[英]Grails inheritance and conflicting one-to-many and many-to-many relationship
我通過嘗試創建簡單的Twitter副本來學習grails。 我目前正在嘗試納入關注者和群組。 我最初想出了一個非常基本的數據庫結構,但在實現它方面卻沒有運氣。 關系的設計如下:
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成為User的實例,然后創建一個將User映射到Person的表。 這樣,僅一個表被創建/用於Group2Person和Person2Person之間的關系。
詳細信息:組是由人員創建的,因此它應該具有“所有者”(person_id)。 它還有許多關注者(即成員)。 組不能跟隨其他組,但是一個人可以跟隨另一個人或一個組。
下面是我如何在grails中實現此功能:
用戶
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'
}
}
組
class Group extends User {
Person owner
String description
def getTweets() {
return followers.tweets.flatten()
}
static transients = {
tweets
}
}
鳴叫 (以防萬一?)
class Tweet {
static belongsTo = [author: Person]
String text
Date dateCreated
static constraints = {
text shared: "mustFill", maxSize: 140
}
}
當我運行cmd grails schema-export
,出現以下錯誤:“ |錯誤加載插件管理器時出錯:域類[tweeter.Group]和[tweeter.Person類]不能多對多擁有關系。兩者都包含相互引用的belongsTo定義。(使用--stacktrace查看完整的跟蹤)”
我能夠使數據庫創建幾乎正確的架構。 不幸的是,使用了User2Person(又名追隨者)聯接表的主鍵(user_id,person_id)。 那意味着我不能有兩個記錄,例如:(1、2)和(2、1)(例如,兩個用戶彼此關注)。 下面是更新的類( commit ):
用戶
class User {
static belongsTo = Person
static hasMany = [followers: Person]
String name
Date dateCreated
Date lastUpdated
static constraints = {
name shared: "mustFill", size: 3..20
}
}
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'
}
}
模式中的關注者表如下所示:
create table user_follows (
user_id bigint,
follows__id bigint,
primary_key(user_id, follows__id)
)
我在網上搜索了有關更改聯接表主鍵的信息。 我能找到的最好的方法是使用類似以下的代碼:
static mappedBy = { followers joinTable: [name:"someName", ...] }
不幸的是,我很難找到有關joinTable映射的良好文檔,並且大多數資料似乎都表明不可能輕易更改連接表的主鍵。 然后,我決定按照本指南使用單獨的域類來定義聯接表:沒有Hibernate XML的多對多映射。 以下是最終更新的代碼( commit ):
用戶
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)
}
}
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)
}
}
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()
}
}
}
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