Got ActiveRecord::AssociationTypeMismatch on model

I got a problem on saving model with my rest API. I have a Card model with many tasks and one Customer associated :

class Card < ActiveRecord::Base
  belongs_to :customer

  has_many :card_tasks
  has_many :tasks, :through => :card_tasks

  accepts_nested_attributes_for :tasks
  accepts_nested_attributes_for :card_tasks
  accepts_nested_attributes_for :customer

end


class CardTask < ActiveRecord::Base
  belongs_to :task
  belongs_to :card

  accepts_nested_attributes_for :task
  accepts_nested_attributes_for :card
end

class Task < ActiveRecord::Base
    has_many :cards, :through => :card_tasks
    has_many :card_tasks
end

When i send a json like this :

{
    "card" = > {
        "miscellaneous" = > "Obervations diverses", 
        "heater" = > "0", 
        "water_quality" = > "", 
        "customer" = > {
          "id" = > "2", "house_name" = > "house_name2", "city" = > "city_2", "lastname" = > "lastname2", "sci" = > "sci2", "postal_code" = > "potal_code_2", "address_line_1" = > "address_line_2", "updated_at" = > "2012-03-05 18:20:57 +0000", "created_at" = > "2012-03-05 18:20:54 +0000", "firstname" = > "firstname2", "address_line_2" = > "address_line_3", "water_used" = > "0"
         },
         "tasks" = > [
          {
            "title" = > "Nettoyage ligne eau", "id" = > "6", "updated_at" = > "2012-02-17 08:40:47 +0000", "created_at" = > "2012-02-17 08:40:47 +0000"
          }, 
          {
            "title" = > "Surveillance", "id" = > "4", "updated_at" = > "2012-02-17 08:40:47 +0000", "created_at" = > "2012-02-17 08:40:47 +0000"
          }
         ]
    }
}

My create action :

  def create
      card = Card.new(params[:card])
      if (card.save)
        respond_with({ :card => card} , :location => nil, status: :created) and return
      end
      respond_with({ :errors => card.errors }, :location => nil, status: :unprocessable_entity)  and return
  end

When doing this, i got a :

ActiveRecord::AssociationTypeMismatch (Task(#70249431354580) expected, got ActiveSupport::HashWithIndifferentAccess(#70249421573300)):
  app/controllers/cards_controller.rb:14:in `new'
  app/controllers/cards_controller.rb:14:in `create'

What did i do wrong?

The problem is in the JSON structure, Rails expects tasks_attributes, not tasks . Check this question for details.