Ash Elixir•3y ago

Calculations and AshPhoenix.Form

First post! So thanks @Zach Daniel for this awesome library. Looks really promising! I'm working on a side project where I have to store employee contracts. For each contract I store the employment (as factor between 0 and 1) and the number of vacation days per year (when working full time).

defmodule MyApp.Users.Contract do
  attributes do
    # other attributes
    attribute :emp_factor, :decimal, allow_nil?: false, default: Decimal.new(1)
    attribute :vac_days, :decimal, allow_nil?: false, default: Decimal.new(25)
  end

  create :create do
    primary? true
    argument :user_id, :integer, allow_nil?: false
    argument :emp_percent, :decimal, allow_nil?: false, default: Decimal.new(100)
  end

  changes do
    change manage_relationship(:user_id, :user, type: :replace), on: :create
    change &change_emp_factor/2
  end

  calculations do
    calculate :vac_days_eff,
              :decimal,
              expr(vac_days * emp_factor)
  
    calculate :emp_percent,
              :decimal,
              expr(emp_factor * 100)
  end

  defp change_emp_factor(changeset, _) do
    case Ash.Changeset.get_argument(changeset, :emp_percent) do
      nil ->
        changeset

      emp_percent ->
        emp_factor = Decimal.div(emp_percent, 100)
        Ash.Changeset.change_attribute(changeset, :emp_factor, emp_factor)
    end
  end

  # relationships etc
end

defmodule MyApp.Users.Contract do
  attributes do
    # other attributes
    attribute :emp_factor, :decimal, allow_nil?: false, default: Decimal.new(1)
    attribute :vac_days, :decimal, allow_nil?: false, default: Decimal.new(25)
  end

  create :create do
    primary? true
    argument :user_id, :integer, allow_nil?: false
    argument :emp_percent, :decimal, allow_nil?: false, default: Decimal.new(100)
  end

  changes do
    change manage_relationship(:user_id, :user, type: :replace), on: :create
    change &change_emp_factor/2
  end

  calculations do
    calculate :vac_days_eff,
              :decimal,
              expr(vac_days * emp_factor)
  
    calculate :emp_percent,
              :decimal,
              expr(emp_factor * 100)
  end

  defp change_emp_factor(changeset, _) do
    case Ash.Changeset.get_argument(changeset, :emp_percent) do
      nil ->
        changeset

      emp_percent ->
        emp_factor = Decimal.div(emp_percent, 100)
        Ash.Changeset.change_attribute(changeset, :emp_factor, emp_factor)
    end
  end

  # relationships etc
end

I'm using AshPhoenix.Form.for_action() and LiveView to create a form (see screenshot) where the user can enter the employment as percentage (argument emp_percent) and the number of vacation days (attribute vac_days) and the hint should show the calculated effective vacation days based on the employment factor. I'm already validating the form on change but I would like vac_days_eff to be recalculated each time (so I can show it in the hint below the field). I might also be a bit confused about action attributes/arguments. Could this be solved using a private action argument?

5 Replies

hannesOP•3y ago

I already got it working with the following helper but I'm wondering if this is the right way:

defp vacation_days_hint(%AshPhoenix.Form{} = form) do
  with {:ok, contract} <- Ash.Changeset.apply_attributes(form.source, force?: true),
       {:ok, contract} <- MyApp.Users.load(contract, [:emp_percent, :vac_days_eff]),
       %Decimal{} <- contract.vac_days_eff,
       %Decimal{} <- contract.emp_percent do
    "#{format(contract.vac_days_eff, :day)} at #{format(contract.emp_percent, :percent)}"
  else
    _ -> nil
  end
end

defp vacation_days_hint(%AshPhoenix.Form{} = form) do
  with {:ok, contract} <- Ash.Changeset.apply_attributes(form.source, force?: true),
       {:ok, contract} <- MyApp.Users.load(contract, [:emp_percent, :vac_days_eff]),
       %Decimal{} <- contract.vac_days_eff,
       %Decimal{} <- contract.emp_percent do
    "#{format(contract.vac_days_eff, :day)} at #{format(contract.emp_percent, :percent)}"
  else
    _ -> nil
  end
end

hannesOP•3y ago

Sorry, forgot to add the screenshot

ZachDaniel•3y ago

Ah, yeah so there aren't any helpers for recalculating on submit, but there is a helper for running calculations without needing the underlying record. Here is how you'd use it:

MyApp.Users.calculate(
  MyApp.Users.Contract, 
  :vac_days_off, 
  refs: [
    vac_days: 10, emp_factor: 10
  ]
)

MyApp.Users.calculate(
  MyApp.Users.Contract, 
  :vac_days_off, 
  refs: [
    vac_days: 10, emp_factor: 10
  ]
)

So then in your UI you could do something like this:

vac_days = AshPhoenix.Form.get_value(form, :vac_days)
emp_factor = AshPhoenix.Form.get_value(form, :emp_factor)
# probably make sure the above are integers
MyApp.Users.calculate(
  MyApp.Users.Contract, 
  :vac_days_off, 
  refs: [
    vac_days: vac_days, emp_factor: emp_factor
  ]
)

vac_days = AshPhoenix.Form.get_value(form, :vac_days)
emp_factor = AshPhoenix.Form.get_value(form, :emp_factor)
# probably make sure the above are integers
MyApp.Users.calculate(
  MyApp.Users.Contract, 
  :vac_days_off, 
  refs: [
    vac_days: vac_days, emp_factor: emp_factor
  ]
)

To clean this up a bit, you can use the code interface

code_interface do
  define_for MyApp.Users # This is required, we need to know what API to call functions on
  define_calculation :vac_days_off, args: [:vac_days, :emp_factor]
end

code_interface do
  define_for MyApp.Users # This is required, we need to know what API to call functions on
  define_calculation :vac_days_off, args: [:vac_days, :emp_factor]
end

And then the above looks like this:

vac_days = AshPhoenix.Form.get_value(form, :vac_days)
emp_factor = AshPhoenix.Form.get_value(form, :emp_factor)
# probably make sure the above are integers
MyApp.Users.Contract.vac_days_off(vac_days, emp_factor)

vac_days = AshPhoenix.Form.get_value(form, :vac_days)
emp_factor = AshPhoenix.Form.get_value(form, :emp_factor)
# probably make sure the above are integers
MyApp.Users.Contract.vac_days_off(vac_days, emp_factor)

hannesOP•3y ago

Thanks for the quick reply! It works! 🙂

ZachDaniel•3y ago

🥳 that helper is relatively new, so there may be some rough edges. The main thing to remember: it can only do expressions that can be done without doing a sql query (or other data layer operations) So if you want to use that helper, you can't have things like fragment/1, for example

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Calculations and AshPhoenix.Form

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