Defining Foreign Key in Entity Framework Code First

Question

In the following Model example (taken from this official ASP.NET site) I understand they are defining a Foreign Key relationship between Blogs (parent) and Posts (Child) tables. But in the Blog class below what is the use of public List<Post> Posts { get; set; } property? If I remove this property and generate SQL Server database out of this Model, I can still see database was successfully created with BlogId in Posts table as Foreign Key to Blog table

namespace EFGetStarted.AspNetCore.NewDb.Models
{
    public class BloggingContext : DbContext
    {
        public BloggingContext(DbContextOptions<BloggingContext> options)
            : base(options)
        { }

        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; }
    }
}

List<Post> Posts is a navigation property which makes it convenient to access Posts from a Blog. — Jasen
– Jasen, Commented Nov 7, 2016 at 17:26
@Jasen your link is broken, it should be msdn.microsoft.com/en-us/library/jj713564(v=vs.113).aspx — Tim
– Tim, Commented Nov 7, 2016 at 17:34
Seems I copied in an additional letter at the end of that URL. — Jasen
– Jasen, Commented Nov 7, 2016 at 17:39

Vivien Chevallier · Accepted Answer · 2016-11-07 17:29:26Z

3

Nam,

The Posts property in the Blog class could be useful if you have the need to access the posts directly from an instance of a Blog object.

Loading the posts could be done via lazy loading or Entity Framework Eager loading using the Include method:

    var blogs1 = context.Blogs 
                      .Include(b => b.Posts) 
                      .ToList();

If you remove it will not prevent the database to be properly created.

answered Nov 7, 2016 at 17:29

Vivien Chevallier

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5 Comments

alanh Over a year ago

Actually EfCore doesn't support lazy loading yet, you have to use Include. And just to clarify the FK is created based on BlogId, the Posts property is merely a navigation property.

nam Over a year ago

@VivienChevallier So your query is saying give me all posts of which blog? Or, your query is saying give me each blog along with all the posts associated with each blog. In other words, it's another way of writing a LINQ JOIN between Blogs and Posts tables. I'm trying to understand your response before marking it as an Answer.

Vivien Chevallier Over a year ago

@nam My query is saying: give me each blog along with all the posts associated with each blog.

nam Over a year ago

@VivienChevallier And this query would have not worked without the property public List<Post> Posts { get; set; } used in the Blog Class?

Vivien Chevallier Over a year ago

Exactly, you would not have been able to write it.

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