带有ASP.NET的实体框架为一对多集合返回null

Question

I'm trying to create a one to many relationship with the following objects.

using System;
using System.Collections.Generic;
using System.Linq;
using System.Threading.Tasks;
using System.Data.Common;
using System.ComponentModel.DataAnnotations.Schema;
using Microsoft.EntityFrameworkCore;

namespace Program.Models
{
    public class Log
    {
        public int ID { get; set; }
        public string Source { get; set; }
        public string Title { get; set; }
        public DateTime Timestamp { get; set; }
        public ICollection<Category> Categories { get; set; }

        public Log()
        {
            this.Categories = new List<Category>();
        }
    }

    public class Category
    {
        public int ID { get; set; }
        public string Key { get; set; }
        public string Value { get; set; }
    }

    public class LogDBContext : DbContext
    {
        public DbSet<Log> Logs { get; set; }
        public DbSet<Category> Categories {get; set;}

        public LogDBContext(DbContextOptions<LogDBContext> options) : base(options)
        {
        }
        public LogDBContext() : base()
        {

        }
    }
}

On startup, I put the following entities into the database

        var cat1 = new Category { Key = "seed", Value = "seed" };
        var cat2 = new Category { Key = "seed2", Value = "seed2" };

        var log = new Log {
            Source = "seed",
                Timestamp = DateTime.Now,
                Title = "seed" };
        log.Categories.Add(cat1);
        log.Categories.Add(cat2);

        context.Logs.Add(log);

        context.SaveChanges();

I obtain all the entities by calling the function:

        [HttpGet]
        [Route("getAll")]
        public IEnumerable<Log> GetAll()
        {

            return _context.Logs.Include(c => c.Categories).AsEnumerable();


        }

I have tried putting the virtual keyword on the collection of categories (in the Log class) but no matter what I try I get null back for categories from the server.

[{"id":0,"source":"seed","title":"seed","timestamp":"2018-03-19T13:21:27.0034628","categories":null}]

Any help will be greatly appreciated.

Edit:

Based on the feedback I created a constructor for Log that initializes the ICollection.

public Log()
{
    this.Categories = new List<Category>();
}

Now, instead of null I'm getting an empty list [{"id":0,"source":"seed","title":"seed","timestamp":"2018-03-19T13:21:27.0034628","categories":[]}]

A step in the right direction but not there yet.

edit: For clarification, I'm using EF-Core 2.0.2 edit: Also, I am using .NET Core 2.05

"solution": I switched my project to use Asp.net with the full .Net Framework and it appears to work. I don't know why it refused to work with the asp.net with .net core.

Answer 1

Problem is that your One-to-Many relationship is not configured correctly. Write your model classes as follows:

public class Log
{
    public Log()
    {
        this.Categories = new List<Category>();
    }

    public int ID { get; set; }
    public string Source { get; set; }
    public string Title { get; set; }
    public DateTime Timestamp { get; set; }
    public ICollection<Category> Categories { get; set; }


}

public class Category
{
    public int ID { get; set; }

    [ForeignKey("Log")]
    public int LogID { get; set;}
    public string Key { get; set; }
    public string Value { get; set; }

    public Log Log {get; set;}
}

Hope, this will solve your problem..