数据映射器如何映射2+ dbms表之间的关系？

Question

I have two data domains, Student and Course, and relative data mappers, StudentMapperMySQL and CourseMapperMySQL. Now I want to implement a third data mapper for fetching both data, especially for taking advantage of SQL JOIN (SELECT ... FROM students JOIN courses ON .... WHERE ...). What should the select()/fetch() method return? and how should it work via a repository pattern implementation?

Answer 1

It looks like you are creating new aggregate root just for query purpose, which is totally fine if you would like to use CQRS.

Every AR should have its own repository, so you should make AR StudentCourses and StudentCoursesRepository (something from your ubiqutious language).

You should call that repository to get student course data repo.getCoursesForStudent(student_id) or for all at once repo.getStudentsWithCourses().

Repositories should return only aggregate roots, so in return you would get an aggregate root or list of them.

Such AR would look like that:

class StudentCourses:
    studentName Name (VO)
    studentId AggregateId (VO)
    courseList Course[0..*] (Entity)

class Course:
    your course data

What is important here is that you have different models for command and for query, so you can have two different Course model entities in your system. Course in command model is probably an aggregate root, while in query model it's not.

I suppose that this answer might be a bit hard to understand, but I did my best. Anyway I'm not a DDD expert so there might be other ways to do that.