Startup.ConfigureServices.cs

public void ConfigureServices(IServiceCollection services)
{
    services.AddMvc().AddJsonOptions(
        options => options.SerializerSettings.PreserveReferencesHandling = PreserveReferencesHandling.All);
    services.AddDbContext<NorthwindSlimContext>(
        options => options.UseSqlite("Data Source=northwindslim.db"));
}