Associations validation

:validates_presence_of works for belongs_to, has_one and has_many associations in Rails3, so you can safely have:

belongs_to :draft
validates :draft, :presence => true # you will need one associated object
# validates :draft_id, :presence => true would also work, but is not recommended (validation of foreign key).

has_one :team
validates :team, :presence => true # you will need one associated object

has_many :comments
validates :comments, :presence => true # you will need at least one associated object

RailsAdmin will look on both :draft and :draft_id for validation when checking for :required? and for displaying errors messages.

Polymorphic belongs_to will work as well, the same way.

validates_associated is a different thing: it validates the opposite object(s), and nil (no associated object) is valid for belongs_to associations.