validates_exclusion_of

Validates that the value of the specified attribute is not in a particular enumerable object.

  class Person < ActiveRecord::Base
    validates_exclusion_of :username, :in => %w( admin superuser ), :message => "You don't belong here"
    validates_exclusion_of :age, :in => 30..60, :message => "This site is only for under 30 and over 60"
    validates_exclusion_of :format, :in => %w( mov avi ), :message => "extension %{value} is not allowed"
  end

Configuration options:

• :in - An enumerable object of items that the value shouldn’t be part of.
• :message - Specifies a custom error message (default is: "is reserved").
• :allow_nil - If set to true, skips this validation if the attribute is nil (default is false).
• :allow_blank - If set to true, skips this validation if the attribute is blank (default is false).
• :if - Specifies a method, proc or string to call to determine if the validation should occur (e.g. :if => :allow_validation, or :if => Proc.new { |user| user.signup_step > 2 }). The method, proc or string should return or evaluate to a true or false value.
• :unless - Specifies a method, proc or string to call to determine if the validation should not occur (e.g. :unless => :skip_validation, or :unless => Proc.new { |user| user.signup_step <= 2 }). The method, proc or string should return or evaluate to a true or false value.