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'
}
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.
If I understand you then you can use other version of belongsTo which does not store back reference of Owner Class e.g
class Book {
static belongsTo = [Student, Teacher]
}
Ref
Related
I'm making an e-learing platform in Grails and I'm not sure how to go about modelling the domain classes. I have the following classes: User, Role (student, instructor, admin...), UserRole and Course.
I want to make it so that every Course belongs to one instructor(User), and every Instructor can have many Courses. But also every student(also User) can be enrolled in many courses. My problem is that instructor and student are both one class (User).
I tried to make it like this by making an additional class Enrolment which keeps record of every student enrolled in each class, but I'm not sure it's ok.
class User {
String username
String password
...
class Course {
String title
String description
static belongsTo = [ instructor : User ]
...
class Enrolment {
User student
Course course
...
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?
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
}
Let's say i have 3 domain classes:
class Book {
static belongsTo = [Author,User]
static hasMany = [authors:Author,usersWhomReadThisBook:User]
String title
}
class Author {
static hasMany = [books:Book]
String name
}
class User {
static hasMany = [booksRead:Book]
String name
}
belongsTo defines a cascading relationship so deleting a parent will delete all objects that belong to it.
Question is: when I delete a User does it cascade up and delete the Books the user have read? Even if it still belongs to an existing Author? Or does it only delete from the join table?
The documentation is not clear on this use case.
Your question is:
When I delete a User does it cascade up and delete the Books the user have read? Even if it still belongs to an existing Author? Or does it only delete from the join table?
The answer is:No deletes are not cascaded for many to many,here if we delete the user only it will be deleted because the books is associated with other class Author.
I am a beginner in GRAILS so i am hoping some help on the issue i am facing.
I have read the documentation but i am still vague on the idea of relationships in grails. In grails, you could have 4 types of relationship between domain classes.
1 to 1
1 to many
many to 1
many to many
Grails has three constructs to define relationships
static hasMany =
static belongsTo =
static hasOne =
My question and dilemma is why do we need these three constructs to define a relation when we could just specify what type of objects each class has that would automatically define relationship between domain classes.
for example
To define many to many i could have two classes designed this way
class Author{
Set<Book> books
}
class Book{
Set<Author> authors
}
For 1 to many and many to 1
class Author{
Set<Book> books
}
class Book{
String title
}
for one to one
class Author{
Book book
}
class Book{
Author author
}
I appreciate it if anyone can give me a clear, easy to understand explanation. Thank you!
Everything you defined there should work fine. You don't have to use any of the other stuff that you mentioned that GORM offers, but there are reasons that you might want to. For example, you can write a class like this:
class Author{
Set<Book> books
}
That is not the same thing as this:
class Author {
static hasMany = [books: Book]
}
When you use hasMany, Grails generates this for you...
class Author {
Set<Book> books
def addToBooks(Book b) {
books.add(b)
this
}
def addToBooks(Map m) {
books.add(new Book(m))
this
}
def removeFromBooks(Book b) {
books.remove(b)
this
}
}
That isn't exactly what is generated, but that is some of the stuff that you might care about.
There is more to it than is represented there. For example, if the Book has a reference back to the Author, the addToBooks methods will hook that back reference up for you.
There are other behaviors associated with the other properties you mentioned. For example, the hasOne property switches the direction in which the foreign key points on the persistence model. The belongsTo property enforces cascading of certain events. etc.
Take a look at the GORM docs at http://grails.org/doc/latest/guide/GORM.html for more information.
UPDATED
I have a domain classes as below
class Training{
// has one createdBy object references User domain and
// has one course object references Course domain
// has One Trainer1 and Trainer2 objects refernces Trainer Object.
}
class Trainer{
String name
//can have many trainings.
//If Trainer gets deleted, the trainings of him must be deleted
}
Class User{
String name
// can have many trainings.
}
class Course{
String name
//If Course gets deleted, Trainings of this course must be deleted
// can have many trainings.
}
I have got a training create page, Where I have to populate already saved Course, User, Trainer1 and Trainer2. I am not saving them while Creating the training.
So, How to specify the relationship in grails
You did not put any effort to searching answer for yourslef. There are plenty basic examples and blog posts how to map relations in Grails. You should start with Grails documentation of GORM - Grails' object relational mapping. You can find it here.
I see some minor flaws in yout initial design: ie why should training be deleted if user is deleted when trainings will obviously tie with many users. Can training exists without trainers or vice versa ?
I would start with something like this:
Class Training {
static hasMany = [users: User, trainers: Trainer]
static belongsTo = Course
}
Class Trainer {
String name
}
Class User {
String name
}
Class Course {
String name
static hasMany = [trainings: Training]
}
EDIT: I have to agree with Tomasz, you have jumped here too early without searching for answers yourself. Grails.org has good documentation about GORM with examples too.