InMemory implicitly deletes entity when changed to an invalid state

[gojanpaolo]

A Post with null Blog can be saved using the in-memory provider even if the Post.BlogId is non-nullable. But when a Post is saved with a Blog instance and then later changed to null, the said Post entity will be implicitly deleted.

Steps to reproduce

using Microsoft.EntityFrameworkCore;
using System;
using System.Linq;
using Xunit;

public class Tests
{
    private readonly DbContextOptions<Context> _options;

    public Tests()
    {
        _options = new DbContextOptionsBuilder<Context>()
            .UseInMemoryDatabase(Guid.NewGuid().ToString())
            .Options;
    }

    [Fact]
    public void Can_save_Post_with_null_Blog()
    {
        using (var ctx = new Context(_options))
        {
            ctx.Add(new Post { Blog = null });
            ctx.SaveChanges();
        }

        using (var ctx = new Context(_options))
        {
            Assert.NotEmpty(ctx.Post);
        }
    }

    [Fact]
    public void In_memory_implicitly_deletes_Post_when_Blog_is_set_to_null()
    {
        using (var ctx = new Context(_options))
        {
            ctx.Add(new Post { Blog = new Blog() });
            ctx.SaveChanges();
        }

        using (var ctx = new Context(_options))
        {
            ctx.Post.Include(p => p.Blog).First().Blog = null;
            ctx.SaveChanges();
        }

        using (var ctx = new Context(_options))
        {
            Assert.NotEmpty(ctx.Post);
        }
    }
}

public class Context : DbContext
{
    public Context(DbContextOptions<Context> options) : base(options) { }
    public DbSet<Post> Post { get; set; }
    public DbSet<Blog> Blog { get; set; }
}

public class Post
{
    public int PostId { get; set; }
    public int BlogId { get; set; }
    public Blog Blog { get; set; }
}

public class Blog
{
    public int BlogId { get; set; }
}

Further technical details

EF Core version: 2.1.3
Database Provider: Microsoft.EntityFrameworkCore.InMemory
Operating system: Win7
IDE: Visual Studio 2017 15.4

[ajcvickers]

@gojanpaolo For the first case, the save is not throwing because the in-memory provider currently doesn't enforce relational constraints--see #2166.

For the second case, Post.BlogId is not nullable, which implies a required relationship between Blog and Post. This means that when the relationship is severed the default behavior is to delete the dependent (Post) since it can't exist without it's parent. See https://docs.microsoft.com/en-us/ef/core/saving/cascade-delete

[gojanpaolo]

I see now. Thank you @ajcvickers.