I have a Mission model that has_many Task, and the Task belongs_to Mission

For security I've made this validation on the Task Model:

  validates_presence_of :mission_id
  validates_numericality_of :mission_id

But the problem is that when create a Mission and add tasks like this:

 @mission.tasks.build

The validation returns error, because the mission id on the task is null ( the mission wasn't yet created )

If I delete the validation, the Mission and Task is created successfuly, but how can I keep the validation and still have this work? I could do a callback after the save, but I don't think that's right, because I don't want to save Tasks without a mission_id.

P.S. I'm hidding my mission field on the form. If I have it visible, it will show the currect mission and everything is ok. But if I hidde it the error happens.

<%=  f.hidden_field :mission, :label => "Missão" %>

Is the form reseting the attributes given by the controller on the new action?