validates_inclusion_of

Validates whether the value of the specified attribute is available in a particular enumerable object.

class Person < ActiveRecord::Base
  validates_inclusion_of :gender, :in => %w( m f )
  validates_inclusion_of :age, :in => 0..99
  validates_inclusion_of :format, :in => %w( jpg gif png ), :message => "extension %{value} is not included in the list"
  validates_inclusion_of :states, :in => lambda{ |person| STATES[person.country] }
end

Configuration options:

• :in - An enumerable object of available items. This can be supplied as a proc or lambda which returns an enumerable. If the enumerable is a range the test is performed with Range#cover? (backported in Active Support for 1.8), otherwise with include?.

• :message - Specifies a custom error message (default is: “is not included in the list”).

• :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).

• :on - Specifies when this validation is active. Runs in all validation contexts by default (nil), other options are :create and :update.

• :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.