Relationship Diagram<\/figcaption><\/figure>\n <\/p>\n
Part 2 \u2013 Setting Up the Application<\/strong><\/p>\nIn this part, we would set up a Spring Boot Starter application. Follow the steps below:<\/p>\n
Step 1:<\/strong> Create a Spring application (you should be able to do this by now). Add the starter web dependency. Also add the starter-jpa dependency.<\/p>\nStep 2:<\/strong> Create three classes corresponding to Post, User and Location. Put them in the models package<\/p>\nStep 3:<\/strong> Also create all the repositories in the repositories package<\/p>\nStep 4:<\/strong> Create the services in the Services package<\/p>\nStep 5:<\/strong> Finally, create the RestControllers in the controllers package<\/p>\n <\/p>\n
Part 3 – @ManyToOne Relationship<\/strong><\/p>\nWe would now configure Many-to-one relationship between the User and the Location entities.\u00a0Similarly, we do the same between the Post and User entities.<\/p>\n
Follow the steps below:<\/p>\n
Step 1:<\/strong> Annotate the Location field of the User entity with @ManyToOne annotation.<\/p>\nStep 2:<\/strong> Annotate the User field of the Post entity with @ManyToOne annotation<\/p>\nWhen you use these annotation, the entity decides to handle the relationship. In the first case, the User entity<\/strong> does this by creating a column called location_id in the User table. Similarly, the Post entity<\/strong> does this by creating a column called user_id<\/p>\n <\/p>\n
Part 4 – Configure the H2 Database<\/strong><\/p>\nH2 Database is an in-memory database that is normally used during development. Now we configure the H2 database following the steps below:<\/p>\n
Step 1:<\/strong> Open the application.properties file. It is in the src\/main\/resources folder<\/p>\nStep 2:<\/strong> Add the following code in the application.properties file<\/p>\n <\/p>\n
spring.<\/span>h2<\/span>.<\/span>console<\/span>.<\/span>enabled<\/span>=<\/span>true<\/span>\nspring.<\/span>datasource<\/span>.<\/span>platform<\/span>=<\/span>h2\nspring.<\/span>datasource<\/span>.<\/span>url<\/span>=<\/span>jdbc:h2:mem:<\/span>socialdb\nspring.<\/span>datasource<\/span>.<\/span>driverClassName<\/span>=<\/span>org.<\/span>h2<\/span>.<\/span>Driver<\/span>\n<\/pre>\n <\/p>\n
Step 3:<\/strong> Run the the application and access the H2 Console<\/p>\n <\/p>\n
Part 5 – Database Initialization (data.sql)<\/strong><\/p>\nWe would write a script to insert some initial data into the database. Follow the steps below:<\/p>\n
Step 1:<\/strong> Create a file inside the src\/main\/resource<\/em>s folder. Name it data.sql<\/p>\nStep 2:<\/strong> Open the file and add the following code<\/p>\ninsert into Location(id, name) values(1, 'Budapest, Hungary');\ninsert into Location(id, name) values(2, 'Owerri, Nigeria');\ninsert into Location(id, name) values(3, 'Califonia, USA');\n\ninsert into User(id, firstname, lastname, email, location_id) values(1, 'Kindson', 'Munonye', 'kany@gmail.com', 1);\ninsert into User(id, firstname, lastname, email, location_id) values(2, 'Jeffrey', 'Yuba', 'yuba@gmail.com', 2);\ninsert into User(id, firstname, lastname, email, location_id) values(3, 'Solace', 'Okeke', 'solace@gmail.com', 3);\n\ninsert into Post(id, post_date, details, user_id) values(1, CURRENT_TIMESTAMP(), 'Very good post', 1);\ninsert into Post(id, post_date, details, user_id) values(2, CURRENT_TIMESTAMP(), 'A rainy day', 2);\ninsert into Post(id, post_date, details, user_id) values(3, CURRENT_TIMESTAMP(), 'nice tutorials', 3);\n<\/pre>\n <\/p>\n
Step 3:<\/strong> Run the application. Access h2-console and verify that the data is inserted. The H2-Console window is shown below<\/p>\nH2 Console<\/figcaption><\/figure>\n <\/p>\n
Part 6 – @OneToMany Relationship<\/strong><\/p>\nThis relationship allows us to get list of child entities under a parent entity. For examples, we could get the list of posts for a user. Or we could get list of users in a location. Follow the steps below:<\/p>\n
Step 1:<\/strong>\u00a0 In the Location class, add a private field of List<User> type. (This means list of all users under that location)<\/p>\nStep 2:\u00a0<\/strong> Annotate this field with the @OneToMany annotation<\/p>\nStep 3:\u00a0<\/strong> In the User class, add a private field of List<Post> type. (This means list of all posts for that user)<\/p>\nStep 4:<\/strong> Annotate this field with the @OneToMany annotation<\/p>\nStep 5:<\/strong> Run the application and access the H2 Console<\/p>\nNow, you will notice in the H2 console that there are additional tables that have been created.<\/p>\n
What happens here is this:<\/p>\n
for the User-Location relationship, the Location\u00a0entity<\/strong> decides to handle the relationship mapping by creating an additional table called LOCATION_USERS.<\/p>\nAgain, for the Post-User relationship, the User entify decides the handle the relationship by creating an additional table called POST_USERS.<\/p>\n
But remember that the relationship mapping has already been handled using by the @ManyToOne using the location_id and the user_id columns.<\/p>\n
This duplicate mapping can be resolved using the mappedBy attibute<\/p>\n
<\/p>\n
Part 7 – The mappedBy Attribute<\/strong><\/p>\nThe mappedBy is an attribute of the @OneToMany relationship. You use the mappedBy to tell the @OneToMany annotation that the relationship has already been handled using a foreign key in the corresponding entity. In this way, an additional table is not created.<\/p>\n
To do this:<\/p>\n
Step 1:<\/strong> In the @OneToMany annotation in the Location entity, add mappedBy=”location”<\/p>\nStep 2:<\/strong> In the @OneToMany annotation in the User entity, add mappedBy=”user”<\/p>\nStep 3:<\/strong> Launch the application, access H2-Console and check that there are no more duplicate tables;<\/p>\nLet’s now move on the the next section…<\/p>\n","protected":false},"excerpt":{"rendered":"
I’m happy, that finally we have a complete tutorial that addresses the challenges faced by many programmers. Entity relationships(@OneToMany and @ManyToOne). So everything about relationships … <\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"pagelayer_contact_templates":[],"_pagelayer_content":"","footnotes":""},"categories":[414],"tags":[],"class_list":["post-1942","post","type-post","status-publish","format-standard","hentry","category-programming"],"_links":{"self":[{"href":"https:\/\/kindsonthegenius.com\/blog\/wp-json\/wp\/v2\/posts\/1942","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/kindsonthegenius.com\/blog\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/kindsonthegenius.com\/blog\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/kindsonthegenius.com\/blog\/wp-json\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/kindsonthegenius.com\/blog\/wp-json\/wp\/v2\/comments?post=1942"}],"version-history":[{"count":1,"href":"https:\/\/kindsonthegenius.com\/blog\/wp-json\/wp\/v2\/posts\/1942\/revisions"}],"predecessor-version":[{"id":2110,"href":"https:\/\/kindsonthegenius.com\/blog\/wp-json\/wp\/v2\/posts\/1942\/revisions\/2110"}],"wp:attachment":[{"href":"https:\/\/kindsonthegenius.com\/blog\/wp-json\/wp\/v2\/media?parent=1942"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/kindsonthegenius.com\/blog\/wp-json\/wp\/v2\/categories?post=1942"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/kindsonthegenius.com\/blog\/wp-json\/wp\/v2\/tags?post=1942"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}
![\"Relationship](\"https:\/\/www.kindsonthegenius.com\/wp-content\/uploads\/2020\/09\/Relationship-Diagram-1024x428.jpg\")
<\/a>![\"H2](\"https:\/\/www.kindsonthegenius.com\/wp-content\/uploads\/2020\/09\/H2-Console.jpg\")