module ActiveModel::Validations::ClassMethods

NOTE: Calling +validate+ multiple times on the same method will overwrite previous definitions.

value.
method, proc, or string should return or evaluate to a +true+ or +false+
or unless: Proc.new { |user| user.signup_step <= 2 }). The
determine if the validation should not occur (e.g. unless: :skip_validation,
* :unless - Specifies a method, proc, or string to call to
proc or string should return or evaluate to a +true+ or +false+ value.
or if: Proc.new { |user| user.signup_step > 2 }). The method,
if the validation should occur (e.g. if: :allow_validation,
* :if - Specifies a method, proc, or string to call to determine
on: [:create, :custom_validation_context])
on: :custom_validation_context or
or an array of symbols. (e.g. on: :create or
Runs in all validation contexts by default +nil+. You can pass a symbol
* :on - Specifies the contexts where this validation is active.
Options:

It's not possible to halt the validate callback chain.
Note that the return value of validation methods is not relevant.

end
end
errors.add(:base, 'Must be friends to leave a comment') unless commenter.friend_of?(commentee)
validate do

include ActiveModel::Validations
class Comment

Or with a block where +self+ points to the current record to be validated:

end
end
errors.add(:base, 'Must be friends to leave a comment') unless commenter.friend_of?(commentee)
def must_be_friends

end
comment.must_be_friends
validate do |comment|

include ActiveModel::Validations
class Comment

With a block which is passed with the current record to be validated:

end
end
errors.add(:base, 'Must be friends to leave a comment') unless commenter.friend_of?(commentee)
def must_be_friends

validate :must_be_friends

include ActiveModel::Validations
class Comment

This can be done with a symbol pointing to a method:

you're looking for more descriptive declaration of your validations.
overriding the +validate+ instance method becomes too unwieldy and
Adds a validation method or block to the class. This is useful when

def validate(*args, &block)
  options = args.extract_options!
  if args.all?(Symbol)
    options.each_key do |k|
      unless VALID_OPTIONS_FOR_VALIDATE.include?(k)
        raise ArgumentError.new("Unknown key: #{k.inspect}. Valid keys are: #{VALID_OPTIONS_FOR_VALIDATE.map(&:inspect).join(', ')}. Perhaps you meant to call `validates` instead of `validate`?")
      end
    end
  end
  if options.key?(:on)
    options = options.dup
    options[:on] = Array(options[:on])
    options[:if] = [
      ->(o) { !(options[:on] & Array(o.validation_context)).empty? },
      *options[:if]
    ]
  end
  set_callback(:validate, *args, options, &block)
end