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.
Related
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
I want to create a domain class as like , One user can post many orders [Bidirectional] and one order can be liked by many users [unidirectional].
I have written a domain class as shown below ,
Class User {
String userName;
List orders
static hasMany = [Order]
}
Class Order {
String orderId
String orderName
//Indicates this order belongs to only one user
static belongsTo =[owner : User ] // Bidirectional
//Indicates order can be liked by many users
static hasMany = [likedUser : User] //Unidirectional
}
But I am getting am error saying invalid schema . Any body please help...
This post looks similar to my question but I am not getting , Please help.
First, order is a reserved word in SQL. Since GORM by default creates a table with the same name as your class, you'll need to either rename your class or provide a different name to use when mapping to SQL tables.
For example:
class Order {
static mapping = {
table 'user_order'
}
// ...
}
Another problem is that Order contains two associations to User. You need to tell GORM which one of these that is the bi-directional association from User to Order. That can be achieved using mappedBy, like this:
class User {
String userName
static hasMany = [orders: Order]
static mappedBy = [orders: 'owner']
}
Hope this helps.
Given the following domain classes, it is possible to get the value of author's ID from a Book object without fetching the related Author object like this: Book.get(1).authorId (see this SO question).
class Author {
hasMany = [books: Book]
}
class Book {
belongsTo = [author: Author]
}
However, adding books ignoreNotFound:true to Book static mapping (to get around some legacy issues) causes Book.get(1).authorId to be null.
Is there a way to get the value of AUTHOR_ID column in the BOOK table when the related record is missing in the AUTHOR table?
In my domain I have a many-to-many relationship. The problem is that GORM forces me to define owner entity but I don't think that either side "owns" the relation.
class User {
String username
String password
static hasMany = [organizations: Organization]
static belongsTo = Organization
static constraints = {
}
}
class Organization {
String name;
static hasMany = [members: User]
}
In this case I'm obviously not allowed to delete User who is in some organization (because Organization "owns" the relation). I would like to be able to delete both entities and on delete simply remove the relation (row from user_organization table). Is it possible or do I have to write this logic myself (if so, what would be the best way to implement this)?
You are allowed to delete both sides of the relationship, no matter who is the "owner". The belongsTo just applies the proper cascading so you don't have to.
In your example if you want to delete a user you first have to remove the relationship. So, in order to delete a user you do:
organization.removeFromMembers(user)
user.delete()
And if you delete an organization, since it is the "owner", you just don't need to use removeFrom*.
I have some domain class Incident,Problem, Category, Impact, Urgency etc.
Class Incident
{
Category category
String subject
Impact impact
}
Class Problem
{
Urgency urgency
Category category
String title
}
Class Category
{
String categoryName
String description
}
now, some rows are inserted into this class. now if I am deleting category it throws error like 'grails cannot delete or update a parent row'..
so what I have to do for deleting?
The problem is - you have reference to Category in Incident and Problem classes, so database tables for those classes will have Foreign key on category table, so you can not delete a category untill you either remove those incidents/problems or update those incidents problems and set category to null (you will have to make them as nullable in domain constraints)
So either you do
Problem.executeUpdate('update Problem p set category = null where category = ?', [category])
Same for incidents
Or you can model your domain classes using belongsTo and hasMany and grails will handle every thing automatically
Some thing like
class Problem {
static belongsTo = [category:Category]
}
class Category {
static hasMany = [
problems: Problem
]
static mappings = {
problems cascade: "all-delete-orphan"
}
}
I would prefer to manage relationships using belongsTo, hasMany, hasOne rather then just using references, it expresses the model better.
It depends on your domain model as well, in your business can problems, incidents exist without a category ! or they must belong to some category. If your answer is first option, your dont want to cascade delete, but update those incidents/problems with null category, if your answer is second option - you needs cascade all-delete-orphan
How your Category looks like, is it belongsTo Incident domain class,if category belongs to some domain class you can not delete it.
Ref : See here