Grails / Gorm : Difference between declaring object and describing relationship? - grails

Question

I'm having trouble understanding the difference between declaring a domain-object in another domain and specifying the relationship between the domains.
Sample code:
class User {
Book book
}
versus
class User {
static hasOne = Book
}
class Book {
String name
}

Answer 1

The hasOne relationship will put the key on the child object, so in the db you'll find book.user_id with hasOne rather than user.book_id if you just declare Book book on User. You'll see the difference in the DDL generated if you use grails schema-export.
Here's the DDL with hasOne in place:
create table book (id bigint generated by default as identity (start with 1), version bigint not null, user_id bigint not null, primary key (id), unique (user_id));
create table user (id bigint generated by default as identity (start with 1), version bigint not null, primary key (id));
alter table book add constraint FK2E3AE98896CD4A foreign key (user_id) references user;
Here's the DDL with just Book book on User:
create table book (id bigint generated by default as identity (start with 1), version bigint not null, primary key (id));
create table user (id bigint generated by default as identity (start with 1), version bigint not null, book_id bigint not null, primary key (id));
alter table user add constraint FK36EBCB952E108A foreign key (book_id) references book;
Notice that the book table has the reference in the first example and the user has it in the 2nd.
Long answer: I strongly recommend watching Burt Beckwith's presentation on GORM/collections/mapping. Lots of great info around GORM and the consequences of various advantages/problems with describing relationships with hasMany/belongsTo, etc.

Answer 2

The main difference is that when using hasOne the foreign key reference is stored in the child table instead of the parent table, i.e. a user_id column would be stored in the book table instead of a book_id column being stored in the user table. If you didn't use hasOne, then a book_id column would be generated in the user table.
There is an explanation and example in the Grails documentation for hasOne.
Hope this helps.