Question

我正在尝试使用其Question

获取Answers

问题

@Entity
public class Question extends Model {
  @Id
  public Long id;

  @Constraints.Required
  @OneToMany(cascade = CascadeType.ALL)
  public ArrayList<Answer> answers = new ArrayList<>(4);
  ...
}

答案

@Entity
public class Answer extends Model{
  @Id
  public Long id;

  @ManyToOne
  public Question question;
  ...
}

查询

public static Result getQuestion() {
    return ok(
        Json.toJson(Question.find
        .fetch("answers")
        .orderBy("question.id")
        .setMaxRows(1)
        .findList()
        .get(0)
        )
    );
}

fetch("answers")会抛出Nullpointerexception。

数据库表question和answer是一致的。表answer包含question表的正确外键。

堆栈跟踪

play.api.Application$$anon$1: Execution exception[[NullPointerException: null]]
at play.api.Application$class.handleError(Application.scala:296) ~[play_2.11-2.3.8.jar:2.3.8]
at play.api.DefaultApplication.handleError(Application.scala:402) [play_2.11-2.3.8.jar:2.3.8]
at play.core.server.netty.PlayDefaultUpstreamHandler$$anonfun$3$$anonfun$applyOrElse$4.apply(PlayDefaultUpstreamHandler.scala:320) [play_2.11-2.3.8.jar:2.3.8]
at play.core.server.netty.PlayDefaultUpstreamHandler$$anonfun$3$$anonfun$applyOrElse$4.apply(PlayDefaultUpstreamHandler.scala:320) [play_2.11-2.3.8.jar:2.3.8]
at scala.Option.map(Option.scala:145) [scala-library-2.11.1.jar:na]
Caused by: java.lang.NullPointerException: null
at com.avaje.ebeaninternal.server.deploy.BeanDescriptor.getBeanDescriptor(BeanDescriptor.java:1411) ~[avaje-ebeanorm-3.3.4.jar:na]
at com.avaje.ebeaninternal.server.querydefn.OrmQueryDetail.setDefaultSelectClause(OrmQueryDetail.java:414) ~[avaje-ebeanorm-3.3.4.jar:na]
at com.avaje.ebeaninternal.server.querydefn.DefaultOrmQuery.setDefaultSelectClause(DefaultOrmQuery.java:842) ~[avaje-ebeanorm-3.3.4.jar:na]
at com.avaje.ebeaninternal.server.core.DefaultServer.createQueryRequest(DefaultServer.java:1105) ~[avaje-ebeanorm-3.3.4.jar:na]
at com.avaje.ebeaninternal.server.core.DefaultServer.createQueryRequest(DefaultServer.java:1093) ~[avaje-ebeanorm-3.3.4.jar:na]

Answer 1

您可以查询非空的“答案”

这应该有效

Question.find.fetch("answers").where().isNotNull("answers");

这可能有效

Question.find.fetch("answers").where().ne("answers.id", null);

Answer 2

问题是Question类中的这一行：

public ArrayList<Answer> answers = new ArrayList<>(4);

在Play Framework中，必须使用列表界面而不是实现：

public List<Answer> answers = new ArrayList<>(4);

Play Framework EBean使用关系获取对象

2 个答案: