belongs_to.rb

#!/usr/bin/env ruby
#
# encoding: utf-8

require 'dm-sqlite-adapter'
require 'dm-migrations'
require 'dm-validations'

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

class Group
  include DataMapper::Resource

  property :id,   Serial
  property :name, String, :length => 10..100

  has n, :students
end

class Student
  include DataMapper::Resource

  property :id,   Serial
  property :name, String, :required => true

  belongs_to :group
  has n, :scores

  def _save
    result = super

    unless result
      validate          if dirty_self?
      validate_parents  if dirty_parents?
      validate_children if dirty_children?
    end

    result
  end

  def validate
    valid?
  end

  def validate_parents
    parent_relationships.each do |relationship|
      parent = relationship.get(self)
      unless parent.valid?
        errors[relationship.name] = parent.errors
      end
    end
  end

  def validate_children
    child_associations.each do |collection|
      if collection.dirty?
        collection.each do |child|
          unless child.valid?
            (errors[collection.relationship.name] ||= []) << child.errors
          end
        end
      end
    end
  end
end

class Score
  include DataMapper::Resource

  property :id,     Serial
  property :result, String, :length => 1..2

  belongs_to :student
end

DataMapper.finalize
DataMapper.auto_migrate!

group   = Group.new :name => 'foo'
student = Student.new :group => group
student.scores.new :result => 'A'
student.scores.new :result => ''

student.save

puts student.errors.inspect

puts "*"*80

puts student.errors[:group].inspect

puts "*"*80

puts student.errors[:scores].inspect