gistfile1.txt

ruby-1.9.2-p0 > u = User.first
 => #<User ...

ruby-1.9.2-p0 > u.children_valid? 
 => true 

ruby-1.9.2-p0 > u.user_roles << UserRole.new
 => [#<UserRole @user_id=1 @role_id=10 @division_id=7>, #<UserRole @user_id=1 @role_id=nil @division_id=nil>] 

ruby-1.9.2-p0 > u.children_valid? 
 => false 

ruby-1.9.2-p0 > u.errors[:user_roles]
 => [#<DataMapper::Validations::ValidationErrors:0xa647e90 @resource=#<UserRole @user_id=1 @role_id=nil @division_id=nil>, @errors={:role_id=>["Role can't be blank"], :division_id=>["Division can't be blank"]}>]