Instancio中根据关联Course字段赋值Lesson报错，如何解决？

我想要创建一个Lesson模型，使其字段值基于关联Course的subject字段：

Lesson类代码：

@Data
public class Lesson {
    private Long id;
    private Course course;
    private String title;
    private String description;
}

Course类代码及Lesson测试模型：

@Data
@AllArgsConstructor
@NoArgsConstructor
@Builder
public class Course {
    private Long code;
    private String subject;
    private String description;
    private Set<User> users;
}

public static final Model<Lesson> LESSON_TEST_MODEL = Instancio.of(Lesson.class)
            .supply(field(Lesson::getId), () -> START_LESSON_ID.getAndSet(START_LESSON_ID.get() + DEFAULT_INCREMENT_STEP))
            .supply(field(Lesson::getCourse), () -> Instancio.of(COURSE_TEST_MODEL).create())
            .assign(given(field(Course::getSubject), field(Lesson::getTitle))
                    .set(When.is("Mathematics"), "Math Lesson")
                    .set(When.is("History"), "History Lesson")
                    .set(When.is("Literature"), "Literature Lesson")
                    .set(When.is("Physics"), "Physics Lesson")
                    .set(When.is("Computer Science"), "Computer Science Lesson"))
            .assign(given(field(Course::getSubject), field(Lesson::getDescription))
                    .set(When.is("Mathematics"), "Lesson on mathematics concepts")
                    .set(When.is("History"), "Lesson on world history")
                    .set(When.is("Literature"), "Lesson on classical literature")
                    .set(When.is("Physics"), "Lesson on fundamental physics")
                    .set(When.is("Computer Science"), "Lesson on computer programming"))
            .toModel();

运行代码时抛出异常：

Reason: unresolved assignment expression

The following assignments could not be applied:

 -> from [field(Course, "subject")] to [field(Lesson, "title")]
 -> from [field(Course, "subject")] to [field(Lesson, "description")]

As a result, the following targets could not be assigned a value:

 -> field Lesson.description (depth=1)
 -> field Lesson.title (depth=1)

问题出在你用.supply()方法直接生成Course实例，这会跳过Instancio的内部字段追踪机制，导致它无法关联Course的subject字段和Lesson的title、description字段。

修改方式是去掉.supply()生成Course的代码，改用.withModel()来关联Course的测试模型，让Instancio能完整追踪对象间的关联关系：

public static final Model<Lesson> LESSON_TEST_MODEL = Instancio.of(Lesson.class)
            .supply(field(Lesson::getId), () -> START_LESSON_ID.getAndSet(START_LESSON_ID.get() + DEFAULT_INCREMENT_STEP))
            // 替换supply为withModel，让Instancio管理Course实例的创建
            .withModel(field(Lesson::getCourse), COURSE_TEST_MODEL)
            .assign(given(field(Course::getSubject), field(Lesson::getTitle))
                    .set(When.is("Mathematics"), "Math Lesson")
                    .set(When.is("History"), "History Lesson")
                    .set(When.is("Literature"), "Literature Lesson")
                    .set(When.is("Physics"), "Physics Lesson")
                    .set(When.is("Computer Science"), "Computer Science Lesson"))
            .assign(given(field(Course::getSubject), field(Lesson::getDescription))
                    .set(When.is("Mathematics"), "Lesson on mathematics concepts")
                    .set(When.is("History"), "Lesson on world history")
                    .set(When.is("Literature"), "Lesson on classical literature")
                    .set(When.is("Physics"), "Lesson on fundamental physics")
                    .set(When.is("Computer Science"), "Lesson on computer programming"))
            .toModel();

如果你的COURSE_TEST_MODEL需要自定义生成逻辑，确保它是通过Instancio的Model API构建的，这样Instancio就能正确识别并关联嵌套对象的字段，完成条件赋值。

内容的提问来源于stack exchange，提问作者yaroslav96