Grails / Gorm：如何在不影响数据库的情况下过滤域对象列表

Question

Say we have something like the standard Book domain object and bookCategory object. In my controller I want to return a subset of list of books to the view. That subset is not achievable using a find query. When I try to filer the return object, it deletes relationships from the database!

I tried this:

class BookCategory{
     String name
     static hasMany = [books:Book]
}

class Book{
    String title
}

def myController() {
    def categories
    categories = BookCategory.list()
    def user = getCurrentUser()

    categories.each { category ->
         category.books.removeAll { book ->
             !isBookBannedForThisUser(book.title, user)
        }
    [bookCategories: categories]
    }
}

The problem is that it permanently removes these books from the categories for all users from the database!!!

I tried putting the method in a service and using a readonly transaction, but this did not help.

I assume that even if I copy all the categories and books into new list, they will still update the DB as they will still have the book IDs (which I need)

Saving to the database when you dont say save() is very dangerous. is there a way to disable this feature completely?

Answer 1

There is a fundamental flaw in your approach. Do not modify your domain instances if you don't intend to have the changes persisted. Doing so is going to cause you headaches.

Your domain model is suppose to be your system of record. Any changes to it are suppose to be persisted.

If you need to gather up data and manipulate it without having it reflected in your domain model then use a DTO (data transfer object) or similar pattern.

Simply calling .discard() will discard the changes you have made from being persisted when the session automatically flushes.

Instead of working against the framework, and disabling behavior, change your approach to be correct.