e.g. [#inst "2021-01-01" :GBP :USD 10] but [#inst "2021-01-01" :USD :GBP 8]?

@lanejo01 yes, this kind of situation is possible: [#inst "2021-01-01" :GBP :USD 2] [#inst "2021-01-01" :USD :GBP 0.4] In addition there might be more then one source of rates, so there might be: [#inst "2021-01-01" :GBP :USD [:name "BoE"] 2.01] [#inst "2021-01-01" :GBP :USD [:name "HMRC"] 2.03]

What's the best way to get two strings like this into an instant that i can save in datomic? "2021-06-08T16:30:12" "America/New_York" I tried using clj-time but no dice

(let [datetime-vec (take 8 (parse-timestamp vector date-str))]
     (-> (apply zoned-date-time datetime-vec)
         (with-zone UTC-offset)
         (instant)))

:db.error/wrong-type-for-attribute Value 2021-06-08T16:30:12-04:00[America/New_York] is not a valid :inst for attribute :session/start_time

#inst "2021-06-08T20:30:12.000000000-00:00"

Perhaps this https://forum.datomic.com/t/java-time/1406 is relevant?

@jdkealy For now you gotta turn that into a j.u.Date before you persist it in Datomic.

what's a j.u.date a google search is giving me linkes to jdate 🙂

i guess, Java Util Date ?

(java.util.Date. (tz/to-instant "2021-06-08T16:30:12" "America/New_York"))

This seems to work

([date-str, UTC-offset]
   (let [datetime-vec (take 8 (parse-timestamp vector date-str))]
     (-> (apply zoned-date-time datetime-vec)
         (with-zone UTC-offset)
         (instant)
         inst-ms
         (java.util.Date.))))

input

"2021-06-08T16:30:12" "America/New_York"

#inst "2021-06-08T20:30:12.000-00:00"

To give better advice I'd need to know more about the rest of the app, but for those fx entities I'd lean towards representing :GBP->:USD as an entity with a composite tuple of :loc/from and :loc/to where the composite tuple represents identity. Then for the rates i'd want to know what the access patterns will be and the growth of the dimensions. The rate entity could be something like:

{:time/at #inst "2021-01-01
 :rate/from-to {:loc/from :GBP :loc/to :USD}
 :rate/source {:source/name "BoE}
 :rate/amount 2.01}

[#inst "2021-01-01" 123 456 2.01]

FWIW, if you care about the TZ when you query obviously you'll need to stash that as another attribute and reconstitute a ZonedDateTime on the way out. Avoid querying with zones if you can, it can make simple queries slow and complicated.

Oh also, you can use (Date/from your-inst)

cool thanks! Yes, I'm saving 3 attrs, the timezone, the original string, the converted instant (just in case i screw something up, i can regenerate them )

I'll only be querying for stuff in the immediate future, so i think I'll just be passing (> (java.util.Inst))

When accessing raw index (listing datoms using ), is there a way of substituting numbers by entities itself? I was checking https://docs.datomic.com/cloud/query/raw-index-access.html, but there is nothing about such a thing. Example: when I preview datoms for an example data set

(d/datoms db-solar-system {:index :eavt})

...
 #datom[74766790688845 73 "Sun" 13194139533319 true]
 #datom[74766790688845 74 696000.0 13194139533319 true]
 #datom[74766790688846 73 "Jupiter" 13194139533319 true]
 #datom[74766790688846 74 69911.0 13194139533319 true]
 #datom[74766790688847 73 "Saturn" 13194139533319 true]
 #datom[74766790688847 74 58232.0 13194139533319 true]
...