public NullKeysSqliteFixture()
            {
                var serviceProvider = new ServiceCollection()
                                      .AddEntityFrameworkSqlite()
                                      .AddSingleton(TestModelSource.GetFactory(OnModelCreating))
                                      .BuildServiceProvider(validateScopes: true);

                _options = new DbContextOptionsBuilder()
                           .UseSqlite(SqliteTestStore.CreateConnectionString("StringsContext"))
                           .UseInternalServiceProvider(serviceProvider)
                           .Options;

                CreateContext().Database.EnsureClean();
                EnsureCreated();
            }