How to use EF Core concurrency token created by ForNpgsqlUseXminAsConcurrencyToken

Question

I have found npgsql provider extension to set up concurrency token for entity framework core entity, which should do something like this:

modelBuilder.Entity<MyEntity>(b =>
{
    b.Property<uint>("xmin")
        .HasColumnType("xid")
        .ValueGeneratedOnAddOrUpdate()
        .IsConcurrencyToken();
});

If I understand it well, it creates shadow property on entity.

How can I use this property to track concurrent updates (more users try to update the same entity) in ASP.NET Core, for example? Should I try to to map xmin column to normal property and put it to hidden input tag as it is showed in asp.net core documentation? Or is there another way?

Answer 1

Discussing with Olivier MATROT I realized how to do what I need.

The solution is not ideal because it is tied up with provider (SQL server provider needs byte[] as concurrency token property), but works as expected:

public class MyEntity
{
    public int Id { get; set; }
    public string Name { get; set; }
    public uint ConcurrencyStamp { get; set; }
}

In the context (If migrations are used, property need to be removed from migration code to eliminate column creation attempt)

protected override void OnModelCreating(ModelBuilder builder)
{
    base.OnModelCreating(builder);

    // ...

    builder.Entity<MyEntity>()
        .Property(e => e.ConcurrencyStamp)
            .ForNpgsqlHasColumnName("xmin")
            .ForNpgsqlHasColumnType("xid")
            .ValueGeneratedOnAddOrUpdate()
            .IsConcurrencyToken();
}

Edit view

@model Namespace.MyEntity

<form asp-action="Edit">
    <div class="form-horizontal">
        <h4>Person</h4>
        <hr />
        <div asp-validation-summary="ModelOnly" class="text-danger"></div>

        <input type="hidden" asp-for="Id" />
        <input type="hidden" asp-for="ConcurrencyStamp" />

        <div class="form-group">
            <label asp-for="Name" class="col-md-2 control-label"></label>
            <div class="col-md-10">
                <input asp-for="Name" class="form-control" />
                <span asp-validation-for="Name" class="text-danger"></span>
            </div>
        </div>

        <div class="form-group">
            <div class="col-md-offset-2 col-md-10">
                <input type="submit" value="Save" class="btn btn-default" />
            </div>
        </div>

    </div>
</form>

and default scaffolded action (just to complete the example)

[HttpPost]
[ValidateAntiForgeryToken]
public async Task<IActionResult> Edit(int id, [Bind("Id,Name,ConcurrencyStamp")] MyEntity model)
{
    if (id != model.Id)
    {
        return NotFound();
    }

    if (ModelState.IsValid)
    {
        try
        {
            _context.Update(model);
            await _context.SaveChangesAsync();
        }
        catch (DbUpdateConcurrencyException)
        {
            if (!MyEntityExists(model.Id))
            {
                return NotFound();
            }
            else
            {
                throw;
            }
        }
        return RedirectToAction("Index");
    }
    return View(model);
}

So the solution is to make xmin value accessible as the entity property.