Entity Framework Core 无法更新包含相关对象的 object

Question

I am trying to implement a WebApi with Entity Framework Core. I have modelled an implicit many-to-many relationship , and scaffolded the controllers based on the DbContext.

But this does not allow persisting an object with it's linked objects.

A simplified version demonstrating my problem:

using Microsoft.EntityFrameworkCore;
using Microsoft.VisualStudio.TestTools.UnitTesting;
using System.Collections.Generic;
using System.Linq;

namespace Tests
{
    public class Author
    {
        public long Id { get; set; }
        public string Name { get; set; }
        public ICollection<Book> Books { get; set; }
    }

    public class Book
    {
        public long Id { get; set; }
        public string Title { get; set; }
        public ICollection<Author> Authors { get; set; }
    }

    public class LibraryDbContext : DbContext
    {
        public DbSet<Book> Books { get; set; }
        public DbSet<Author> Authors { get; set; }

        protected override void OnConfiguring(DbContextOptionsBuilder optionsBuilder)
        {
            optionsBuilder.UseSqlServer(@"Server=localhost\mssqlserver01;Database=Library;Trusted_Connection=True;MultipleActiveResultSets=true");
        }
    }

    [TestClass]
    public class Test
    {
        [TestMethod]
        public void TestMethod1()
        {
            // Assign
            Author author_buzz = new Author()
            {
                Name = "Buzz",
                Books = new List<Book>()
                {
                    new Book()
                    {
                        Title = "Book 1",
                    },
                    new Book()
                    {
                        Title = "Book 2",
                    }
                }
            };

            using (LibraryDbContext context = new LibraryDbContext())
            {
                context.Database.EnsureDeleted();
                context.Database.EnsureCreated();

                context.Authors.Add(author_buzz);
                context.SaveChanges();
            }

            // Act
            author_buzz.Name = "Buzz Lightyear";
            //author_buzz.Books = null; // Uncommenting this will make the test pass.

            using (LibraryDbContext context = new LibraryDbContext())
            {
                context.Entry(author_buzz).State = EntityState.Modified;
                //context.Update(author_buzz); // Using this way of updating doesn't work either.

                context.SaveChanges();
            }

            // Assert
            using (LibraryDbContext context = new LibraryDbContext())
            {
                author_buzz = context.Authors
                    .Include(author => author.Books)
                    .Single();

                Assert.AreEqual("Buzz Lightyear", author_buzz.Name);
                Assert.AreEqual(2, author_buzz.Books.Count);
            }
        }
    }
}

Running this test results in the following exception on the last SaveChanges():

SqlException: Violation of PRIMARY KEY constraint 'PK_AuthorBook'. Cannot insert duplicate key in object 'dbo.AuthorBook'. The duplicate key value is (1, 1).

However, when I set the property Books to null, the test passes.

It seems to me that Entity Framework does not recognize the fact that the relationship between the author and the books is an existing one, and tries to add it again. Then it fails because that link already exists in the entity AuthorBook.

How can I update an entity, without clearing it's relationship properties? In a more complex domain, this is something that I don't want to go trough, and I feel there should be an easier way.

(This is the same implementation as the PUT method in the controller that is scaffolded by visual studio, based on Entity Framework. Also, using context.Update() instead doesn't seem to work either.)

Answer 1

The problem was that the entity was not attached to the context anymore. Solved it by calling context.Attach(author_buzz);before setting the State to Modified.