Creating a One-to-Many Relationship

The previous section established a persistent many-to-one relationship between Event objects and EventCategory objects. This section demonstrates how to create the inverse one-to-many relationship.

Setting Up the Relationship - Video

Setting Up the Relationship - Text

Creating a one-to-many relationship from categories to events means that each EventCategory object will have access to each event that belongs to it. Near the top of EventCategory, add a new field to store these events.

Model Configuration

private final List<Event> events = new ArrayList<>();

The events field will be initialized when an EventCategory object is created. It is marked final to ensure that the collection isn’t deleted or replaced. Since it is final, only a getter is needed; add one near the bottom of the class.

To establish a persistent relationship, we use the @OneToMany annotation on events. This annotation requires a parameter to let Hibernate know how to determine which events belong to a given category object.

@OneToMany(mappedBy = "eventCategory")
private final List<Event> events = new ArrayList<>();

By specifying mappedBy = "eventCategory", we are telling Hibernate that for a given category object, someCategory, the events collection should be populated by all events for which the eventCategory field is set to someCategory. To determine this, Hibernate will look at the foreign key column on the events table.

Controller Updates

In order to use this newly established relationship, we need a controller and view that access the events for a given category. We will create a new view that lists events by category. In other words, given a category ID, the view will display a list of all events in the given category.

The end result will look like this:

Notice that this view is extremely similar to our main view listing all events. The only differences are the title and the specific events in the list. Since both title and events are values that we pass into our events/index.html template, we can use the same template for this view as we have been using for the view at route /events.

Taking this one step further, we will use the same controller as well. This will allow us to have a route, /events that displays all events if no eventId is specified, but displays a filtered list when a valid eventId is provided. Let’s get to work in EventController.

The method at the route /events is displayAllEvents. Since this handler will no longer always display all events, we rename it to displayEvents.

Let’s view the finished handler method and break down the changes one-by-one:

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@GetMapping
public String displayEvents(@RequestParam(required = false) Integer categoryId, Model model) {

   if (categoryId == null) {
      model.addAttribute("title", "All Events");
      model.addAttribute("events", eventRepository.findAll());
   } else {
      Optional<EventCategory> result = eventCategoryRepository.findById(categoryId);
      if (result.isEmpty()) {
            model.addAttribute("title", "Invalid Category ID: " + categoryId);
      } else {
            EventCategory category = result.get();
            model.addAttribute("title", "Events in category: " + category.getName());
            model.addAttribute("events", category.getEvents());
      }
   }

   return "events/index";
}

Update 1: Optional categoryId Parameter

In order to filter events by category, we need the ID of a category to filter by. The handler now has a new parameter:

@RequestParam(required = false) Integer categoryId

This allows requests to URLs like /events?categoryId=1. By specifying required = false we are telling Spring that it should call this handler for requests to the path /events even if no such ID is specified. This allows us to preserve our existing behavior that lists all events at /events. If no categoryId is specified in the request, then the categoryId parameter will be null when the handler is called.

Update 2: null Check

Much of the method consists of a large if/else if/else block.

Before filtering by category, we check for the existence of the categoryId parameter. The first part of the conditional block is:

if (categoryId == null) {
   model.addAttribute("title", "All Events");
   model.addAttribute("events", eventRepository.findAll());
}

If no categoryId is passed in, we carry out the same behavior as before, passing all events into the view.

Update 3: Ensuring the Category Object Exists

If the conditional check above fails, then we have a non-null categoryId. However, this does not guarantee that an object with the given ID exists in the database. We must manually check for the existence of such an object.

Here’s our conditional block with the next piece added:

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if (categoryId == null) {
   model.addAttribute("title", "All Events");
   model.addAttribute("events", eventRepository.findAll());
} else {
   Optional<EventCategory> result = eventCategoryRepository.findById(categoryId);
   if (result.isEmpty()) {
         model.addAttribute("title", "Invalid Category ID: " + categoryId);
   } else {
         // TODO 
   }
}

Line 36 queries our repository for a category object with the given ID. Notice that the findById method returns not an EventCategory object, but something of type Optional<EventCategory>. Huh?

Referring to the documentation for the CrudRepository.findById method, we see that it returns an Optional instance. We don’t currently know what the heck that is, but referring to the documentation for Optional describes how to use this class.

Essentially, by returning an Optional<EventCategory> object, the findById method is taking into account the exact situation we are in, that an object with such an ID may not exist. An Optional object can be thought of as a container for the results of our query. If such an object does exist, we can get it using Optional.get(). Otherwise, Optional.isEmpty() will return true.

On lines 37-38 we use this fact to check to see if an EventCategory object was found. If not, we display an error message in the title.

Update 4: Retrieving Events For the Given Category

We are finally ready to consider the full conditional:

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if (categoryId == null) {
   model.addAttribute("title", "All Events");
   model.addAttribute("events", eventRepository.findAll());
} else {
   Optional<EventCategory> result = eventCategoryRepository.findById(categoryId);
   if (result.isEmpty()) {
         model.addAttribute("title", "Invalid Category ID: " + categoryId);
   } else {
         EventCategory category = result.get();
         model.addAttribute("title", "Events in category: " + category.getName());
         model.addAttribute("events", category.getEvents());
   }
}

Lines 39-42 will only execute if we know for sure that a category with the given ID exists. In this case, we extract the category from the Optional object result using the get() method. Then we create an appropriate title using the category name, and pass the events belonging to the category into the view.

Line 42 is what we have been building up to this entire time. Our category object now has an events field that is populated using the @OneToMany annotation that we added near the beginning of this section, allowing the inverse relationship to be established.

Now, not only do events have categories, but categories know which events they contain.

Linking to Filtered Views

With our new controller logic in place, we have one small update to make before we can fully use this new functionality.

We need to provide a way for the user to access our new filtered views. In the eventCategories/index.html template, modify the loop within the table to include a link to the filtered view for each category:

<tr th:each="category : ${categories}">
   <td><a th:text="${category.name}" th:href="'/events?categoryId=' + ${category.id}"></a></td>
</tr>

The logic in th:href uses string concatenation to create the appropriate URL for each category.

With this code in place, start up our application and test out the new functionality! Be sure to navigate to /eventCategories to see your new links to the filtered views. Also, be sure to test that /events still displays all events.

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