title: Owned type mapping EF Core fails when saving
body: <p>I want to make TableSplitting using Owned Types. I have the following model:</p>
  
  <pre><code>public class Account 
  {
    public GUID Id { get; set; }
    public string Email { get; set; }
    public StreetAddress Address { get; set; }
  }
  
  public class StreetAddress
  {
    public string Name { get; set; }
    public string Address { get; set; }
    public string Zipcode { get; set; }
    public string City { get; set; }
    public string  Country { get; set; }
    public Location Location { get; set; }
  }
  
  public class Location
  {
    public double Lat { get; set; }
    public double Lng { get; set; }
  }
  </code></pre>
  
  <p>And I defined my mapping of the Account like this:</p>
  
  <pre><code>public override void Map(EntityTypeBuilder&lt;Account&gt; map)
  {
      // Keys
      map.HasKey(x =&gt; x.Id);
  
      // Indexs
      map.HasIndex(x =&gt; x.Email).IsUnique();
  
      // Property mappings.
      map.Property(x =&gt; x.Email).HasMaxLength(255).IsRequired();
  
      // Owned types.
      map.OwnsOne(x =&gt; x.Address, cb =&gt; cb.OwnsOne(a =&gt; a.Location));
  }
  </code></pre>
  
  <p>When I run the migration things are working and the columns are created in the database. But when I try to insert and save an address like so:</p>
  
  <pre><code>var account1 = new Account("e@mail.com", "First", "Last")
      {
        Address = new StreetAddress()
                    {
                          Address1 = "Street 1",
                          City = "City",
                          Zipcode = "2000",
                          Country = "Denmark",
                          Location = new Location()
                          {
                              Lat = 0.0,
                              Lng = 5.5
                          }
  
                      }
                  };
  this.Context.Accounts.Add(account1);
  </code></pre>
  
  <p>I get this error</p>
  
  <blockquote>
    <p>Message   "The entity of 'Account' is sharing the table 'Accounts' with
    'Account.Address#StreetAddress', but there is no entity of this type
    with the same key value 'Id:b7662057-44c2-4f3f-2cf0-08d504db1849' that
    has been marked as 'Added'."</p>
  </blockquote>