title: EF relation - cannot map due to list<guid> being primitive
body: <p>I'd want to store in User class collection of Items</p>
  
  <p>But I cannot overcome this problem:</p>
  
  <blockquote>
    <p>System.InvalidOperationException: 'The property 'User.ItemsIds' could not be mapped, because it is of type 'List' which is not a supported primitive type or a valid entity type. Either explicitly map this property, or ignore it using the '[NotMapped]' attribute or by using 'EntityTypeBuilder.Ignore' in 'OnModelCreating'.'</p>
  </blockquote>
  
  <p>I'm using ASP.NET Core 2.0 + EFCore</p>
  
  <pre><code>public class User
  {
      [Key]
      public Guid Id { get;  set; }
      [ForeignKey("Items")]
      public virtual List&lt;Guid&gt; ItemsIds{ get; set; }
      (...)
  }
  
  public class Item
  {
      [Key]
      public Guid Id { get;  set; }
      public string ItemName { get; set; }
      public Item(string name)
      {
          Id = new Guid();
          ItemName = name;
      }
  }
  
  public class AppContext : DbContext
  {
      public AppContext(DbContextOptions&lt;AppContext&gt; options) : base(options)
      {   
          Database.SetCommandTimeout(35000);
      }
  
      public DbSet&lt;User&gt; Users { get; set; }
      public DbSet&lt;Item&gt; Items { get; set; }
  
      protected override void OnModelCreating(ModelBuilder modelBuilder)
      {
          modelBuilder.Entity&lt;User&gt;().ToTable("Users");
          modelBuilder.Entity&lt;Item&gt;().ToTable("Items");
      }
  }
  </code></pre>
  
  <p>Here's similar problem, but still it hasn't been "fixed"?</p>
  
  <p><a href="https://stackoverflow.com/questions/20711986/entity-framework-code-first-cant-store-liststring">Entity Framework - Code First - Can&#39;t Store List&lt;String&gt;</a></p>