Posted on Apr 8, 2021 • Updated on Apr 15, 2021

Entity Relationships Part 1: One-To-Many

Adding a Car Class

One-To-Many Relation

Clarifications

At the end of the video, the person column in the car table (the table to the right) is called a foreign key column as the values in the column are ids on the person table (table to the left)

One-To-Many Implementation

GitHub Repo Tag

Clarification

Car Controller

The annotation @Controller was mistakenly used instead of @RestController in class CarController. @RestController is a newer annotation that removes the need of using a @ResponseBody annotation to wrap the return type in a response class. However, if you are returning the type ResponseEntity<T> it shouldn't make a difference.

CascadeType

When using a one-to-many relationship it is often relevant to set a cascade type. This decides which actions on an entity should be propagated to its collection of another entity. In our case, if we want the cars to be deleted when the person who owns them gets deleted we should set a value on the @ManyToOne annotation cascade = CascadeType.ALL for all actions (including deletion, persisting) to affect the cars. If you only want deletion specifically to be propagated then you would set the value to cascade = CascadeType.REMOVE. See Overview of JPA/Hibernate Cascade Types

Avoiding Recursive JSON Representation

Snippets & Commands

JSON annotations

Ignore the property completely:

@JsonIgnore

or make the property only be rendered as the id:

@JsonIdentityInfo(generator = ObjectIdGenerators.PropertyGenerator.class, property = "id")
@JsonIdentityReference(alwaysAsId = true)

Updating Car & Validation

GitHub Repo Tag

Clarifications

Car Field Validation

At the very end of the video. The field topSpeed is annotated with @NotBlank but that annotation actually doesn't work on int fields because compared to a String, an int has no concept of being empty (int value 0 is not considered empty) so there you would have to use @NotNull instead.

Validation Groups

If you would like to validate an object differently depending on which method it is used (for example in one controller method you might want to enforce a non-null reference to another entity but in another you want the reference to be able to be omitted) you can use Validation Groups, see this link.

Snippets & Commands

build.gradle

plugins {
    id 'org.springframework.boot' version '2.4.4'
    id 'io.spring.dependency-management' version '1.0.11.RELEASE'
    id 'java'
}

repositories {
    mavenCentral()
}

dependencies {
    implementation 'org.springframework.boot:spring-boot-starter-web'
    implementation 'org.springframework.boot:spring-boot-starter-data-jpa'
    implementation 'org.springframework.boot:spring-boot-starter-validation'
    runtimeOnly 'org.postgresql:postgresql'
}