LINQ：向上遍历层次结构以检索父层次结构答案

【问题标题】：LINQ: Traverse up a hierarchy to retrieve parent hierarchyLINQ：向上遍历层次结构以检索父层次结构
【发布时间】：2019-12-25 01:34:24
【问题描述】：

我有一些数据代表同一实体上的父子关系。给定一个节点，我需要找到它的整个上层（父、祖父、曾祖父等）

我的实体是这样的：

 public partial class Location{
        public int LocationId { get; set; }
        public int? FkParentLocationId { get; set; }
        ..... more properties here.......
        public virtual Location FkParentLocation { get; set; }
        public virtual ICollection<Location> InverseFkParentLocation { get; set; }
}

我指的是建议的层次结构遍历实现here，但是当你沿着层次结构向下时它会起作用。如何使用 LINQ 检索上层层次结构？

样本数据：

            List<Location> locations = new List<Location> {
            new Location { LocationId = 5, FkParentLocationId = 3, LocationName = "Windsor", LocationDisplayName = "Windsor"},
            new Location { LocationId = 15, FkParentLocationId = 3, LocationName = "Hampshire", LocationDisplayName = "Hampshire" },
            new Location { LocationId = 12, FkParentLocationId = 3, LocationName = "Sussex", LocationDisplayName = "Sussex"},
            new Location { LocationId = 13, FkParentLocationId = 3, LocationName = "Willowood", LocationDisplayName = "Willowood"},
            new Location { LocationId = 1, FkParentLocationId = 3, LocationName = "Gerbshire", LocationDisplayName = "Gerbshire"},
            new Location { LocationId = 3, FkParentLocationId = 2, LocationName = "Lincoln", LocationDisplayName = "Lincoln"},
            new Location { LocationId = 2, LocationName = "Mains", LocationDisplayName = "Mains" }            };

预期输出：给定位置 ID：5，我应该得到一个包含位置 3 和 2 的列表（因为它们是父位置）。

【问题讨论】：

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标签： linq parent-child hierarchical-data ef-core-3.0

【解决方案1】：

这是您可以使用的一种方法，通过控制台应用程序进行了演示。大量借用Jon Skeet。

using System;
using System.Collections.Generic;
using System.Linq;

namespace Locations
{
    public partial class Location 
    {
        public int LocationId { get; set; }
        public int? FkParentLocationId { get; set; }
        public virtual Location FkParentLocation { get; set; }
        public virtual ICollection<Location> InverseFkParentLocation { get; set; }
        public string LocationName { get; set; }
        public string LocationDisplayName { get; set; }      
    }  

    class Program
    {
        static void Main(string[] args)
        {
            List<Location> locations = new List<Location> {
            new Location { LocationId = 5, FkParentLocationId = 3, LocationName = "Windsor", LocationDisplayName = "Windsor"},
            new Location { LocationId = 15, FkParentLocationId = 3, LocationName = "Hampshire", LocationDisplayName = "Hampshire" },
            new Location { LocationId = 12, FkParentLocationId = 3, LocationName = "Sussex", LocationDisplayName = "Sussex"},
            new Location { LocationId = 13, FkParentLocationId = 3, LocationName = "Willowood", LocationDisplayName = "Willowood"},
            new Location { LocationId = 1, FkParentLocationId = 3, LocationName = "Gerbshire", LocationDisplayName = "Gerbshire"},
            new Location { LocationId = 3, FkParentLocationId = 2, LocationName = "Lincoln", LocationDisplayName = "Lincoln"},
            new Location { LocationId = 2, LocationName = "Mains", LocationDisplayName = "Mains" }            };


            var result  = GetAncestorsIds(locations, 5);
            foreach (var id in result)
            {
                System.Console.WriteLine(id);
            }          
        }

        private static IEnumerable<int> GetAncestorsIds(List<Location> locations, int id)
        {
            Location location = locations.SingleOrDefault(l => l.LocationId == id);
            if(location != null)
            {   
                while(location != null && location.FkParentLocationId != null)
                {
                    location = locations.SingleOrDefault(l => l.LocationId == location.FkParentLocationId);
                    if(location != null)
                    {
                        yield return location.LocationId;
                    }
                }
            }   
        }
    }
}

而且这种方法可以变成你自己的 Linq 扩展。这是它的外观。

public static class MyExtensions
{
    public static IEnumerable<int> GetAncestorIds<TSource>(this IEnumerable<TSource> source, Func<TSource, int> pk, Func<TSource, int?> fk, int id)
    {
        TSource currentObj = source.SingleOrDefault(s => pk(s) == id);

        while(currentObj != null && fk(currentObj) != null)
        {
            currentObj = source.SingleOrDefault(s => pk(s) == fk(currentObj));
            if(currentObj != null)
            {
                yield return pk(currentObj);
            }
        }
    }
}

然后为你的场景调用这个，你会这样做

var result = locations.GetAncestorIds(l => l.LocationId, l => l.FkParentLocationId, 5);

【讨论】：