dry-struct How to conditionally validate one attribute?

Question

I'm using dry-types and dry-struct and I would like to have a conditional validation.

for the class:

class Tax < Dry::Struct
  attribute :tax_type, Types::String.constrained(min_size: 2, max_size: 3, included_in: %w[IVA IS NS])
  attribute :tax_country_region, Types::String.constrained(max_size: 5)
  attribute :tax_code, Types::String.constrained(max_size: 10)
  attribute :description, Types::String.constrained(max_size: 255)
  attribute :tax_percentage, Types::Integer
  attribute :tax_ammount, Types::Integer.optional
end

I want to validate tax_ammount as an Integer and mandatory if `tax_type == 'IS'.

Answer 1

dry-struct is really for basic type assertion and coercion.

If you want more complex validation then you probably want to implement dry-validation as well (as recommended by dry-rb)

See Validating data with dry-struct which states

Please don’t. Structs are meant to work with valid input, it cannot generate error messages good enough for displaying them for a user etc. Use dry-validation for validating incoming data and then pass its output to structs.

The conditional validation using dry-validation would be something like

TaxValidation = Dry::Validation.Schema do

  # Could be:
  #   required(:tax_type).filled(:str?, 
  #      size?: 2..3, 
  #      included_in?: %w(IVA IS NS)) 
  # but since we are validating against a list of Strings I figured the rest was implied
  required(:tax_type).filled(included_in?: %w(IVA IS NS)) 
  optional(:tax_amount).maybe(:int?)

  # rule name is of your choosing and will be used 
  # as the errors key (i just chose `tax_amount` for consistency)
  rule(tax_amount:[:tax_type, :tax_amount]) do |tax_type, tax_amount|
    tax_type.eql?('IS').then(tax_amount.filled?) 
  end
end

• This requires tax_type to be in the %w(IVA IS NS) list;
• Allows tax_amount to be optional but if it is filled in it must be an Integer (int?) and;
• If tax_type == 'IS' (eql?('IS')) then tax_amount must be filled in (which means it must be an Integer based on the rule above).

Obviously you can validate your other inputs as well but I left these out for the sake of brevity.

Examples:

TaxValidation.({}).success?
#=> false
TaxValidation.({}).errors
# => {:tax_type=>["is missing"]}
TaxValidation.({tax_type: 'NO'}).errors
#=>  {:tax_type=>["must be one of: IVA, IS, NS"]}
TaxValidation.({tax_type: 'NS'}).errors
#=>  {}
TaxValidation.({tax_type: 'IS'}).errors
#=> {:tax_amount=>["must be filled"]}
TaxValidation.({tax_type: 'IS',tax_amount:'NO'}).errors
#=> {:tax_amount=>["must be an integer"]}
TaxValidation.({tax_type: 'NS',tax_amount:12}).errors 
#=> {}
TaxValidation.({tax_type: 'NS',tax_amount:12}).success?
#=> true 

Answer 2

There is an alternative solution - without validation rules that essentially duplicate the struct attributes.

module TaxChore
  class BaseTax < Dry::Struct
    attribute :type, Types::String.enum('IVA','NS')
    # ...
    attribute? :amount, Types::Integer.default(0)
  end

  class MandatoryTax < BaseTax
    attribute :type, Types::String.enum('IS')
    attribute :amount, Types::Integer
  end

  Tax = BaseTax | MandatoryTax

  def self.run
    tax = Tax.(type: 'IVA')
    p tax
    tax = Tax.(type: 'IVA', amount: 21)
    p tax
    tax = Tax.(type: 'IS', amount: 42)
    p tax
    begin
      tax = Tax.(type: 'IS')
      p tax
    rescue Dry::Struct::Error => e
      puts e
    end
  end

  run
end

Outputs:

#<TaxChore::BaseTax type="IVA" amount=0>
#<TaxChore::BaseTax type="NS" amount=21>
#<TaxChore::MandatoryTax type="IS" amount=42>
[TaxChore::MandatoryTax.new] :amount is missing in Hash input

(N.B: I've removed the redundant tax_ prefix, since tax.type is just as clear but shorter.)