Rails conditional validation through one to many relationship - ruby-on-rails

Question

I am trying to make a conditional validation of a field. Such that it only validates if another field is a specific value. The problem here is, that this other field is a one to many relation, and I can't seem to get it working.
Here is the relevant code:
class CreateInvolvedPartyTypes < ActiveRecord::Migration
def change
create_table :involved_party_types do |t|
t.string :code
t.string :name
t.timestamps null: false
end
end
end
class CreateInvolvedParties < ActiveRecord::Migration
def change
create_table :involved_parties do |t|
t.string :first_name
t.string :last_name
t.references :involved_party_type
t.timestamps null: false
end
end
end
class InvolvedParty < ActiveRecord::Base
def ipt_cd?
self.involved_party_type.code == 'I'
end
validates :first_name, presence: { message: "Please insert first name" }
validates :last_name, presence: { message: "Please insert last name" }, :if => :ipt_cd?
validates :involved_party_type, presence: { message: "Please select involved party type" }
belongs_to :involved_party_type
end
The above code fails with:
undefined method `code' for nil:NilClass
Thanks for your help

Answer 1

The error means that self.involved_party_type in InvolvedParty#ipt_cd? is nil. You should test the presence of involved_party_type before calling #code on it, or use #try.
def ipt_cd?
return false if involved_party.nil?
involved_party_type.code == 'I'
end
def ipt_cd?
self.involved_party_type.try(:code) == 'I'
end
Or you can avoid the problem by only invoking the validation if involved_party_type exists.
validates :last_name, presence: { message: "Please insert last name" }, if: -> { involved_party_type && ipt_cd? }

Answer 2

I think the issue is that you're getting confused with calling instance and class level data.
"Instance" data is populated each time a class is invoked
"Class" data is static, always appending to the class
The cosmetic difference between the two is that class data is typically called through self (EG def self.method & self.attribute), whilst instance data is called with "naked" attributes (IE without self).
You're calling the following:
def ipt_cd?
self.involved_party_type.code == 'I'
end
The problem is that you're referencing self as if it's a piece of class data. What you want is the instance equivalent:
def ipt_cd?
involved_party_type.code == 'I'
end
As the other answer states, your error is caused by a piece of data having no method for code, meaning it's nil.
The casue of this is here (solution is above -- IE remove self):
involved_party_type.code == 'I'
Thus, if you want to make sure you don't receive this error, you'll have to ensure that involved_party_type is present. This can be done by first ensuring you're referencing the instance variant of the data, followed by ensuring it's there anyway. The other answer provided the best way to achieve that.
Finally, I think your structure could be improved.
Referencing the actual data representation of an associated field is bad practice in my opinion. You're trying to create a new piece of data, and yet you're referencing an associated attribute?
Why not do the following:
#app/models/party_type.rb
class PartyType < ActiveRecord::Base
has_many :involved_parties
end
class InvolvedParty < ActiveRecord::Base
belongs_to :party_type
validates :first_name, :party_type, presence: true
validates :last_name, presence: { message: "Please insert last name" }, if: :cd?
private
def cd?
party_type == PartyType.find_by(code: "I").pluck(:id)
end
end
This will send another DB query but it removes the dependency on specific data. Your current setup is not relying on foreign keys, but on a value which may change.
Whilst this recommendation also relies on data (IE code == I), it uses it as a quantifier within ActiveRecord. That is, you're not comparing the data, but the relationship.