Grails / Gorm：声明对象和描述关系之间的区别？

发布于 2024-10-21 16:07:59 字数 205 浏览 6 评论 0原文

我无法理解在另一个域中声明域对象和指定域之间的关系之间的区别。

示例代码：

class User { 
Book book
}

与

class User { 
static hasOne = Book
}

class Book {
String name
}

原文

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
}

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我是有多爱你 2024-10-28 16:07:59

hasOne 关系会将键放在子对象上，因此在数据库中，如果您只是在 User 上声明 Book book ，您会发现带有 hasOne 的 book.user_id 而不是 user.book_id 。如果您使用grails schema-export，您将看到生成的 DDL 的差异。

这是带有 hasOne 的 DDL：

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;

这是在 User 上只有 Book book 的 DDL：

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;

请注意，book 表在第一个示例中具有引用，而用户在第二个示例中具有引用。

长答案：我强烈建议观看 Burt Beckwith 在 GORM/collections/mapping 上的演示。关于 GORM 的大量重要信息以及描述与 hasMany/belongsTo 等关系的各种优点/问题的后果。

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.

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