Grails 3：查找以多对多关系添加到另一个域的域（未为参数 1 指定值

Question

I'm a Grails newbie and I found the following obstacle:

I have 2 domains: Course and Student, they have a many-to-many relationship (a Course can have several students, a student can enroll in several courses) and the student belongs to the course.

So, when I add a student to a course, I want to be able to find what Courses have added a specific student.

I tried to use:

def s = Student.get(id)

def c = Course.findAllByStudents(s)

But grails keeps telling me "No value specified for parameter 1".

Can you guys throw some light into this?

Answer 1

Course.findAllByStudents expects as parameter Set of Students but you are supplying it with single instance of Student, that's why you are getting "No value specified for parameter 1" .

To find in what Courses is Student. If you created domain classes like this:

class Course {
    //some Course attributes
    static hasMany = [students: Student] 
}

class Student {
     //some Student attributes
     static hasMany = [courses: Course]
     static belongsTo = Course
} 

then you can simply use s.courses .

If you are not two-way mapping that relationship. You can create criteria like this:

Course.withCriteria {
    createAlias 'students', 's'
    eq 's.elements', s
}