Question

我有以下代码：

ICommand _navigateAddNewAddress;
    public ICommand NavigateAddNewAddress => _navigateAddNewAddress ?? (_navigateAddNewAddress = new Command(async (sender, args) =>
    {
        try
        {
            App.PushPage(AppPage.PROFILE_ADD_NEW_RESIDENTIAL_ADDRESS);
        }
        catch (Exception ex)
        {
            App.LogError(ex);
        }
    }));

我为Host设置了一个属性：if (await TryUpdateModelAsync<Host>( hostToUpdate, "Host", s => s.Name, s => s.Description, s => s.Address, s => s.Postcode, s => s.Suburb, s => s.State)) { try { await _context.SaveChangesAsync(); return RedirectToPage("./Index"); } catch (DbUpdateConcurrencyException ex) { var exceptionEntry = ex.Entries.Single(); var clientValues = (Host)exceptionEntry.Entity; var databaseEntry = exceptionEntry.GetDatabaseValues(); if (databaseEntry == null) { ModelState.AddModelError(string.Empty, "Unable to save. " + "The host was deleted by another user."); return Page(); } var dbValues = (Host)databaseEntry.ToObject(); await setDbErrorMessage(dbValues, clientValues, _context); // Save the current RowVersion so next postback // matches unless an new concurrency issue happens. Host.RowVersion = (byte[])dbValues.RowVersion; // Must clear the model error for the next postback. ModelState.Remove("Host.RowVersion"); } }和LastDateModified，该属性是计算得出/预定义的值

即{{1}的LastModified和DateTime.Now的{{1}}。

那么如何将其传递给此代码？

LastDateModified

Answer 1

您可以在保存对象之前设置（替代）值：

var hostToUpdate = await _context.Host.FindAsync(s => s.Id == id);

if (await TryUpdateModelAsync(
    hostToUpdate,
    "host", // empty with MVC
    s => s.Name, s => s.Description, s => s.Address, 
    s => s.Postcode, s => s.Suburb, s => s.State))
{
    try
    {
        hostToUpdate.LastModified = DateTime.Now;
        hostToUpdate.LastDateModifiedBy = _userManager.GetUserId(User);

        await _context.SaveChangesAsync();
        return RedirectToPage("./Index");
    }
    // ...
}

请注意，LastModified和LastDateModifiedBy不是TryUpdateModelAsync语句的一部分。但是如果是的话，这些值将被操作覆盖。

从剃须刀页面documentation：

DB上下文跟踪内存中的实体是否同步以及它们在数据库中的对应行。数据库上下文同步信息确定调用SaveChangesAsync时会发生什么。

通过Mvc documentation（不再更新）：

实体框架的自动更改跟踪设置了已修改在由表单输入更改的字段上标记。当。。。的时候调用SaveChanges方法后，实体框架创建SQL 语句以更新数据库行。

要解释为什么这样做，首先TryUpdateModelAsync更新用户更新的字段，然后该操作更新其他字段。所有这些都由实体框架进行跟踪和保存。这是默认的实体框架行为。

请注意，您可以选择添加自动更新字段的代码。在那种情况下，您不会忘记设置它们，并且节省了几行代码。而且您根本不必更改代码。

该策略是实体实现基本字段并在保存更改时对其进行更新。这是更扩展的版本：

public interface IBaseEntity
{
    DateTime LastDateModified { get; set; }
    string LastDateModifiedBy { get; set; }

    DateTime DateCreated { get; set; }
    string DateCreatedBy { get; set; }
}

public class Host : IBaseEntity
{
    // the other fields
    // ...

    public DateTime LastDateModified { get; set; }
    public string LastDateModifiedBy { get; set; }

    public DateTime DateCreated { get; set; }
    public string DateCreatedBy { get; set; }
}

上下文：

public partial class MyContext : DbContext
{
    // Reference to the name of the current user.
    private readonly string _userName;

    public MyContext(DbContextOptions<MyContext> options, IHttpContextAccessor httpContext)
        : base(options)
    {
        // Save the name of the current user so MyContext knows
        // who it is. The advantage is that you won't need to lookup
        // the User on each save changes.
        _userName = httpContext.HttpContext.User.Identity.Name;
    }

    public virtual DbSet<Host> Host { get; set; }

    // You'll only need to override this, unless you are
    // also using non-async SaveChanges.
    public override Task<int> SaveChangesAsync(bool acceptAllChangesOnSuccess, CancellationToken cancellationToken = default(CancellationToken))
    {
        UpdateEntries();
        return base.SaveChangesAsync(acceptAllChangesOnSuccess, cancellationToken);
    }

    // You can move this to another partial class.
    private void UpdateEntries()
    {
        // Modified
        var modified = ChangeTracker.Entries().Where(v => v.State == EntityState.Modified && typeof(IBaseEntity).IsAssignableFrom(v.Entity.GetType())).ToList();
        modified.ForEach(entry =>
        {
            ((IBaseEntity)entry.Entity).LastDateModified = DateTime.UtcNow;
            ((IBaseEntity)entry.Entity).LastDateModifiedBy = _userName;
        });

        // Added
        var added = ChangeTracker.Entries().Where(v => v.State == EntityState.Added && typeof(IBaseEntity).IsAssignableFrom(v.Entity.GetType())).ToList();
        added.ForEach(entry =>
        {
            ((IBaseEntity)entry.Entity).DateCreated = DateTime.UtcNow;
            ((IBaseEntity)entry.Entity).DateCreatedBy = _userName;
            ((IBaseEntity)entry.Entity).LastDateModified = DateTime.UtcNow;
            ((IBaseEntity)entry.Entity).LastDateModifiedBy = _userName;
        });
    }

    // ...
}

在启动中添加HttpContextAccessor：

public void ConfigureServices(IServiceCollection services)
{
    services.AddHttpContextAccessor();

现在，每次保存实现IBaseEntity的对象时，这些字段都会自动更新。

请注意，我没有在此处注入UserManager。如果用户包含名称声明，则可以使用该名称声明。这将保存对数据库的调用。

作为一项改进，您可以编写一个新服务来解决用户名并注入用户名。

如何将计算的值添加到此TryUpdateModelAsync <>

1 个答案: