Linq Join：使用映射表连接并关联对象

Question

我正在尝试编写一个 linq 查询来进行非常基本的连接。我有两个数组

Park[] parks = new Park[]{
                new Park() {ID = 1, Name = "Free Park"},
                new Park() {ID = 2, Name = "Cost Park"},
                new Park() {ID = 3, Name="Sneak in Park"}
};

和

Facility[] facilities = new Facility[] {
                new Facility() { ID = 1, Name = "Swing", MinimumAge = 1, MaximumAge = 120},
                new Facility() { ID = 2, Name = "Slide", MinimumAge = 1, MaximumAge = 200},
                new Facility() { ID = 3, Name = "See-Saw", MinimumAge = 1, MaximumAge = 300}
};

每个公园可以有0...n设施，因此我们有一组映射对象

ParkFacility[] associations = new ParkFacility[] {
                new ParkFacility() {ParkID = 1, FacilityID = 1},
                new ParkFacility() {ParkID = 1, FacilityID = 2},
                new ParkFacility() {ParkID = 1, FacilityID = 3},
                new ParkFacility() {ParkID = 2, FacilityID = 1},
                new ParkFacility() {ParkID = 3, FacilityID = 2}
};

这是Park类的定义

class Park
{
        public int ID { get; set; }
        public String Name { get; set; }
        public Facility[] Facilities { get; set; }
}

是否可以仅使用连接并将适当的设施与公园相关联？即将 Park 中的 Facilities 数组设置为使用 associations 适当映射的数组？

编辑：到目前为止我的研究..

var x_temp = from g in parks
             join j in associations on g.ID equals j.ParkID into h
             select new Park()
             {
                Name = g.Name,
                ID = g.ID,
                Facilities = (from u in h join m in facilities on u.FacilityID equals m.ID select m).ToArray()
             };

我尝试使用 sub-linq 查询并且它可以工作，但我正在寻找一个只有 连接的解决方案

Answer 1

您可以创建从公园 ID 到设施的查找，并使用它来填充每个 Park 对象的 Facilities 属性。请注意，执行此操作的最佳位置是在 Park 的构造函数中，但根据您现有的代码，此 sn-p 将在对象初始化器中执行此操作：

var lookup = associations.ToLookup(pf => pf.ParkID, pf => facilities.Single(f => f.ID == pf.FacilityID));
Park[] parks = new Park[]{
    new Park() {ID = 1, Name = "Free Park", Facilities = lookup[1].ToArray()},
    new Park() {ID = 2, Name = "Cost Park", Facilities = lookup[2].ToArray()},
    new Park() {ID = 3, Name="Sneak in Park", Facilities = lookup[3].ToArray()}
};

此外，将所有Facility 和Park 实例存储在将ID 映射到实例的Dictionary 中会很有帮助。在这种情况下，您的查找不需要对每个关联的所有设施进行线性扫描。