如何使用 JPA EntityManager select 包括外部实体的多个列

Question

I am trying to select multiple columns from 2 tables with OneToOne relationship, where the users will send the columns they want to search and the sever returns the result list contains only those columns. I have two entities like these:

@Table(name = "user")
public class User implements Serializable{
    private static final long serialVersionUID = 1L;

    @Column(name = "id" )
    @Id
    private String id;

    private String name;

    @OneToOne(mappedBy = "user", cascade = CascadeType.ALL)
    @JsonManagedReference
    private Address address;

    //Getters and Setters
}

public class Address  implements Serializable{
    private static final long serialVersionUID = 1L;
    
    @Column(name = "user_id" )
    @Id
    private String id;

    private String houseNr;
    
    @OneToOne
    @JoinColumn(name = "user_id")
    @JsonBackReference
    private Networkdata networkdata;
    
    //Getters and Setters

With search functions using EntityManager with Tuple

public List<?> find(String[] neededFields){
   if(neededFields.length > 0){
      String queryStr = this.createQueryString(neededFields);

      TypedQuery<Tuple> query = em.createQuery(queryStr, Tuple.class);

      List<Map<String, Object>> resultList = new ArrayList<>();
        
      query.getResultList().forEach(tuple -> {
         Map<String, Object> map = new HashMap<>();
         List<TupleElement<?>> elements = tuple.getElements();
         for (TupleElement<?> element : elements ) {
            String alias = element.getAlias();
                    
            map.put(alias, tuple.get(alias));
         }
                
         resultList.add(map);
      });

      return resultList; 
   }
   else{
      return em.createQuery(this.FIND_ALL_STR, User.class).getResultList();
   }
}

When i search using SELECT u FROM User u, Address a WHERE u.id = a.id . It returns result like this:

[
   {
      "id": "5e4e3c95cd8b290008db6f3c",
      "name": "name",
      "address": {
         "id": "5e4e3c95cd8b290008db6f3c",
         "houseNr": "123ABC"
      }
   }
]

Another case is SELECT u.address as address FROM User u, Address a WHERE u.id = a.id which returns

[
   {
      "address": {
         "id": "5e4e3c95cd8b290008db6f3c",
         "houseNr": "123ABC"
      }
   }
]

But when i add a little spicy ingredient like for example, to select the id and the whole address entity like this SELECT u.id as id, u.address as address FROM User u, Address a WHERE u.id = a.id it returns

[
   {
      "id": "5e4e3c95cd8b290008db6f3c",
      "address": null
   }
]

Why the first two queries are perfectly fine but the last just returns a null address? How can i solve this?

Other ideas are also welcome.

Answer 1

If you searching from two column, you can actually use INNER JOIN key words with FOREIGN KEY on the Adress. If you still want to trim down your search you can always use WHERE clause. SELECT u.id as Id, a.address as Adress from User u INNER JOIN Address a ON u.id=a.user.id. to trim down your search you can add “WHERE u LIKE %?1% AND a LIKE %?1% ORDER BY u.id “

Better still to save yourself from headache you can use RESPONSE CLASS to map out the column you want to search from something like SELECT new UserAddress(u.id, a.address)......

I hope I'm able to help.