Grails的Many-to-Many改良

標準的M-M做法：

class User{
    static hasMany=[roles:Role]
    String name
}

class Role{
    static belongsTo = User
    static hasMany=[users:User]
    String name
}

使用方法：

Role roleUser=new Role(name: "ROLE_USER").save()
Role roleAdmin==new Role(name: "ROLE_ADMIN").save()

User user1= new User(name: "bruce_lin_chn")
user1.addToRoles(roleUser)
user1.save()

存在的問題：
[list]
[*]hasMany會在User和Role中創建Set(org.hibernate.collection.PersistentSet)
[*]爲了唯一性檢查，增加一個Role到User要求加載所有用戶角色的實例和所有其他擁有該權限的用戶
[*]存在樂觀鎖異常的風險
解決方案：

class User{
    String name
}

class Role{
    String name
}

class UserRole implements Serializable{
    User user
    Role role
    static mapping = {
        table 'user_role_mapping'
        version false
        id composite: ['user', 'role']
    }

    static Set<Role> getRoles(User user){
        UserRole.findAllByUser(user).collect{it.role} as Set
    }

    static boolean hasRole(User user, Role role){
        return UserRole.countByUserAndRole(user,role)>0
    }

    static beforeDeleteUser(User user){

    }

    static beforeDeleteRole(Role role){

    }
}

[list]
[*]
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