JPA 与存储在不同数据库中的用户实体的多对多关系

Question

TLDR;

I'm trying to create a relationship to an entity (users) stored in a different database and can't figure out how to do this in JPA.

Use Case

I am implementing a car service in Spring Boot/JPA. I have created an entity/table called Car . A car may have multiple owners (users) and a user may own multiple cars. However, the User table is stored in a separate user service.

Since this is a many-many relationship, I felt a JoinTable would be appropriate. The problem here is that there is no User table to join to. I just need to store the users' UUIDs in the join table, so GET methods can fetch all cars for a given user, or all owners of a given car.

Source Code

Here's an attempt at the Car entity. Removing the ForeignKey constraint allows entries to be created in the join table without causing an error due to the missing child table.

@Data
@Entity
@Table(name = "car")
@EqualsAndHashCode(callSuper = true)
public class Car {
  @Id @GeneratedValue private UUID id;

  @ManyToMany
  @JoinTable(
      name = "car_owner",
      joinColumns = @JoinColumn(name = "car_id"),
      inverseJoinColumns = @JoinColumn(name = "user_id", insertable = false, updatable = false),
      foreignKey = @ForeignKey(name="user_id", value = ConstraintMode.NO_CONSTRAINT))
  private Set<User> users;

  private String model;
  private double price;
}

Here's the User entity - required for the JPA JoinTable , but only used for its ID:

@Data
@Entity
@EqualsAndHashCode()
public class User {

  @Id private UUID id;

  @ManyToMany(mappedBy = "users")
  @JsonIgnore
  private Set<Car> cars;
}

Sending the model below to a JPA Repository save() method creates the car and entries in the join table successfully:

{
  "model": "Ford Model T",
  "price": 850.00,
  "users": [
    {"id": "e048b593-aad9-4285-b3e6-49475ad9bd1d"},
    {"id": "1d0f7b1e-bc36-4b99-80a1-8835779598ca"}
  ]
}

Problem Statement

However, when I try to retrieve all cars belonging to a specific user ID:

  @Query(
      value = "select * from car c inner join car_owner co on c.id = co.car_id where co.user_id = :userId",
      nativeQuery = true)
  List<Car> findByUserId(UUID userId);

I get the following error:

ERROR: column user1_.id does not exist Resolved [org.springframework.http.converter.HttpMessageNotWritableException: Could not write JSON: could not extract ResultSet; nested exception is com.fasterxml.jackson.databind.JsonMappingException: could not extract ResultSet (through reference chain: java.util.ArrayList[0]->Car["users"])]

How can I read back the list of cars for a given user? Am I going about this the wrong way? I'm open to other implementations.

Answer 1

As @vincendep mentioned, the solution is to drop the User entity and replace the users property in the Car entity with the following:

@ElementCollection
@CollectionTable(name = "car_owner", joinColumns = @JoinColumn(name = "car_id"))
@Column(name = "user_id")
private Set<UUID> owners;

This will still use the car_owner join table. A car can be created with multiple users with the following model:

{
  "model": "Ford Model T",
  "price": 850.00,
  "owners": [
    "32a7967d-8580-418d-b53a-221ed0c52222",
    "a708455c-c37c-4712-a7ce-59c9be5a5eb5"
  ]
}

You can query cars by user ID as follows (credit to @vincendep):

@Query(value = "from Car c where :ownerId member of c.owners")
List<Car> findByUserId(UUID ownerId);