Models and Data
In order to work with data, we need to add another element to our MVC application. Say, for example, we want to do things like remove an event from our list. Well, if two events both have the same name, how might we identify which of those items to delete? We can’t yet. So we need to tweak how we store event data.
Add a Unique Id - Video
Add a Unique Id - Text
Identifying data by a user-defined string called
name is not a sustainable or scalable method
of handling data in most situations. Consider the address book example. How can
we distinguish between two contact entries with the same name field? It is a frequent
practice to add a unique identifier field (sometimes called, or even labelled, uid) to a class
responsible for modeling data. This ensures that our address book can contain two separate entries for
our contacts who have the same name as one another.
To accomplish the same data clarity with events, we’ll add a few things to the event model class:
- A private
- A static counter variable,
- Additional constructor code that:
a. Sets the
idfield to the
nextIdvalue. b. Increments
- A getter method for the
The result in
With these additions, every time a new event object is created, it is assigned a unique integer to its
Create a Data Layer - Video
Create a Data Layer - Text
Now that we’ve begun building a model, it’s a good time to remind ourselves that models are not designed to be data storage containers. Rather, models are meant to shape the data stored in another location into objects that can be used in our application. As we work our way into learning about database usage and service calls, however, we’ll use a Java class to store some data temporarily.
Create a new package called
data and add a class
Event is responsible for organizing
user-inputted information into a Java object,
EventData is responsible for maintaining those objects once they
EventData is itself a Java class that stores events. It contains several methods for managing and
maintaining the event data that simply extend built-in HashMap methods.
The contents of
EventData managing event data, we must once again refactor
EventController to update the items stored in
EventData. In keeping with the objective to remove data handling from the controller, we’ll remove the list
of events at the top of the class. Consequently, for the
displayAllEvents handler, we’ll now use events from
And back to
processCreateEventForm, we’ll make use of the
.add() method from
EventData.add(new Event(eventName, eventDescription));
Delete an Event - Video
Delete an Event - Text
Now that we’ve refined our events storage method, we are able to tackle the task of deleting an object.
To delete an event object from storage, we’ll grab the event’s id and use that
information to call the
remove method of
Since the delete event is user-initiated, a controller will be involved to pass
the information from the user-accessible view to the data layer. So our first step
with this task is to create a controller method to get a view to delete events.
Onto the end of
EventController, add the following method:
We’ll now need to create a new view for the path mapped in the method above. Add a new template,
events/delete.html. This view will reference event id fields in order to distinguish which items the user
will request to delete via checkbox inputs.
We also need a
POST handler to take care of what to do when the delete event information
is submitted by the user. We’ll have this post handler redirect the user back to the home
page once they have selected which event, or events, to remove from storage.
EventController, add another controller method:
This handler method uses the
required = false parameter of
@RequestParam to make this parameter optional. This allows the user to submit the form without any events selected. Once
eventIds is optional, we must also check that it is not
null before entering the loop.
Check Your Understanding
codingevents, which method can we call to list every event object?
codingevents, breaking up the event storage from the
Event model is an example of which object-oriented