Part 8 \u2013 @JoinColumn Annotation<\/strong><\/p>\nThe @JoinColumn annotation is used to specify the actual column(in the table) used for the mapping. This is done at the entity that owns the mapping.<\/p>\n
In the case of User-Post relationship, the Post owns the relationship and automatically creates the user_id column. This is the JoinColumn.<\/p>\n
Similarly, the\u00a0 Location-User relationship, the Location owns the relationship and provides the location_id column.<\/p>\n
Follow the steps below to add the @JoinColumn annotation<\/p>\n
Step 1:<\/strong> In the Location field of the User entity, add @JoinColumn(name=”location_id”)<\/p>\nStep 2:<\/strong> In the User field of the Post entity, add @JoinColumn(name=”user_id”)<\/p>\nStep 3:<\/strong> Relaunch and check the tables in the h2 console (nothing changes though!)<\/p>\n <\/p>\n
Part 9 – Setup findAll() and findById() for all Entities<\/strong><\/p>\nWrite the methods to in the RestController files for Post, User and Location.<\/p>\n
(You should be able to do this by now, else, watch the video and follow along)<\/p>\n
The three RestController and Services files would contain the following:<\/p>\n
UserController<\/strong><\/p>\n@Autowired<\/span>\nprivate<\/span> UserService userService;<\/span>\n\n@GetMapping<\/span>(<\/span>\"\/users\"<\/span>)<\/span>\npublic<\/span> List<<\/span>User><\/span> getAllUsers<\/span>()<\/span> {<\/span>\n return<\/span> userService.<\/span>findAll<\/span>();<\/span>\n}<\/span>\n\n@GetMapping<\/span>(<\/span>\"\/users\/{id}\"<\/span>)<\/span>\npublic<\/span> Optional<<\/span>User><\/span> getUserById<\/span>(<\/span>@PathVariable<\/span> Integer id)<\/span> {<\/span>\n return<\/span> userService.<\/span>findById<\/span>(<\/span>id);<\/span>\n}<\/span>\n<\/pre>\n <\/p>\n
UserService<\/strong><\/p>\n@Autowired<\/span>\nprivate<\/span> UserRepository userRepository;<\/span>\n\npublic<\/span> List<<\/span>User><\/span> getAllUsers<\/span>()<\/span> {<\/span>\n\treturn<\/span> (<\/span>List<<\/span>User>)<\/span> userRepository.<\/span>findAll<\/span>();<\/span>\n}<\/span>\n\npublic<\/span> Optional<<\/span>User><\/span> getUserById<\/span>(<\/span>Integer id)<\/span> {<\/span>\n\treturn<\/span> userRepository.<\/span>findById<\/span>(<\/span>id);<\/span>\n}<\/span>\n<\/pre>\n <\/p>\n
Step 1:<\/strong> Launch the application and visit http:\/\/localhost\/users to see the list of users.<\/p>\n <\/p>\n
LocationController<\/strong><\/p>\n@Autowired<\/span>\nprivate<\/span> LocationService locationService;<\/span>\n\t\n@GetMapping<\/span>(<\/span>\"\/locations\"<\/span>)<\/span>\npublic<\/span> List<<\/span>Location><\/span> getAllLocations<\/span>()<\/span> {<\/span>\n\treturn<\/span> locationService.<\/span>findAll<\/span>();<\/span>\n}<\/span>\n\t\n@GetMapping<\/span>(<\/span>\"\/locations\/{id}\"<\/span>)<\/span>\npublic<\/span> Optional<<\/span>Location><\/span> getLocationById<\/span>(<\/span>@PathVariable<\/span> Integer id)<\/span> {<\/span>\n\treturn<\/span> locationService.<\/span>findById<\/span>(<\/span>id);<\/span>\n}<\/span>\n<\/pre>\n <\/p>\n
LocationService<\/strong><\/p>\n@Autowired<\/span>\nprivate<\/span> LocationRepository locationRepository;<\/span> \n\npublic<\/span> List<<\/span>Location><\/span> findAll<\/span>()<\/span> {<\/span>\n\treturn<\/span> (<\/span>List<<\/span>Location>)<\/span> locationRepository.<\/span>findAll<\/span>();<\/span>\n}<\/span>\n\npublic<\/span> Optional<<\/span>Location><\/span> findById<\/span>(<\/span>Integer id)<\/span> {<\/span>\n\treturn<\/span> locationRepository.<\/span>findById<\/span>(<\/span>id);<\/span>\n}<\/span>\n<\/pre>\n <\/p>\n
PostController<\/strong><\/p>\n<\/p>\n
@Autowired<\/span>\nprivate<\/span> PostService postService;<\/span>\n\n@GetMapping<\/span>(<\/span>\"\/posts\"<\/span>)<\/span>\npublic<\/span> List<<\/span>Post><\/span> getAllPosts<\/span>()<\/span> {<\/span>\n return<\/span> postService.<\/span>findAll<\/span>();<\/span>\n}<\/span>\n\n@GetMapping<\/span>(<\/span>\"\/posts\/{id}\"<\/span>)<\/span>\npublic<\/span> Optional<<\/span>Post><\/span> getPostById<\/span>(<\/span>@PathVariable<\/span> Integer id)<\/span> {<\/span>\n return<\/span> postService.<\/span>findById<\/span>(<\/span>id);<\/span>\n}<\/span>\n<\/pre>\n <\/p>\n
PostService<\/strong><\/p>\n<\/p>\n
private<\/span> PostRepository postRepository;<\/span>\n\npublic<\/span> List<<\/span>Post><\/span> findAll<\/span>()<\/span> {<\/span>\n\treturn<\/span> (<\/span>List<<\/span>Post>)<\/span> postRepository.<\/span>findAll<\/span>();<\/span>\n}<\/span>\n\npublic<\/span> Optional<<\/span>Post><\/span> findById<\/span>(<\/span>Integer id)<\/span> {<\/span>\n\treturn<\/span> postRepository.<\/span>findById<\/span>(<\/span>id);<\/span>\n}<\/span>\n<\/pre>\n <\/p>\n
Part 10 – Infinite Recursion (StackOverflow Error)<\/strong><\/p>\nWe would now solve a problem that always arises when you have mapping between entities. Let’s see what the problem is first!<\/p>\n
Step 1:<\/strong> In the User entity, right-click and generate Getter and Setters for the posts field<\/p>\n