java - 为什么我会收到 org.hibernate.id.IdentifierGenerationException？

Question

当我运行我的主类 (Runner) 程序时，出现以下异常:

org.hibernate.id.IdentifierGenerationException: attempted to assign id 
from null one-to-one property: country

我不知道原因，为什么我会收到此异常。

映射 xml:

<hibernate-mapping>
  <class name="pojo.Country" table="country">
      <id name="countryID" column="c_id">
          <generator class="increment" />
      </id>
      <property name="countryName" column="c_name" />
      <one-to-one class="pojo.PM" name="pm" cascade="all" />
  </class>

  <class name="pojo.PM" table="pm">
      <id name="countryID" column="c_id">
          <generator class="foreign">
              <param name="property">country</param>
          </generator>
      </id>
      <property name="pmName" column="pm_name" />
      <one-to-one class="pojo.Country" name="country" constrained="true" />
  </class>
</hibernate-mapping>

POJO 类:

国家

public class Country {
    private int countryID;
    private String countryName;
    private PM pm;

    public PM getPm() {
        return pm;
    }

    public void setPm(PM pm) {
        this.pm = pm;
    }

    public int getCountryID() {
        return countryID;
    }

    public void setCountryID(int countryID) {
        this.countryID = countryID;
    }

    public String getCountryName() {
        return countryName;
    }

    public void setCountryName(String countryName) {
        this.countryName = countryName;
    }
}

PM

public class PM {
    private int countryID;
    private String pmName;
    private Country country;

    public int getCountryID() {
        return countryID;
    }

    public void setCountryID(int countryID) {
        this.countryID = countryID;
    }

    public String getPmName() {
        return pmName;
    }

    public void setPmName(String pmName) {
        this.pmName = pmName;
    }

    public Country getCountry() {
        return country;
    }

    public void setCountry(Country country) {
        this.country = country;
    }

}

这是尝试提交事务的类:

public class Runner {
    public static void main(String args[]) {System.out.println("dfdf");
        Configuration config = new Configuration().configure();
        SessionFactory sessFact = config.buildSessionFactory();
        Session session = sessFact.openSession();
        Transaction trans = session.beginTransaction();
        Country c = new Country();
        PM pm = new PM();
        pm.setPmName("Manmohan Singh");
        c.setCountryName("India");
        c.setPm(pm);

        session.save(c);
        trans.commit();

    }
}

创建表的 SQL :

CREATE TABLE country(c_id INTEGER,c_name TEXT,PRIMARY KEY(c_id));
CREATE TABLE pm(c_id INTEGER,pm_name TEXT);

Answer 1

问题出在country变量上。在尝试进行某些事务之前，您应该初始化所有属性。

编辑:在 Hibernate 文件中，您希望根据 country 属性的 ID 生成 PM ID。但是，该属性从未被初始化。

 <class name="pojo.PM" table="pm">
      <id name="countryID" column="c_id">
          <generator class="foreign">
              <param name="property">country</param>
          </generator>
      </id>
      <property name="pmName" column="pm_name" />
      <one-to-one class="pojo.Country" name="country" constrained="true" />
  </class>

因此，将 pm.setCountry(c); 添加到您的代码中。