AF
Ash Framework•5mo ago
RootCA

Persisting values from relationships at creation time

Hi there! I'm working on a demo that allows products to be purchased by users. Since prices might change over time, I'd like to put the price of the product at the time of purchase into the order resource record. Given a product with the following attributes 
  attributes do
    uuid_primary_key :id

    attribute :name, :string do
      allow_nil? false
      public? true
    end

    attribute :price, :money do
      allow_nil? false
      public? true
    end

    create_timestamp :inserted_at
    update_timestamp :updated_at
  end
  attributes do
    uuid_primary_key :id

    attribute :name, :string do
      allow_nil? false
      public? true
    end

    attribute :price, :money do
      allow_nil? false
      public? true
    end

    create_timestamp :inserted_at
    update_timestamp :updated_at
  end
and orders defined with 
  attributes do
    uuid_primary_key :id

    attribute :amount, :money do
      allow_nil? false
      public? true
    end

    create_timestamp :inserted_at
    update_timestamp :updated_at
  end

  relationships do
    belongs_to :user, AppName.Accounts.User do
      allow_nil? false
      public? true
    end

    belongs_to :product, AppName.DomainName.Product do
      allow_nil? false
      public? true
    end
  end
  attributes do
    uuid_primary_key :id

    attribute :amount, :money do
      allow_nil? false
      public? true
    end

    create_timestamp :inserted_at
    update_timestamp :updated_at
  end

  relationships do
    belongs_to :user, AppName.Accounts.User do
      allow_nil? false
      public? true
    end

    belongs_to :product, AppName.DomainName.Product do
      allow_nil? false
      public? true
    end
  end
what is the best way to preserve the price value from the product? My first try adds a purchase action to the Order resource: 
create :purchase_product do
  accept [:product_id]
  change relate_actor(:user)
  change fn changeset, context ->
    product_id = Ash.Changeset.get_attribute(changeset, :product_id)
    product = AppName.DomainName.get_product!(product_id)
    Ash.Changeset.change_attribute(changeset, :amount, product.price)
  end
end
create :purchase_product do
  accept [:product_id]
  change relate_actor(:user)
  change fn changeset, context ->
    product_id = Ash.Changeset.get_attribute(changeset, :product_id)
    product = AppName.DomainName.get_product!(product_id)
    Ash.Changeset.change_attribute(changeset, :amount, product.price)
  end
end
but this causes me to lose the builtin "not found" error response if the product_id is invalid. I also considered an after_action, but it didn't feel right to force change an attribute: 
change set_attribute(:amount, "$0.00")
      
change after_action(fn changeset, context ->
  product_id = Ash.Changeset.get_attribute(changeset, :product_id)
  product = AppName.DomainName.get_product!(product_id, actor: context[:actor])
  Ash.Changeset.force_change_attribute(changeset, :amount, product.price)
end)
change set_attribute(:amount, "$0.00")
      
change after_action(fn changeset, context ->
  product_id = Ash.Changeset.get_attribute(changeset, :product_id)
  product = AppName.DomainName.get_product!(product_id, actor: context[:actor])
  Ash.Changeset.force_change_attribute(changeset, :amount, product.price)
end)
Solution:
``` change before_action(fn changeset, context -> product_id = Ash.Changeset.get_attribute(changeset, :product_id) product = AppName.DomainName.get_product!(product_id, actor: context.actor) ...
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6 Replies
RootCA
RootCAOP•5mo ago
oof, I meant before_action—not after_action
Solution
RootCA
RootCA•5mo ago
change before_action(fn changeset, context ->
  product_id = Ash.Changeset.get_attribute(changeset, :product_id)
  product = AppName.DomainName.get_product!(product_id, actor: context.actor)

  Ash.Changeset.force_change_attribute(changeset, :amount, product.price)
end)
change before_action(fn changeset, context ->
  product_id = Ash.Changeset.get_attribute(changeset, :product_id)
  product = AppName.DomainName.get_product!(product_id, actor: context.actor)

  Ash.Changeset.force_change_attribute(changeset, :amount, product.price)
end)
ZachDaniel
ZachDaniel•5mo ago
That is likely how I'd do it
RootCA
RootCAOP•5mo ago
Got it! I just wasn't sure if there was something obvious I was missing since it seems like a scenario that isn't the most uncommon
ZachDaniel
ZachDaniel•5mo ago
Yeah, ideally in the future we will have expression basd attribute setting on creates, which only works on updates i.e change atomic_update(:amount, expr(product.price)) At some point in the future we'll have change atomc_set(:amount, expr(...))
RootCA
RootCAOP•5mo ago
That sounds awesome! With that not present, I'll proceed with the before_action forced change. Thanks!

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