Validating nested models? - validation

Question

To be more specific, "How do I validate that a model requires at least x valid associated models to be created?". I've been trying to validate nested models that get created in the same form as the parent (and ultimately show immediate validations a la jQuery). As a popular example, lets assume the following models and schema.
class Project
include DataMapper::Resource
property :id, Serial
property :title, String, :nullable => false
has 2..n, :tasks
end
class Task
include DataMapper::Resource
property :id, Serial
property :project_id, Integer, :key => true
property :title, String, :nullable => false
belongs_to :project
end
All the validations are done in the schema definitions, as you can see. The important one here is "has 2..n, :tasks". This validation actually works normally, given that the nested task attributes in the params hash will produce valid tasks. If they produce an invalid task, however, then the task won't get created and you'll end up with a Project that has less than 2 tasks, and thus an invalid project object.
As I understand it, this is because it can't figure out if the task attributes are valid or not until it attempts to save the tasks, and since - as far as I know - the tasks can't get saved before the project, the project is unaware if the tasks will be valid or not. Am I correct in assuming this?
Anyway, I was hoping there would be a quick answer, but it seems a lot less trivial then I'd hoped. If you've got any suggestions at all, that would be greatly appreciated.

Answer 1

I actually found a nice solution here using transactions in DataMapper. Basically this transaction tries to save the parent object as well as all the child objects. As soon as one fails to save, then the transaction stops and nothing gets created. If all goes well, then the objects will save successfully.
class Project
def make
transaction do |trans|
trans.rollback unless save
tasks.each do |task|
unless task.save
trans.rollback
break
end
end
end
end
end
This assures that everything is valid before it anything gets saved. I just needed to change my #save and #update methods to #make in my controller code.

Answer 2

SET CONSTRAINTS DEFERRED might be useful if your database engine supports that.
Otherwise, maybe write a stored procedure to do the inserts, and then say that its the resonsibility of the stored procedure to ensure that only correct, validated data is inserted.

Answer 3

There is a model method valid? that runs the validations on a model object before it is saved. So, the simple way to validate the associations would be to use validates_with_block' or 'validates_with_method to check the validations on the associations.
It would look something like this
validates_with_block do
if #tasks.all?{|t|t.valid?}
true
else
[false, "you have an invalid task"]
end
end
Or you could look at dm-association-validator or dm-accepts-nested-attributes
Edit: for extra crazy. run validations on the tasks, then check to see if the only errors are ones related to the association.
validates_with_block do
if #tasks.all?{|t|t.valid?;!t.errors.any?{|e|e[0]==:project}}
true
else
[false, "you have an invalid task"]
end
end