EF6 Code First Lazy Load results in null collectio

问题:

So the dynamic proxy is created, but I can't figure out what I've done wrong to prevent navigation properties from lazy loading. Here is the exact code I've run to test the issue.

DbContext:

public class MyDbContext : DbContext
{
    public MyDbContext()
        : base("MyConnection")
    {
    }

    public DbSet<One> Ones { get; set; }

    public DbSet<Many> Manies { get; set; }
}

Classes:

public class One
{
    public int Id { get; set; }

    public virtual ICollection<Many> Manies { get; set; }

    public One()
    {
        Manies = new List<Many>();
    }
}

public class Many
{
    public int Id { get; set; }

    public string Value { get; set; }

    public int OneId { get; set; }

    public virtual One One { get; set; }

    public Many()
    {
    }
}

Test:

    [TestMethod]
    public void OneToManyTest()
    {
        One parent1 = new One();
        parent1.Manies.Add(new Many() { Value = "child 1" });
        parent1.Manies.Add(new Many() { Value = "child 2" });

        using (MyDbContext db = new MyDbContext())
        {
            db.Ones.Add(parent1);
            db.SaveChanges();
        }
        Assert.IsTrue(parent1.Id > 0, "Id not set");

        One parent2;
        using (MyDbContext db = new MyDbContext())
        {
            db.Configuration.ProxyCreationEnabled = true;
            db.Configuration.LazyLoadingEnabled = true;
            parent2 = db.Ones.Find(parent1.Id);//parent2 is a dynamic proxy
        }

        Assert.AreEqual(parent1.Id, parent2.Id);
        /*parent2.Manies is null*/
        Assert.AreEqual(parent1.Manies.Count, parent2.Manies.Count);//fails
    }

Database:

I've verified the correct information is being inserted in the database. The relationships look good. I'm sure I'm missing something obvious.

Update

This works:

using (MyDbContext db = new MyDbContext())
{
    db.Configuration.ProxyCreationEnabled = true;
    db.Configuration.LazyLoadingEnabled = true;
    parent2 = db.Ones.Find(parent1.Id);//parent2 is a dynamic proxy
    Assert.AreEqual(parent1.Id, parent2.Id);
    Assert.AreEqual(parent1.Manies.Count, parent2.Manies.Count);
}

This doesn't:

using (MyDbContext db = new MyDbContext())
{
    db.Configuration.ProxyCreationEnabled = true;
    db.Configuration.LazyLoadingEnabled = true;
    parent2 = db.Ones.Find(parent1.Id);//parent2 is a dynamic proxy
}
using (MyDbContext db = new MyDbContext())
{
    Assert.AreEqual(parent1.Id, parent2.Id);
    Assert.AreEqual(parent1.Manies.Count, parent2.Manies.Count);//parent2.Manies is null
}

So the same db context is required for built in lazy loading.

回答1:

To trigger lazy loading you need to access the property in some way, before disposing of the context.

Your test code doesn't acces the property before leaving the context:

    One parent2;
    using (MyDbContext db = new MyDbContext())
    {
        db.Configuration.ProxyCreationEnabled = true;
        db.Configuration.LazyLoadingEnabled = true;
        parent2 = db.Ones.Find(parent1.Id);//parent2 is a dynamic proxy
    }
    // Context disposed: thsi would throw an exception:
    var manies = parent2.Manies.ToList()

At this point, your context has been disposed. If you tried to access the Manies property you'd get an error stating this.

    One parent2;
    using (MyDbContext db = new MyDbContext())
    {
        db.Configuration.ProxyCreationEnabled = true;
        db.Configuration.LazyLoadingEnabled = true;
        parent2 = db.Ones.Find(parent1.Id);//parent2 is a dynamic proxy
        // Context available: this sill lazy load the Manies entities
        var manies = parent2.Manies.ToList(); 
    }

Now, if you check the manies properties, it will be available.

The idea of lazy loading is that, while the context is available, the first time you access a property which wasn't loaded initially, it will be loaded at that moment.

Please, see this article to understand the different ways (eager, lazy, explicit) of loading entities with EF:

Loading Related Entities

回答2:

parent2 = db.Ones.Include(o=>o.Manies).FirstOrDefault(o=>o.Id == parent1.Id);