Validate model for certain action - ruby-on-rails

Question

I need to validate a model only for a certain action (:create). I know this is not a good tactic, but i just need to do this in my case.
I tried using something like :
validate :check_gold, :if => :create
or
validate :check_gold, :on => :create
But i get errors. The problem is that i cannot have my custom check_gold validation execute on edit, but only on create (since checking gold has to be done, only when alliance is created, not edited).
Thanx for reading :)
I'm appending some actual code :
attr_accessor :required_gold, :has_alliance
validate :check_gold
validate :check_has_alliance
This is the Alliance model. :required_gold and :has_alliance are both set in the controller(they are virtual attributes, because i need info from the controller). Now, the actual validators are:
def check_gold
self.errors.add(:you_need, "100 gold to create your alliance!") if required_gold < GOLD_NEEDED_TO_CREATE_ALLIANCE
end
def check_has_alliance
self.errors.add(:you_already, "have an alliance and you cannot create another one !") if has_alliance == true
end
This works great for create, but i want to restrict it to create alone and not edit or the other actions of the scaffold.

Answer 1

All ActiveRecord validators have a :on option.
validates_numericality_of :value, :on => :create
Use the validate_on_create callback instead of validate:
validate_on_create :check_gold
validate_on_create :check_has_alliance
Edit:
If you use validates_each you can use the standard options available for a validator declaration.
validates_each :required_gold, :has_alliance, :on => :create do |r, attr, value|
r.check_gold if attr == :required_gold
r.check_has_alliance if attr == :has_alliance
end

Answer 2

Like Sam said, you need a before_create callback. Callbacks basically mean 'execute this method whenever this action is triggered'. (More about callbacks here : http://api.rubyonrails.org/classes/ActiveRecord/Callbacks.html).
This is what you want in your model:
before_create :check_gold
# other methods go here
private # validations don't need to be called outside the model
def check_gold
# do your validation magic here
end
The above method is the simplest to do what you want, but FYI there's also a way to use a before_save callback to execute additional actions on creation:
before_save :check_gold_levels
# other methods
private
def check_gold_levels
initialize_gold_level if new? # this will be done only on creation (i.e. if this model's instance hasn't been persisted in the database yet)
verify_gold_level # this happens on every save
end
For more info on 'new?' see http://api.rubyonrails.org/classes/ActiveResource/Base.html#method-i-new%3F

Answer 3

You need to look into callbacks. Someone once told me this and I didn't understand what they meant. Just do a search for rails callbacks and you will get the picture.
In your model you need to do a callback. The callback you need is before_create and then before a object is created you will be able to do some logic for check for errors.
model.rb
before_create :check_gold_validation
def check_gold_validation
validate :check_gold
end
def check_gold
errors.add_to_base "Some Error" if self.some_condition?
end