So I have two models here:
class Screen < ActiveRecord::Base
belongs_to :user
validates :screen_size, :numericality =>{:less_than_or_equal_to =>
You passed in a string to the :if executable proc/function parameter. When this is a string, it tries to find a function with that name. What you actually want is an anonymous function here using a lambda.
class Screen < ActiveRecord::Base
belongs_to :user
validates :screen_size, :numericality => {:less_than_or_equal_to =>100, :greater_than_or_equal_to => 0}, :if => lambda {|s| s.user.access == 1 }
end
class User < ActiveRecord::Base
has_many :screens
attr_accessible :access
end