DataMapper plugin providing access to validation errors of associated parent and children objects. See the example below to get an idea on how it works.

DataMapper.setup :default, "sqlite::memory"

class User
  include DataMapper::Resource

  property :id, Serial
  property :name, String

  belongs_to :group
  has n, :roles
end

class Group
  include DataMapper::Resource

  property :id, Serial
  property :name, String, :length => 10..255

  has n, :users
end

class Role
  include DataMapper::Resource

  property :id, Serial
  property :name, String, :length => 4..10

  belongs_to :user
end

DataMapper.finalize
DataMapper.auto_migrate!

user  = User.new(:name => "John")
group = Group.new(:name => "Too Short")
role  = Role.new(:name => "Way Too Long")

user.group = group
user.roles << role

user.save
# => false

user.errors[:group].inspect
# => <DataMapper::Validations::ValidationErrors:0xa098a30 @resource=#<Group @id=nil @name="Too Short">, @errors={:name=>["Name must be between 10 and 255 characters long"]}>

user.errors[:roles].inspect
# => [#<DataMapper::Validations::ValidationErrors:0xa0931ac @resource=#<Role @id=nil @name="Way Too Long" @user_id=nil>, @errors={:name=>["Name must be between 4 and 10 characters long"], :user_id=>["User must not be blank"]}>]