dry-struct How to conditionally validate one attribute? - ruby

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.)