使用 Entity Framework Core 时无法在实体中保留列表

Question

Using Microsoft.EntityFrameworkCore.InMemory (5.0.0), I have the following code:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Threading.Tasks;
using Microsoft.EntityFrameworkCore;
using BlogExample;

namespace BlogExample.Data
{
    public class BlogsContext : DbContext
    {
        public BlogsContext (DbContextOptions<BlogsContext> options)
            : base(options)
        {
        }

        public DbSet<BlogExample.Blog> Blogs { get; set; }
        public DbSet<BlogExample.Post> Posts { get; set; }

        protected override void OnModelCreating(ModelBuilder modelBuilder)
        {
            modelBuilder.Entity<Blog>()
                .HasMany(b => b.Posts)
                .WithOne();

            modelBuilder.Entity<Blog>()
                .Navigation(b => b.Posts)
                    .UsePropertyAccessMode(PropertyAccessMode.Property);
        }
    }
}

namespace BlogExample
{
    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; }
    }
}

Now I have the following controller:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Threading.Tasks;
using Microsoft.AspNetCore.Http;
using Microsoft.AspNetCore.Mvc;
using Microsoft.EntityFrameworkCore;
using BlogExample;
using BlogExample.Data;

namespace BlogExample.Controllers
{
    [Route("api/[controller]")]
    [ApiController]
    public class BlogsController : ControllerBase
    {
        private readonly BlogsContext _context;

        public BlogsController(BlogsContext context)
        {
            _context = context;
        }

        // GET: api/Blogs
        [HttpGet]
        public async Task<ActionResult<IEnumerable<Blog>>> GetBlog()
        {
            return await _context.Blogs.ToListAsync();
        }

        // GET: api/Blogs/5
        [HttpGet("{id}")]
        public async Task<ActionResult<Blog>> GetBlog(int id)
        {
            var blog = await _context.Blogs.FindAsync(id);

            if (blog == null)
            {
                return NotFound();
            }

            return blog;
        }

        // POST: api/Blogs
        // To protect from overposting attacks, see https://go.microsoft.com/fwlink/?linkid=2123754
        [HttpPost]
        public async Task<ActionResult<Blog>> PostBlog(Blog blog)
        {
            _context.Blogs.Add(blog);
            await _context.SaveChangesAsync();

            return CreatedAtAction("GetBlog", new { id = blog.BlogId }, blog);
        }
    }
}

So, when I POST the following JSON for a blog:

{
  "blogId": 1,
  "url": "MyUrl",
  "posts": [
    {
      "postId": 1,
      "title": "title1",
      "content": "content1"
    }
  ]
}

Then if I try to perform the GET call to get all my Blogs, I get the blog I just entered above with the posts list property set to null as shown in the response body below:

[
  {
    "blogId": 1,
    "url": "MyUrl",
    "posts": null
  }
]

My code can be found here

I am using the following documentation as a reference:

https://docs.microsoft.com/en-us/ef/core/modeling/relationships?tabs=fluent-api%2Cfluent-api-simple-key%2Csimple-key#single-navigation-property-1

Answer 1

You need to request that EF include related entities. Otherwise fetching a single Entity could end up bringing the whole database back.

Should be

// GET: api/Blogs
[HttpGet]
public async Task<ActionResult<IEnumerable<Blog>>> GetBlogs()
{
    return await _context.Blogs
                         .Include(blog => blog.Posts)
                         .ToListAsync();
}

See

Eager loading

You can use the Include method to specify related data to be included in query results. In the following example, the blogs that are returned in the results will have their Posts property populated with the related posts.

Eager Loading of Related Data