cheat sheets.

validate - define it with a custom validation to run on all saves 

validate_on_create - define it with a custom validation to run on all creates

validate_on_update - define it with a custom validation to run on all updates

All validation helpers can take :if, :on, and :message unless otherwise stated.

validates_acceptance_of(:attribute, :value) - validates that the gived attribute
is set to "1" or the passed value.  Does not have to be a database attribute.

validates_associated(*attributes) - validates whether the associated object or
objects are all valid themselves. works with any kind of association.  does not
take message

validates_confirmation_of(:attribute) - validates that
attributename_confirmation is equal to the attribute.  The confirmation variable
does not need to be in the database

validates_exclusion_of(:attribute, :if) - validates that the value of the
specified attribute is not in a particular enumerable object specified by the
:if argument

validates_format_of(:attribute, :with) - validates whether the value of the
specified attribute is of the correct form by matching it against the regular
expression provided in the :with argument

validates_inclusion_of(:attribute, :if) - validates that the value of the
specified attribute is in a particular enumerable object specified by the :if
argument

validates_length_of(:attribute, *options) - validates that the specified
attribute matches the length restrictions supplied. only one option can be used
at a time.  options: maximum, minimum, is, within, in, allow_nil. message
options: too_long, too_short, wrong_length

validates_numericality_of(:attribute) - validates whether the value of the
specified attribute is numeric by trying to convert it to a float with
Kernel.Float (if integer is false) or applying it to the regular expression
/^[+\-]?\d+$/ (if integer is set to true). options: allow_nil, only_integer

validates_presence_of(:atrribute) - validates that the specified attributes are
not blank (as defined by Object#blank?)

validates_uniqueness_of(:attribute) = calidates whether the value of the
specified attributes are unique across the system. options: scope (one or more
columns by which to limit the scope of the uniquness constraint_