Search code examples
grailsgrails-ormbelongs-to

Grails belongsTo multiple classes

I have a domain class that can belong to one of several classes. I'm seeing validation errors when I try to save.

class Teacher {
    Book book
}

class Student {
    Book book
}

// book can belong to either a student or a teacher
class Book {
    static belongsTo = [student : Student, teacher : Teacher]
}

The validation error suggests that a book must belong to BOTH a student and a teacher (neither can be null), but I want to model it so that it can belong to either. How do I do this please?

Please disregard the fact that for my example you could change it so that a Person owns a book and a Teacher and a Student are both types of Person - I want to know how how to create the correct belongsTo.

Edit to explain reasoning behind requirement:

There will be 3 tables created: Book, Student and Teacher. I need to be able to create an index on the Book class that refers to Student and Teacher. This is so the query that is "find all books that belong to Teacher A" can be made as fast as possible.

If there was just a single belongsTo (example shown if for owner teacher)then this is done like this:

static mapping = { teacher index: 'teacher_idx' }

Solution

  • Well this is very much doable, its just that your approach is wrong here.

    belongsTo is used in a way when an entity must and must be mapped with some other entity. There is nothing like either of them.

    What you can do is 

    1. create an Abstract Domain `Book`
2. create an Domain `StudentBook` it belongs to `Student`
3. create an Domain `TeacherBook` it belongs to `Teacher`

    So here only one table will be created for the three Domains, named as Book. This table will contain a field class which will determine if the book belongs to Student or Teacher.