Defaults and validation

DefaultsCopy Anchor Link

New records are created with the entity's parameterless constructor. To start them with other values, use UseFactory:

resource.AllowCreate(create =>
{
    create.UseFactory(() => new Booking
    {
        Reference = $"VT-{Random.Shared.Next(0, 1_000_000):D6}",
        DepartureDate = DateOnly.FromDateTime(DateTime.Today.AddDays(14)),
        Travellers = 2,
    });
    create.Fields(fields =>
    {
        fields.Add(booking => booking.CustomerId);
        fields.Add(booking => booking.DepartureDate);
        fields.Add(booking => booking.Travellers);
    });
});

Without a factory, DateOnly and DateTime fields start at 0001-01-01, which is rarely what you want.

Values that aren't fields, like Reference here, are set by the factory and saved as they are. You don't need hidden fields. Values your database generates, such as identity keys, need nothing at all.

The factory runs when the form is shown and again when it's saved. A generated value you show on the form, as a display field, is generated again on save. If what's saved has to match what people saw, generate it in a create handler instead.

ValidationCopy Anchor Link

Every save runs ASP.NET Core model validation: data annotations like [Required], [StringLength], [Range] and [EmailAddress], and IValidatableObject. If something's wrong, the form comes back with what was entered and the errors, and nothing is saved.

public class Booking
{
    [Required, StringLength(12)]
    public string Reference { get; set; } = "";

    [Range(1, 20)]
    public int Travellers { get; set; } = 1;
}

For checks that need the database, like "this reference is already taken", return an error from a handler or data source:

return ResourceOperationResult.ValidationFailed(
    nameof(Booking.Reference), "That reference is already in use.");

It shows under the field, like any other validation error.

When EF Core can't save, for example because of a unique index or a concurrency conflict, the form shows a general error and the exception is logged.