实体框架SQLite无法检索相关对象

Question

我试着用WPF重新创建windows 10中的便笺应用程序。我将实体框架与SQLite数据库结合使用。EF无法检索另一个对象的子对象。我怎么能这么做？

我基本上有两个类，我想存储在这个数据库中，StickyNote和SitckyNoteGroup。每个StickyNote都有一个StickyNoteGroup。为了让您完全理解，我将发布与我的问题相关的所有类，但是如果您想要https://github.com/Kamigoro/StickyNotes，这里是github。

namespace Todo.Models
{
    public class StickyNote
    {
        public int Id { get; set; }
        public string Name { get; set; }
        public string Text { get; set; }
        public StickyNoteGroup Group { get; set; }

        public override string ToString()
        {
            return $"Name : {Name}\nText : {Text}";
        }

    }
}

namespace Todo.Models
{
    public class StickyNoteGroup
    {
        public int Id { get; set; }
        public string Name { get; set; }
        public string Color { get; set; }

        public override string ToString()
        {
            return $"Name : {Name}\nColor : {Color}";
        }
    }
}

我使用EntityFramework的类如下所示，名为NoteContext

using Microsoft.EntityFrameworkCore;

namespace Todo.Models.DataAccess
{
    public class NoteContext : DbContext
    {
        public DbSet<StickyNote> Notes { get; set; }
        public DbSet<StickyNoteGroup> NoteGroups { get; set; }


        protected override void OnConfiguring(DbContextOptionsBuilder options)
            => options.UseSqlite("Data Source=Todo.db");

    }
}

最后，我用来执行CRUD操作的类名为DataAccessor，如下所示。

namespace Todo.Models.DataAccess
{
    public static class DataAccessor
    {
        public static List<StickyNote> GetStickyNotes()
        {
            List<StickyNote> notes;
            using (var database = new NoteContext())
            {
                notes = database.Notes.ToList();
            }
            return notes;
        }

        public static void SaveStickyNote(StickyNote stickyNote)
        {
            using (var database = new NoteContext())
            {
                database.Notes.Add(stickyNote);
                database.SaveChanges();
            }
        }

        public static List<StickyNoteGroup> GetStickyNoteGroups()
        {
            List<StickyNoteGroup> noteGroups;
            using (var database = new NoteContext())
            {
                noteGroups = database.NoteGroups.ToList();
            }
            return noteGroups;
        }

        public static void SaveStickyNoteGroup(StickyNoteGroup stickyNoteGroup)
        {
            using (var database = new NoteContext())
            {
                database.NoteGroups.Add(stickyNoteGroup);
                database.SaveChanges();
            }
        }

    }
}

我的问题是为什么这部分代码告诉我当前的StickyNote没有StickyNoteGroup，尽管sqlite数据库中有？。

        private void btnAddStickyNote_Click(object sender, RoutedEventArgs e)
        {
            StickyNoteGroup group = new StickyNoteGroup() { Name = "Test Group", Color = "Red" };
            StickyNote note = new StickyNote() { Name = "Note 1", Text = "An attempt to write a note", Group = group };
            DataAccessor.SaveStickyNote(note);

            foreach (StickyNote currentNote in DataAccessor.GetStickyNotes())
            {
                Debug.WriteLine(currentNote.Group.ToString());
            }
        }

StickyNote表：

​

StickyNoteGroup表：

​

非常感谢你的回答。如果您对我的代码还有其他评论要做，它们是受欢迎的，因为我真的不知道使用这样的ORM的好模式。

Answer 1

似乎您需要添加Include调用：

 public static List<StickyNote> GetStickyNotes()
    {
        List<StickyNote> notes;
        using (var database = new NoteContext())
        {
            notes = database.Notes
                .Include(n => n.Group)
                .ToList();
        }
        return notes;
    }

顺便说一句，您可以从内部的ToList语句返回计算过的查询结果数据(例如通过ToArray、First等)：

 public static List<StickyNote> GetStickyNotes()
    {
        using (var database = new NoteContext())
        {
            return database.Notes
                .Include(n => n.Group)
                .ToList();
        }
    }