Ash Elixir•3y ago

Can a calculation use attributes of a related resource?

E.g., I have belongs_to :brand, App.Portfolio.Brand and calculate :tagline, :string, expr(brand.name <> " — " <> name) The result:

App.Portfolio.Family
|> Ash.Query.load(:brand)
|> Ash.Query.load(:tagline)
|> App.Portfolio.read!()
** (RuntimeError) Error while building reference: brand.name

App.Portfolio.Family
|> Ash.Query.load(:brand)
|> Ash.Query.load(:tagline)
|> App.Portfolio.read!()
** (RuntimeError) Error while building reference: brand.name

18 Replies

ZachDaniel•3y ago

It cannot 🙂 To do that, you'd define a first type aggregate, and refer to that

aggregates do
  first :brand_name, :brand, :name
end

calculate :tagline, :string, expr(brand_name <> " - " <> name)

aggregates do
  first :brand_name, :brand, :name
end

calculate :tagline, :string, expr(brand_name <> " - " <> name)

waseigoOP•3y ago

Thank you, clear! Then, in iex, I do App.Portfolio.load(f, :tagline), where f is a record of type App.Portfolio.Family. (For reference, for others.) Thanks, @Zach Daniel!

ZachDaniel•3y ago

my pleasure

waseigoOP•3y ago

And is it possible to set an identity on an aggregate? (Probably not, huh?)

ZachDaniel•3y ago

Correct, that is not possible Well...it might be? but it won't create a unique index when making migrations, for example What would you want the identity for?

waseigoOP•3y ago

I'm rewriting a Product Data Management thingy I built last year in Python/FastAPI, now with Ash, and instead of copying text between tables, I build my database schema properly. So, a sales :item has a unique 7-digit code. The first 3 digits come from the :code field of the product :category it belongs to. The last 4 digits come from the :code field of the product :variant it is linked with. Within App.Portfolio.Item I define two aggregates (:category_code, :variant_code) as you explained above. Then I was thinking of defining a calculation that concatenates the two aggregates into an :item_code calculated field. The idea is to be able to lookup an App.Portfolio.Item by this :item_code. Possible?

ZachDaniel•3y ago

Yes, definitely possible 🙂 I think the identity may work for you in that case, actually But you also don't necessarily need it for look ups

waseigoOP•3y ago

Ah, so it works on the calculation

ZachDaniel•3y ago

You can do:

Resource
|> Ash.Query.filter(item_code == <whatever>)
|> Api.read_one()

Resource
|> Ash.Query.filter(item_code == <whatever>)
|> Api.read_one()

and things like that. There are various ways to do "get" style actions with or without a corresponding identity

waseigoOP•3y ago

I also noticed that Ash.Resource.Relationships.BelongsTo has both attribute_writable? and writable? whereas Ash.Resource.Relationships.HasOne only has writable?

ZachDaniel•3y ago

Yeah, so belongs to relationships add an attribute, i.e belongs_to :user adds user_id You can control the writability of both the relationship and the generated attribute.

waseigoOP•3y ago

Yep, got it -- but HasOne doesn't have the attribute_writable? option

ZachDaniel•3y ago

Correct, because for has_one, the attribute that changes is on the other resource.

waseigoOP•3y ago

Aha... I see what you mean. To associate A with B, if A has_one B, can you set the relationship field during :create / with an :update, or do you write A's :id into B's a_id ?

ZachDaniel•3y ago

You’d write A’s id into B. has_one is actually not very common in the wild. It’s exactly the same as has_many except the destination is unique on the destination attribute.

waseigoOP•3y ago

Great, it works I was trying with :update by passing the record directly and it didn't work @Zach Daniel , actually, I see that I cannot pass the record directly to the relationship attribute. After load, I get nil on the attribute value. Is my understanding correct that :create and :update can only set attributes that are not relationships?

ZachDaniel•3y ago

By default they accept all public, writable attributes.

waseigoOP•3y ago

I think I lean too much on my Django ORM experience, and this confuses me 😆

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Can a calculation use attributes of a related resource?

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