Spring boot application problems with multiple databases

Question

I am trying to have a OneToMany and ManyToOne mapping in my persistence classes. I have following tables:

@Data
@AllArgsConstructor
@NoArgsConstructor
@Entity
@Table(name = "file")
public class File {
    @Id
    @GeneratedValue(strategy = AUTO)
    private Long fileId;
    @NotBlank(message = "File Name not empty or null")
    private String fileName;
    private Instant createdDate;
    private Long customerId;
    @ManyToOne(fetch = FetchType.EAGER, targetEntity = Customer.class)
    @Column(name = "files")
    private Customer customer; 

@AllArgsConstructor
@NoArgsConstructor
@Entity
@Table(name = "customerDB")
public class Customer {
    @Id
    @GeneratedValue(strategy = AUTO)
    private Long id;
    @NotBlank(message = "Customer name is not null")
    private String customerName;
    @NotBlank(message = "İnfo is not null")
    private String info;
    private Instant createdDate;
    private Long createdUser;


    @OneToMany(targetEntity = File.class, mappedBy = "customer", cascade = CascadeType.ALL, fetch = FetchType.LAZY)
    private Set<File> files;

application.properties

app.datasource.file.url=jdbc:mysql://localhost:3306/filedb?createDatabaseIfNotExist=true
app.datasource.file.username=root
app.datasource.file.password=password
app.datasource.file.driverClassName=com.mysql.cj.jdbc.Driver

app.datasource.customer.url=jdbc:mysql://localhost:3306/customerdb?createDatabaseIfNotExist=true
app.datasource.customer.username=root
app.datasource.customer.password=password
app.datasource.customer.driverClassName=com.mysql.cj.jdbc.Driver

spring.jpa.hibernate.ddl-auto=update
spring.jpa.generate-ddl=true
spring.jpa.show-sql=true
spring.jpa.database=mysql
spring.jpa.properties.hibernate.dialect=org.hibernate.dialect.MySQL8Dialect 

That is, I am getting errors from not trying many one-to-many operations with a lot of problems online. In version in one software. In applications with multiple versions. I couldn't find the solution.I get pretty much the same error with every solution I try. I tried many methods but the result is always an error.

org.springframework.beans.factory.BeanCreationException: Error creating bean with name 'customerEntityManagerFactory' defined in class path resource [com/nishbs/cas/configuration/CustomerSourceConfiguration.class]: Invocation of init method failed; nested exception is org.hibernate.AnnotationException: Use of @OneToMany or @ManyToMany targeting an unmapped class: com.nishbs.cas.model.customer.Customer.files[com.nishbs.cas.model.file.File]
    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.initializeBean(AbstractAutowireCapableBeanFactory.java:1804) ~[spring-beans-5.3.18.jar:5.3.18]
    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.doCreateBean(AbstractAutowireCapableBeanFactory.java:620) ~[spring-beans-5.3.18.jar:5.3.18]
    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBean(AbstractAutowireCapableBeanFactory.java:542) ~[spring-beans-5.3.18.jar:5.3.18]
    at org.springframework.beans.factory.support.AbstractBeanFactory.lambda$doGetBean$0(AbstractBeanFactory.java:335) ~[spring-beans-5.3.18.jar:5.3.18]
    at org.springframework.beans.factory.support.DefaultSingletonBeanRegistry.getSingleton(DefaultSingletonBeanRegistry.java:234) ~[spring-beans-5.3.18.jar:5.3.18]
    at org.springframework.beans.factory.support.AbstractBeanFactory.doGetBean(AbstractBeanFactory.java:333) ~[spring-beans-5.3.18.jar:5.3.18]
    at org.springframework.beans.factory.support.AbstractBeanFactory.getBean(AbstractBeanFactory.java:208) ~[spring-beans-5.3.18.jar:5.3.18]
    at org.springframework.context.support.AbstractApplicationContext.getBean(AbstractApplicationContext.java:1154) ~[spring-context-5.3.18.jar:5.3.18]
    at org.springframework.context.support.AbstractApplicationContext.finishBeanFactoryInitialization(AbstractApplicationContext.java:908) ~[spring-context-5.3.18.jar:5.3.18]
    at org.springframework.context.support.AbstractApplicationContext.refresh(AbstractApplicationContext.java:583) ~[spring-context-5.3.18.jar:5.3.18]
    at org.springframework.boot.web.servlet.context.ServletWebServerApplicationContext.refresh(ServletWebServerApplicationContext.java:145) ~[spring-boot-2.6.6.jar:2.6.6]
    at org.springframework.boot.SpringApplication.refresh(SpringApplication.java:740) [spring-boot-2.6.6.jar:2.6.6]
    at org.springframework.boot.SpringApplication.refreshContext(SpringApplication.java:415) [spring-boot-2.6.6.jar:2.6.6]
    at org.springframework.boot.SpringApplication.run(SpringApplication.java:303) [spring-boot-2.6.6.jar:2.6.6]
    at org.springframework.boot.SpringApplication.run(SpringApplication.java:1312) [spring-boot-2.6.6.jar:2.6.6]
    at org.springframework.boot.SpringApplication.run(SpringApplication.java:1301) [spring-boot-2.6.6.jar:2.6.6]

Answer 1

It seems that you are trying to join two tables that are in different databases. I am not 100% certain but i would say that is not possible the way you are doing it.

Even without spring as a framework you have to link your databases first. I would guess that if you have achieved this then you can access the Database with just one datasource in spring.

Start of with linking your databases and trying to run a manual SQl query, if this works the try again with code. I am not an expert on this but i would start with looking for something linke this Oracle Database Link - MySQL Equivalent?

Answer 2

Firstly be sure that for @Entity annotation you have imported this javax.persistence.Entity package.

When you are joining two tables there will be one more table created. So, I you should specify the name of new table while mapping and also provide the referencedColumnName.

@OneToMany(cascade = CascadeType.ALL)
    @JoinColumn(name = "post_id", referencedColumnName = "id")
    private List<Comment> comments;

Above is one such example which I am trying to say. Try to implement in this way.