Going back to the handler. We saw in a previous post how the repository recreates the aggregate state re-executing the events. Now that we have the aggregate reference we can call methods on it:
class AddToBasketHandler
def execute(add_to_basket_command)
#repository creates the basket reloading all the events
basket = @repository.get_basket(add_to_basket_command.basket_id)
# now we can operate on basket
basket.add_item (add_to_basket_command.article_id)
basket.commit
end
# more code here...
end
In the above example we are adding a new item to the basket using the method add_item. Now we will see how the add_item is implemented in the basket object:
def add_item (item)
raise_event :item_added, item
end
The implementation is quite curious, all that it does is raising an event calling the method raise_event passing a symbol that specify the event name and the item to add.
The raise_event method is provided by the AggregateRootHelper a module included in the Basket class:
def raise_event(event_name, *args)
uncommited_events << {name: event_name, args: args}
send "on_#{event_name}", *args
end
It collect the event information inside an array of “uncommited events” and then dynamically call a method called on_item_added passing the event arguments.
def on_item_added item
get_item(item).try(:increase_quantity) || @items << BasketItem.new(item)
end
The on_item_added event on basket class is where the job is really done: it increase the quantity of the item if it is already present in the basket otherwise it creates a new item.
You may ask why should be so complecated add an item the the basket?
The raise_event is necessary because the storage is event based not status based, so raising the event is important to track the fact that a new item is being added and the uncommitted_events array in the module is there to store this event (and to persist them later). The event raised could also be useful if we wan to inform other aggregates of the fact the a new item is added: for example a warehouse aggregate could receive the message to manage the available quantity.
