java - 简单的@ManyToOne 关系 : could not resolve property (org. hibernate.QueryException)

Question

我有这个简单的关系 - 如图所示:

当我尝试读取具有特定 Country_id 的特定城市时:

Country austria = (Country) session.load(Country.class, 1L); 
// Works as expected
System.out.println(austria.toString());        

Criteria crCities = session.createCriteria(City.class);
crCities.add(Restrictions.eq("Country_id", austria.getId()));
// Crashes ..
List<City> cities = crCities.list();

System.out.println("Show cities of Austria:");
for (City next : cities) {
    System.out.println(" - " + next.getName());
}  

我收到以下错误:

Exception in thread "main" org.hibernate.QueryException: could not resolve property: Country_id of: com.mahlzeit.datamodel.geographic.City

这是 POJO:

国家.java

package com.mahlzeit.datamodel.geographic;

import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.Id;
import javax.persistence.Table;

@Entity
@Table
public class Country {

    @Id
    @GeneratedValue
    private Long id;

    private String country_code;

    public String getCountryCode() {
        return country_code;
    }

    public void setCountryCode(String countryCode) { 
        this.country_code = countryCode;
    }

    public Long getId() {
        return id;
    }

    public void setId(Long id) {
        this.id = id;
    }

    @Override
    public String toString() {
        return "Country [id=" + id.toString() + ", country_code=" + country_code.toString()  +"]";
    }
}

City.java

package com.mahlzeit.datamodel.geographic;

import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.Id;
import javax.persistence.JoinColumn;
import javax.persistence.ManyToOne;
import javax.persistence.Table;

@Entity
@Table
public class City {

    @Id
    @GeneratedValue
    private Long id;

    @ManyToOne
    @JoinColumn(name = "Country_id")
    private Country country;

    private String name;

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public Country getCountry() {
        return country;
    }

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

    public Long getId() {
        return id;
    }

    public void setId(Long id) {
        this.id = id;
    }

     @Override
    public String toString() {
        return "City [id=" + id.toString() + ", country="
                + country.toString() + ", name=" + name + "]";
    }
}

我需要做什么才能使这项工作正常进行？ @JoinColumn(name = "Country_id") 在 City.java 中是否正确？

Answer 1

您的条件有以下限制:

crCities.add(Restrictions.eq("Country_id", austria.getId()));

这意味着 hibernate 搜索名为 Country_id 的属性。您的类 City 没有这样的属性。它只有一个名为 country 的属性。

所以要么使用对象模型

crCities.add(Restrictions.eq("country", austria));

这是 hibernate 下的首选方式。

或者，您可以使用更SQL的方式

crCities.add(Restrictions.eq("country.id", austria.getId()));

您明确表示要使用 country 的属性 id。

结果和创建的SQL语句应该是一样的。