实体框架导航属性为空

Question

我有两个使用实体框架的模型。

public class Player
{

    public int PlayerId { get; set; }
    public string Name { get; set; }
    public string Sex { get; set; }
    public string Plays { get; set; }
    public string FavouriteSurface { get; set; }

}

public class SinglesMatch
{
    public int SinglesMatchId { get; set; }
    public int Player1Id { get; set; }
    public int Player2Id { get; set; }
    public int PlayerIdWinner { get; set; }
    public DateTime Date { get; set; }
    public string Venue { get; set; }
    public string Score { get; set; }
    public List<Player> Players { get; set; } 
}

我正在使用下面的代码尝试根据 SinglesMatch 模型中的 PlayerId 与 Player 模型中的 PlayerID 匹配来显示玩家的名称。

      @foreach (var item in @Model)
                    {

                        <ul id="Players" class="bg-success"></ul>
                        <br/>
                        <h3>Date - @Html.DisplayFor(@modelItem => item.Date)</h3>
                        <li>Venue - @Html.DisplayFor(@modelItem => item.Venue)</li>
                        <li>Player 1 - @Html.DisplayFor(@modelItem => item.Players.First(p => p.PlayerId == item.Player1Id).Name)</li>
                        <li>Player 2 - @Html.DisplayFor(@modelItem => item.Players.First(p => p.PlayerId == item.Player2Id).Name)</li>
                        <li>Score- @Html.DisplayFor(@modelItem => item.Score)</li>



                    }

调试时，从我的存储库中检索模型时，导航属性始终显示为 null。

我是否以正确的方式使用导航属性？我的查询有问题吗？

编辑以包含 DbContext：

    public TennisTrackerContext() : base("name=TennisTrackerContext")
    {

    }

    public DbSet<Player> Players { get; set; }
    public DbSet<PlayerRecord> PlayerRecords { get; set; }
    public DbSet<SinglesMatch> SinglesMatches { get; set; }
    public DbSet<DoublesMatch> DoublesMatches { get; set; }
    public DbSet<Venue> Venues { get; set; } 
}

}

Answer 1

您需要添加一个桥接表。 Sql 将自动创建它，但除非您在 c# 中创建它，否则您将无法访问这些变量。

public class Player
    {
        public int PlayerId { get; set; }
        public string Name { get; set; }
        public string Sex { get; set; }
        public string Plays { get; set; }
        public string FavouriteSurface { get; set; }
        List<PlayerInMatch> Matches { get; set; }

        public Player()
        {
            Matches = new List<PlayerInMatch>();
        }
    }

    public class PlayerInMatch
    {
        public int Id { get; set; }
        public int PlayerId { get; set; }
        [ForeignKey("PlayerId")]
        public Player Player { get; set; }
        public int SinglesMatchId { get; set; }
        [ForeignKey("SinglesMatchId")]
        public SinglesMatch SinglesMatch { get; set; }

    }

    public class SinglesMatch
    {
        public int SinglesMatchId { get; set; }
        public int PlayerIdWinner { get; set; }
        public DateTime Date { get; set; }
        public string Venue { get; set; }
        public string Score { get; set; }
        public List<PlayerInMatch> Players { get; set; }

        public SinglesMatch()
        {
            Players = new List<PlayerInMatch>();
        }
    }

    static void Main(string[] args)
    {
        var match = new SinglesMatch();
        match.Players.Select(c => c.Player.Name);
    }

Answer 2

您需要将导航属性设为virtual 以启用延迟/急切加载：

public class SinglesMatch
{
public int SinglesMatchId { get; set; }
public int Player1Id { get; set; }
public int Player2Id { get; set; }
public int PlayerIdWinner { get; set; }
public DateTime Date { get; set; }
public string Venue { get; set; }
public string Score { get; set; }

public virtual List<Player> Players { get; set; } 
}

另外，你在fluent api中定义了SinglesMatch和Singles之间的关系吗？

编辑：我看到你没有通过注释或流利的 api 映射任何关系，我建议你看看这个：

https://msdn.microsoft.com/en-us/data/jj591617.aspx