====== Warenkorb ======

  * Produkt (Weiterbildung) wird dem Einkaufskorb hinzugefügt \\ //membership/purchase/%, %=mpid//
  * :?: Nettopreis (amount) wird aus ms_membership_plan.main_amount genommen
  * FIXME->WichtigeFelder
  * Datensatz in db:ms_cart_products
  * Ändern geht nicht, löschen im Warenkorb
  * FIXME Was ist mit den Produkten im Warenkorb, wenn sich der Preis oder MwSt geändert hat?
    * :?: Preis: Produkt aus allen Warenkörben entfernen
    * :?: MwSt: Alle Warenkörbe leeren

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Function (ms_membership.module) **//ms_membership_purchase_membership($mpid)//**
  * $m_plan = ms_membership_load_membership_plan($mpid);
  * ? ms_membership_plan_is_free($m_plan)
    * ja: change_roles, status=3, insert_membership, prepare_mail, notify_admins
    * nein: 
      * $product = ms_core_new_product()
        * **!!!** $product.amount = $m_plan.main_amount;
      * (?recurring): 
        * $product.type='recurring', 
        * $product.recurring_schedule=array(...$m_plan)
      * ms_core_add_cart_product()
        * $cart = ms_core_get_cart()
          * cid = uid || $_SESSION['ms_cart_id']
          * $cart = new MsOrder()
          * $cart.products = ms_core_get_cart_products($cid);
          * $cart.adjustments = ms_core_get_cart_adjustments($cid);
          * $cart.order_type = 'cart';
          * $cart.data = $data; --extra data
        * //hook//_ms_cart_add, //hook//_ms_cart_added_product_alter
        * (?single), (?recurring)
        * **INSERT INTO {ms_cart_products}**
        * //rules//: ms_core_event_product_add_to_cart
        * //hook//_ms_cart_added
      * ms_core_checkout();
        * drupal_goto('ms/checkout')

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