TypeORM cascade option: cascade, onDelete, onUpdate

onDelete: 'CASCADE isn't supported in OneToMany relations yet. More context here: https://github.com/typeorm/typeorm/issues/1913

This is my conclusion of looking into it:

The cascade option does not affect the database column constraints, and I believe is used by TypeORM only in evaluating how to save entity relations to the database. We can define entities like this:

@Entity()
class Book extends BaseEntity {
    @ManyToOne(() => Author, (author) => author.books, {
        onDelete: 'CASCADE',
    })
    public author?: Author
}

@Entity()
class Author extends BaseEntity {
    @OneToMany(() => Book, (book) => book.author, {
        cascade: true,
    })
    public books: Book[];
}

onDelete sets the authorId foreign key to CASCADE onDelete on Book. This means that when the author is deleted, the book is also deleted.

Setting cascade: true on Author tells TypeORM that if a new book is appended on an author and the author is saved, the new book should also be saved to the database. Like this:

const author = await Author.findOne({ id: '123' });
author.books.push(new Book(...));
await author.save();

If cascade is not set on Book, the new book will not be saved to the database.