guys … a quick design question for components in Datomic...

I am trying to model a shopping basket with some items in the basket and figured that the items should be a component of the basket

it all worked out nicely but ...

I noticed that when I add another element to the basket that the items are duplicated

@raymcdermott: so that's a bug right ?

or maybe to put another way… that I’m finding it unintuitive to model the current state of the items in the basket using the component model

I’m not sure...

which is why I asked

maybe I’m doing it wrong

@raymcdermott: I'm not sure I understand what you get

could you put some code somewhere maybe ?

this is my original cart

(def cart [{:db/id     #db/id [:db.part/user -1]
            :cart/id   #uuid "d213198b-36b5-4c19-8cb1-e172f59091d9"
            :cart/name "My Shopping Cart"
            :cart/sku-counts
                       [
                        {:sku-count/sku    12345
                         :sku-count/count  1}
                        {:sku-count/sku    54321
                         :sku-count/count  2}
                        ]
            }
           ])

and then an updated cart

(def new-cart [{
                :db/id 17592186045421
                :cart/sku-counts
                       [
                        {:sku-count/sku   12345
                         :sku-count/count 1}
                        {:sku-count/sku   54321
                         :sku-count/count 2}
                        ]
                }
               ])

and it creates two new records even though the data has not changed

is :sku-count/sku an identity field ?

{:db/id                 #db/id[:db.part/db]
  :db/ident              :sku-count/sku
  :db/valueType          :db.type/long
  :db/cardinality        :db.cardinality/one
  :db/doc                "Number of the SKU"
  :db.install/_attribute :db.part/db}

so no

no

It's normal that it gets duplicated then. 2 strategies here :

{:db/id                 #db/id[:db.part/db]
  :db/ident              :cart/sku-counts
  :db/isComponent        true
  :db/valueType          :db.type/ref
  :db/cardinality        :db.cardinality/many
  :db/doc                "SKUs with counts for this cart"
  :db.install/_attribute :db.part/db}

1) have an identity field on the cart items, and do insert or update based on whether the line item is created or updated

2) On each transaction, erase the past line items and replace with the updated ones (it's a 'document-like' approach)

ok, so in either case I have to take care of the state rather than (my naive understanding) that it would be handled by Datomic because from my view there is no novelty

For the second approach, I ended up writing a database function that does this:

thanks - so I guess it’s a common issue!

and how did you insert the snippet (not a Slackista)

click the + sign in the bar at the bottom of the screen

And it's not that common - don't be too eager to use something like this

I take it you mean the function

yes

isn’t this a classic master / detail

@raymcdermott: what do you mean?

I mean the need to update component entries

basket / items ; order / order-lines ; etc...

Most of the time I would add /update /delete the line items individually, instead of 'resetting' the cart all the time

ok, so just so I understand … why didn’t you do that?

i would like to understand the trade-offs

I didn't have a choice, I was migrating from a Document database, and my legacy clientside code relied on this design

ah, ok. I think the add / update / delete approach is simpler for this use case

@val_waeselynck: thanks for the suggestions

it really depends on what granularity is permitted on the client

One thing to consider is that transaction functions are more costly

in performance

it’s just a toy at the moment

so I can decide

I think it’s going to be easier to update the existing counts

the nested map is probably ideal for a case where the data will not change or only rarely

I guess another benefit is that deleting the outer item will delete the components so that’s less tedious

anyway, I’ll get back to my toys - thanks

@raymcdermott: have fun!

following on from the earlier conversation … real life crept in...

anyway I now have this situation...

(clojure.pprint/pprint (d/pull db '[*] cart-id))
{:db/id 17592186045421,
 :cart/id #uuid "d213198b-36b5-4c19-8cb1-e172f59091d9",
 :cart/name "My Shopping Cart",
 :cart/sku-counts
 [{:db/id 17592186045422, :sku-count/sku 12345, :sku-count/count 1}
  {:db/id 17592186045423, :sku-count/sku 54321, :sku-count/count 2}
  {:db/id 17592186045425, :sku-count/sku 12345, :sku-count/count 1}
  {:db/id 17592186045426, :sku-count/sku 54321, :sku-count/count 2}]}

and I would like to retract the first two sku-counts

but I am struggling to locate the right way to achieve the retraction … docs are not feeling obvious

This is close (I think) but I get errors...

(def retraction [:db/retract 17592186045422 :sku-count/sku 12345 :sku-count/count 1])
=> #'shopping-cart-demo.datomic/retraction
@(d/transact conn retraction)
IllegalArgumentExceptionInfo :db.error/not-transaction-data Transaction data element must be a List or Map, got :db/retract  datomic.error/arg (error.clj:57)

or maybe I’m way off… either way I would appreciate a pointer

ok, I have got as far as

@(d/transact conn [[:db.fn/retractEntity 17592186045422 ]])

@raymcdermott: that's the one you want

although the pull API still reports the same data

@raymcdermott: try to always picture the low-level set of datoms that is involved

do I need to touch?

I think you're querying on an old version of your db

correct!

thanks .. I’m getting there