validate

Is it really deprecated?

I think the deprecation notice is in the wrong place, it is actually the instance method ActiveRecord::Validations#validate that has been deprecated.

The same applies to ActiveRecord::Validations#validate_on_create and ActiveRecord::Validations#validate_on_update: they have both been deprecated in favour of the class methods validate_on_create and validate_on_update.

More on deprecation

This is not deprecated. I think the docs are confused because the validate, validate_on_create, and validate_on_update methods are actually callbacks and not explicitly defined on their own. The correct usage is the same as in the docs above.

You can call several times

You can call it several times, like:

class Comment < ActiveRecord::Base
  validate :must_be_friends
  validate :must_be_awesome
  ...

or with several arguments:

class Comment < ActiveRecord::Base
  validate :must_be_friends, :must_be_awesome
  ...

You can call it with an { :on => ~~~ } as the last argument

For example:

validate :must_be_friends, :on => :create