cheat sheets.

-----------------------------------------------------------------------------
Method           Option                     Message                    Interp
-----------------------------------------------------------------------------
confirmation_of  -                          :confirmation              -
acceptance_of    -                          :accepted                  -
presence_of      -                          :blank                     -
length_of        :within, :in               :too_short                 count
                 :within, :in               :too_long                  count
                 :is                        :wrong_length              count
                 :minimum                   :too_short                 count
                 :maximum                   :too_long                  count
uniqueness_of    -                          :taken                     -
format_of        -                          :invalid                   -
inclusion_of     -                          :inclusion                 -
exclusion_of     -                          :exclusion                 -
associated       -                          :invalid                   -
numericality_of  -                          :not_a_number              -
                 :greater_than              :greater_than              count
                 :greater_than_or_equal_to  :greater_than_or_equal_to  count
                 :equal_to                  :equal_to                  count
                 :less_than                 :less_than                 count
                 :less_than_or_equal_to     :less_than_or_equal_to     count
                 :odd                       :odd                       -
                 :even                      :even                      -
-----------------------------------------------------------------------------

All messages interpolate (in addition to the above):

  model
  attribute
  value

Keys are tried in this order:

  activerecord.errors.models.CLASS.attributes.ATTRIBUTE.MESSAGE
  activerecord.errors.models.CLASS.MESSAGE
  (the above for all classes up to and not including AR::Base)
  activerecord.errors.messages.MESSAGE

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More info: http://guides.rails.info/i18n.html