.NET Core 2 EF, object references are null

Question

I've started to use .NET Core 2 and databases for the first time and looked at examples like: Getting Started with EF Core on .NET Core Console App with a New database. I've got a few models, like

public class BloggingContext : DbContext
{
    public DbSet<Blog> Blogs { get; set; }
    public DbSet<Post> Posts { get; set; }
}

public class Blog
{
    public int BlogId { get; set; }
    public string Url { get; set; }

    public List<Post> Posts { get; set; }
}

public class Post
{
    public int PostId { get; set; }
    public string Title { get; set; }
    public string Content { get; set; }

    public int BlogId { get; set; }
    public Blog Blog { get; set; }
}

But if I load an object, like Post post = context.Post.SingleOrDefault(s => s.PostId == id) then post.Blog is null. But if I before reading from the database, expand context.Blogs in the debugger, then post.Blog is a valid object after reading from the database with the above command.

So it feels like I need to read the blogs from the database to load those so the reference from Post will be correct. What is the correct way of doing this?

Second question: how to get the database context from anywhere? Should I have a default constructor with a connection string set in the constructor and create a new context all the time?

Answer 1

Take a look here for detailed explanation: https://learn.microsoft.com/en-us/ef/core/querying/related-data

A few examples from the link above:

Eager loading

using (var context = new BloggingContext())
{
    var blogs = context.Blogs
        .Include(blog => blog.Posts)
        .ToList();
}

Explicit loading

using (var context = new BloggingContext())
{
    var blog = context.Blogs
        .Single(b => b.BlogId == 1);

    context.Entry(blog)
        .Collection(b => b.Posts)
        .Load();
}

Lazy loading is not yet supported. It should be coming in v2.1.

Answer 2

Just to keep this updated for future reference, EF Core now supports lazy loading.

https://learn.microsoft.com/en-us/ef/core/querying/related-data

protected override void OnConfiguring(DbContextOptionsBuilder optionsBuilder)
=> optionsBuilder
    .UseLazyLoadingProxies()
    .UseSqlServer(myConnectionString);

This should solve the problem.