c# - EF Core 一对多关系列表返回 null

Question

我正在尝试学习如何在 EF Core 中正确使用 DbContext。我有一个团队类(class):

public class Team 
{
    public int ID { get; set; }
    public string Name { get; set; }
    public bool CanSelfManage { get; set; } = false;
    public virtual List<Mileage> Mileages { get; set; }
    public IdentityUser Member { get; set; }
    public string State { get; set; }
    public List<Horse> Horses { get; set; }
}

还有一个 Mileage 类:

public class Mileage
{
    public int ID { get; set; }
    public virtual Team Team { get; set; }
    public virtual int TeamID { get; set; }
    public DateTime Date { get; set; }
    public LogType Type { get; set; }
    public decimal Miles { get; set; }
    public IdentityUser User { get; set; }
    public List<Horse> Horses { get; set; }
}

我的 DbContext 类包含

public DbSet<Team> Teams { get; set; }
public DbSet<Mileage> Mileages { get; set; }
public DbSet<Horse> Horses { get; set; }
public DbSet<SecurityEntry> SecurityEntries { get; set; }
protected override void OnModelCreating(ModelBuilder modelBuilder)
{
    base.OnModelCreating(modelBuilder);
    modelBuilder.Entity<Mileage>()
        .HasOne(t => t.Team)
        .WithMany(b => b.Mileages)
        .HasForeignKey(t => t.TeamID)
        .IsRequired();
    modelBuilder.Entity<Horse>()
        .HasOne(t => t.Team)
        .WithMany(h => h.Horses);
    modelBuilder.Entity<Horse>()
        .HasMany(m => m.Mileages);
    modelBuilder.Entity<Mileage>()
        .HasMany(h => h.Horses);
}

我遇到的问题是，无论我做什么，Team.Mileages 都会返回 null 并且永远不会被填充。
如果我将 List 设置为不映射，注入(inject) DbContext 并尝试在上下文之外运行任何东西，它会抛出以下错误:

A second operation started on this context before a previous operation completed

我是否遗漏了什么明显的东西？我正在使用 MySQL，如果这有什么不同的话。

Answer 1

Entity Framework 默认使用延迟加载，您要么设置为预先加载并始终加载您的引用，要么在数据库请求时请求您的集合。示例:

_dbcontext.Team.Include(team => team.Mileages).ToList();