Question

我正在通过尝试创建一个简单的twitter副本来学习grails。我目前正在尝试加入粉丝和群组。我最初提出了一个非常基本的数据库结构，我没有运气实现它。关系设计如下：

    Person:
        has many: Groups, Tweets, (Person as followers through User2Person)
    Group:
        has many: (Person as followers through User2Person)
        belongs to: Person as owner
    User2Person:
        belongs to: (Person or Group)
        belongs to: Person

基本上，我希望Person和Group成为User的一个实例，然后创建一个将User映射到Person的表。这样，只有一个表被创建/用于Group2Person和Person2Person之间的关系。

更多信息：一个人是由一个人创建的，所以它应该有一个“所有者”（person_id）。它也有许多粉丝（即成员）。小组不能跟随其他小组，但是一个人可以跟随另一个人或小组。

以下是我在grails中实现的方法：

用户

    abstract class User {
        static hasMany = [followers: Person]
        static mappedBy = [followers: "followed"]
        String name
        Date dateCreated
        Date lastUpdated

        static constraints = {
            name shared: "mustFill", size: 3..20
        }
    }

人

    class Person extends User {
        static belongsTo = [followed: User]
        static hasMany = [tweets: Tweet, groups: Group]
        static mappedBy = [groups: "owner"]
        String username
        String email

        static constraints = {
            username shared: "mustFill", unique: true, size: 4..15
            email shared: "mustFill", email: true
        }

        static mapping = {
            tweets sort: 'dateCreated', order: 'desc'
        }

    }

组

    class Group extends User {
        Person owner
        String description

        def getTweets() {
            return followers.tweets.flatten()
        }

        static transients = {
            tweets
        }
    }

推文（以防万一？）

    class Tweet {
        static belongsTo = [author: Person]
        String text
        Date dateCreated

        static constraints = {
            text shared: "mustFill", maxSize: 140
        }
    }

当我运行cmd grails schema-export时，我收到以下错误：“|错误加载插件管理器错误：域类[class tweeter.Group]和[class tweeter.Person]不能在很多中互相拥有-to-many relationship。两者都包含相互引用的belongsTo定义。（使用--stacktrace查看完整的跟踪）“

Answer 1

我能够让数据库创建几乎正确的架构。不幸的是，使用了User2Person（a.k.a.粉丝）的连接表的主键（user_id，person_id）。这意味着我不能有两个记录，如：（1,2）和（2,1）（例如，两个用户互相跟随）。以下是更新的课程（commit）：

用户

class User { static belongsTo = Person static hasMany = [followers: Person] String name Date dateCreated Date lastUpdated static constraints = { name shared: "mustFill", size: 3..20 } }

<强>人

class Person extends User { static hasMany = [tweets: Tweet, groups: Group, follows: User] static mappedBy = [tweets: "author", groups: "owner"] String username String email static constraints = { username shared: "mustFill", unique: true, size: 4..15 email shared: "mustFill", email: true } static mapping = { tweets sort: 'dateCreated', order: 'desc' } }

架构中的跟随者表格如下：

create table user_follows ( user_id bigint, follows__id bigint, primary_key(user_id, follows__id) )

我在网上搜索了有关更改连接表主键的信息。我能找到的最好的是使用如下代码：

static mappedBy = { followers joinTable: [name:"someName", ...] }

不幸的是，我很难找到关于joinTable映射的好文档，大多数来源似乎都表明无法轻松更改连接表的主键。然后我决定使用一个单独的域类来定义本指南后面的连接表：没有Hibernate XML的多对多映射。以下是最终更新的代码（commit）：

用户

class User { static belongsTo = Person static hasMany = [people: UserFollower] static mappedBy = [people: "followed"] String name Date dateCreated Date lastUpdated static constraints = { name shared: "mustFill", size: 3..20 } static transients = { followers } def getFollowers() { return people.collect { it.follower } } void addToFollowers(Person person) { UserFollower.link(this, person) } void removeFromFollowers(Person person) { UserFollower.unlink(this, person) } }

<强>人

class Person extends User { static hasMany = [tweets: Tweet, groups: Group, users: UserFollower] static mappedBy = [tweets: "author", groups: "owner", users:"follower"] String username String email static constraints = { username shared: "mustFill", unique: true, size: 4..15 email shared: "mustFill", email: true } static mapping = { tweets sort: 'dateCreated', order: 'desc' } static transients = { follows } def getFollows() { return users.collect { it.followed } } void addToFollows(User user) { UserFollower.link(user, this) } void removeFromFollows(User user) { UserFollower.unlink(user, this) } }

<强> UserFollower

class UserFollower { User followed Person follower static constraints = { followed nullable: false follower nullable: false } static void link(User user, Person person) { UserFollower f = UserFollower.findByFollowedAndFollower(user, person) if(!f) { f = new UserFollower() user.addToPeople(f) person.addToUsers(f) f.save() } } static void unlink(User user, Person person) { UserFollower f = UserFollower.findByFollowedAndFollower(user, person) if(f) { f = new UserFollower() user.removeFromPeople(f) person.removeFromUsers(f) f.delete() } } }

Grails继承和冲突的一对多和多对多关系

1 个答案: