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 {
private String id;
private String name;
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. mapperpublic interface StudentMapper extends baseMapper {
}
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 );
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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().lambda().eq(Student::getName, "Sandy")));
}
打印的SQL语句:
DELETE
FROM STUDENT
WHERe ID='7';
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DELETe
FROM STUDENT
WHERe ID='8';
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DELETe
FROM STUDENT
WHERe ID='9';
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DELETe
FROM STUDENT
WHERe NAME = 'Sandy';
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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().lambda().set(Student::getAge, 3).eq(Student::getId, 2)));
}
打印的SQL语句:
UPDATE STUDENT SET NAME='李四'
WHERe ID='1';
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UPDATE STUDENT SET AGE=3
WHERe ID = 2;
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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().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';
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SELECt ID,NAME,AGE
FROM STUDENT
WHERe ID = 2;
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7.3.5. AR分页查询
@Test
public void pageTest(){
Student user = new Student();
IPage studentIPage = user.selectPage(new Page(1, 4),new QueryWrapper().like("name", "刘"));
List users = studentIPage.getRecords();
System.out.println(users);
}
打印的SQL语句:
SELECT COUNT(1)
FROM STUDENT
WHERe name LIKE '%刘%';
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SELECt ID,NAME,AGE
FROM STUDENT
WHERe name LIKE '%刘%'
LIMIT 0,4;
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