# * <tt>:too_short</tt> - The error message if the attribute goes under the minimum (default is: "is too short (min is %{count} characters)").
# * <tt>:wrong_length</tt> - The error message if using the <tt>:is</tt> method and the attribute is the wrong size (default is: "is the wrong length (should be %{count} characters)").
# * <tt>:message</tt> - The error message to use for a <tt>:minimum</tt>, <tt>:maximum</tt>, or <tt>:is</tt> violation. An alias of the appropriate <tt>too_long</tt>/<tt>too_short</tt>/<tt>wrong_length</tt> message.
# * <tt>:on</tt> - Specifies when this validation is active (default is <tt>:save</tt>, other options <tt>:create</tt>, <tt>:update</tt>).
# * <tt>:on</tt> - Specifies the context where this validation is active (e.g. <tt>:on => :create</tt> or <tt>:on => :special_rules</tt>)
# * <tt>:if</tt> - Specifies a method, proc or string to call to determine if the validation should
# occur (e.g. <tt>:if => :allow_validation</tt>, or <tt>:if => Proc.new { |user| user.signup_step > 2 }</tt>). The
# method, proc or string should return or evaluate to a true or false value.
# * <tt>:message</tt> - A custom error message (default is: "is not a number").
# * <tt>:on</tt> - Specifies when this validation is active (default is <tt>:save</tt>, other options <tt>:create</tt>, <tt>:update</tt>).
# * <tt>:on</tt> - Specifies the context where this validation is active (e.g. <tt>:on => :create</tt> or <tt>:on => :special_rules</tt>)
# * <tt>:only_integer</tt> - Specifies whether the value has to be an integer, e.g. an integral value (default is +false+).
# * <tt>:allow_nil</tt> - Skip validation if attribute is +nil+ (default is +false+). Notice that for fixnum and float columns empty strings are converted to +nil+.
# * <tt>:greater_than</tt> - Specifies the value must be greater than the supplied value.