7. ActiveRecord

Active Record(活动记录)，是一种领域模型模式，特点是一个模型类对应关系型数据库中的一个表，而模型类的一个实例对应表中的一行记录。ActiveRecord 一直广受动态语言（ PHP 、 Ruby 等）的喜爱，而 Java 作为准静态语言，对于 ActiveRecord 往往只能感叹其优雅，所以 MP 也在 AR 道路上进行了一定的探索，仅仅需要让实体类继承 Model 类且实现主键指定方法，即可开启 AR 之旅（JFinal 也有AR相关的操作）。接下来看具体代码：

7.1. entity

@Data
public class Student extends Model<Student> {
    /**
     * 主键ID
     */
    private String id;

    /**
     * 姓名
     */
    private String name;

    /**
     * 年龄
     */
    private Integer age;


    //复写pkVal，返回主键属性
    @Override
    protected Serializable pkVal() {
        return this.id;
    }

}

注：实体类继承Model类，重写pkVal方法。

7.2. mapper

public interface StudentMapper extends BaseMapper<Student> {
}

注：虽然AR模式用不到该接口，但是一定要定义，否则使用AR时会报空指针异常。

7.3. 使用AR

7.3.1. AR插入操作

@Test
public void insertTest() {
    Student student = new Student();
    student.setName("刘武");
    student.setAge(5);
    System.out.println(student.insert());
}

打印的SQL语句:

INSERT INTO STUDENT ( ID, NAME, AGE ) VALUES ( '201907251416560478106192891', '张三', 5 );
------------------------------------------------------------------------------------------------------------------------

注：可以看到我们并不需要注入mapper接口，不过正如刚才所说，不使用但还是要定义，否则会报错。AR操作是通过对象本身调用相关方法，比如要insert一个user，那就用这个user调用insert方法即可。返回值为布尔类型，由上图可看到返回了true，是操作成功的。

7.3.2. AR删除操作

@Test
public void deleteTest() {
    Student student = new Student();
    student.setId("7");
    System.out.println(student.deleteById());
    System.out.println(student.deleteById("8"));
    System.out.println(new Student().setId("9").deleteById());
    System.out.println(new Student().delete(new QueryWrapper<Student>().lambda().eq(Student::getName, "Sandy")));
}

打印的SQL语句:

DELETE
 FROM STUDENT
 WHERE ID='7';
------------------------------------------------------------------------------------------------------------------------
DELETE
 FROM STUDENT
 WHERE ID='8';
------------------------------------------------------------------------------------------------------------------------
DELETE
 FROM STUDENT
 WHERE ID='9';
------------------------------------------------------------------------------------------------------------------------
DELETE
 FROM STUDENT
 WHERE NAME = 'Sandy';
------------------------------------------------------------------------------------------------------------------------

注：这里介绍了两种删除方法，代码中已有注释说明。需要注意的是，删除数据库中不存在的数据，结果也是true。

7.3.3. AR更新操作

@Test
public void updateTest() {
    System.out.println(new Student().setId("1").setName("李四").updateById());
    System.out.println(new Student().update(new UpdateWrapper<Student>().lambda().set(Student::getAge, 3).eq(Student::getId, 2)));
}

打印的SQL语句:

UPDATE STUDENT SET NAME='李四'
 WHERE ID='1';
------------------------------------------------------------------------------------------------------------------------
UPDATE STUDENT SET AGE=3
 WHERE ID = 2;
------------------------------------------------------------------------------------------------------------------------

7.3.4. AR查询操作

@Test
public void selectTest() {
    System.out.println(new Student().setId("3").selectById().getName());
    Student student = new Student().selectOne(new QueryWrapper<Student>().lambda().eq(Student::getId, 2));
    System.out.println("Name:"+student.getName());
    System.out.println("Age:"+student.getAge());
}

打印的SQL语句:

SELECT ID,NAME,AGE
 FROM STUDENT
 WHERE ID='3';
------------------------------------------------------------------------------------------------------------------------
SELECT ID,NAME,AGE
 FROM STUDENT
 WHERE ID = 2;
------------------------------------------------------------------------------------------------------------------------

7.3.5. AR分页查询

@Test
public void pageTest(){
    Student user = new Student();
    IPage<Student> studentIPage = user.selectPage(new Page<Student>(1, 4),new QueryWrapper<Student>().like("name", "刘"));
    List<Student> users = studentIPage.getRecords();
    System.out.println(users);
}

打印的SQL语句:

SELECT COUNT(1)
 FROM STUDENT
 WHERE name LIKE '%刘%';
------------------------------------------------------------------------------------------------------------------------
SELECT ID,NAME,AGE
 FROM STUDENT
 WHERE name LIKE '%刘%'
 LIMIT 0,4;
------------------------------------------------------------------------------------------------------------------------