Rails and MongoDB with MongoMapper

I'm new to Rails development and I'm starting with MongoDB also.

I have been following this Railscast tutorial about complex forms with Rails but I'm using MongoDB as my database. I'm having no problems inserting documents with it's childs and retrieving the data to the edit form, but when I try to update it I get this error

undefined method `assert_valid_keys' for false:FalseClass

this is my entity class

class Project 
  include MongoMapper::Document

  key :name, String, :required => true
  key :priority, Integer

  many :tasks
  after_update :save_tasks

  def task_attributes=(task_attributes)
    task_attributes.each do |attributes|
    if attributes[:id].blank?
      tasks.build(attributes)
    else
      task = tasks.detect { |t| t.id.to_s == attributes[:id].to_s }  
      task.attributes = attributes
    end
  end    
end

def save_tasks
tasks.each do |t|
  if t.should_destroy?
    t.destroy
  else
    t.save(:validate => false)
  end
end

end end

class Task 
include MongoMapper::EmbeddedDocument

key :project_id, ObjectId
key :name, String
key :description, String
key :completed, Boolean

belongs_to :project
attr_accessor :should_destroy

def should_destroy?
  should_destroy.to_i == 1
end 
end

Does anyone knows whats happening here? Thanks