EF Core One-to-Many relationship list returns null

Question

I'm trying to learn how to properly utilize DbContext in EF Core. I have a Team 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; }
}

And a Mileage class:

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; }
}

And my DbContext class contains

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);
}

The problem that I'm having is that, no matter what I do, Team.Mileages returns null and is never populated.
If I set the List to not be mapped, inject the DbContext and try to run anything off of the context, it throws the following error:

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

Is there something glaring that I'm missing? I am using MySQL, if that makes any difference.

Answer 1

Entity framework by default uses Lazy loading, you either set to eager loading and load your references always or you ask for your collections on a database request. Example:

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