Question

在ASP的世界里肯定是新的，在下面的例子中发现自己迷失了。显然需要专家来引导。

有两个模特团队＆amp;具有以下数据库结构的锦标赛：

public class Tournament
{
    [Key]
    public string TournamentID { get; set; }
    public DateTime TournamentDate { get; set; }
    public string Place { get; set; }

    [ForeignKey("TeamA")]
    public string TeamAID { get; set; }
    public Team TeamA { get; set; }

    [ForeignKey("TeamB")]
    public string TeamBID { get; set; }
    public Team TeamB { get; set; }
}

public class Team
{
    [Key]
    public string TeamID { get; set; }
    public string TeamName { get; set; }
    public string Captain { get; set; }

    [InverseProperty("TeamA")]
    public virtual ICollection<Tournament> TeamA { get; set; }

    [InverseProperty("TeamB")]
    public virtual ICollection<Tournament> TeamB { get; set; }
}

因为锦标赛有两个小组因此使用InverseProperty和外键来建立关系。

然而，因为我试图将结果带回来：

团队详情;
以下列格式显示竞争对手的锦标赛列表：

TeamID

TeamName

CaptainName

TournamentID | TournamentDate | TournamentPlace |竞争者

...

因此，使用如下的辅助模型：

public class TeamVM
{
    public string ID { get; set; }
    public string Name { get; set; }
    public string Captain { get; set; }
    public virtual IEnumerable<TournamentVM> Tournaments { get; set; }
}
public class TournamentVM
{
    public string ID { get; set; }
    public DateTime Date { get; set; }
    public string Place { get; set; }
    public string Competitor { get; set; }
}

并使用如下控制器映射数据库相关表中的值：

public async Task<IActionResult> TheAction(string id)
{
    Team team = await _context.Teams.Where(x => x.TeamID == id).Include(x => x.TeamA).Include(x => x.TeamB).SingleOrDefaultAsync();

    TeamVM model = new TeamVM
    {
        ID = team.TeamID,
        Name = team.TeamName,
        Captain = team.Captain,
        Tournaments = team.TeamA.Where(x => x.TeamAID == team.TeamID).Select(x => new TournamentVM
        {
            ID = x.TournamentID,
            Date = x.TournamentDate,
            Place = x.Place,
            Competitor = x.TeamB.TeamName
        }).Concat(team.TeamB.Where(x => x.TeamBID == team.TeamID).Select(x => new TournamentVM
        {
            ID = x.TournamentID,
            Date = x.TournamentDate,
            Place = x.Place,
            Competitor = x.TeamA.TeamName
        })).OrderBy(x => x.Date)
    };
    return View(model);
}

但结果却是抱怨：

NullReferenceException：对象引用未设置为对象的实例。

因此突出显示部分：

Tournaments = team.TeamA.Where(x => x.TeamAID == team.TeamID).Select(x => new TournamentVM
    {
        ID = x.TournamentID,
        Date = x.TournamentDate,
        Place = x.Place,
        Competitor = x.TeamB.TeamName
    }).Concat(team.TeamB.Where(x => x.TeamBID == team.TeamID).Select(x => new TournamentVM
    {
        ID = x.TournamentID,
        Date = x.TournamentDate,
        Place = x.Place,
        Competitor = x.TeamA.TeamName
    })).OrderBy(x => x.Date)

尝试了相当多的时间终于让我发现，当发生上述错误时，如果我将上述部分修改为：

Tournaments = team.TeamA.Where(x => x.TeamAID == team.TeamID).Select(x => new TournamentVM
    {
        ID = x.TournamentID,
        Date = x.TournamentDate,
        Place = x.Place,
        Competitor = x.TeamA.TeamName //i.e. TeamA instead of TeamB
    }).Concat(team.TeamB.Where(x => x.TeamBID == team.TeamID).Select(x => new TournamentVM
    {
        ID = x.TournamentID,
        Date = x.TournamentDate,
        Place = x.Place,
        Competitor = x.TeamB.TeamName //i.e. TeamB instead of TeamA
    })).OrderBy(x => x.Date)

结果显示。

因此，我不能只提供参赛者的名字，而只能提供球队自己的名字。换句话说，TeamA的部分无法为竞争对手提供TeamB.TeamName。

可能是什么原因和解决办法？

对视图使用以下代码：

@model TeamVM
<div>@Model.ID</div>
<div>@Model.Name</div>
<div>@Model.Captain</div>
<table>
    <thead>
        <tr>
            <th>ID</th>
            <th>Date</th>
            <th>Place</th>
            <th>Competitor</th>
        </tr>
    </thead>
    <tbody>
        @foreach (var item in Model.Tournaments)
        {
            <tr>
                <td>@item.ID</td>
                <td>@item.Date</td>
                <td>@item.Place</td>
                <td>@item.Competitor</td>
            </tr>
        }
    </tbody>
</table>

期待并感谢您的专业意见。

提前致谢。

Answer 1

我想建议另一种方法来解决这个问题，因为你的设置听起来有点令人困惑。

您可以简化：

//The same
public class Tournament
{
    [Key]
    public string TournamentID { get; set; }
    public DateTime TournamentDate { get; set; }
    public string Place { get; set; }

    [ForeignKey("TeamA")]
    public string TeamAID { get; set; }
    public Team TeamA { get; set; }

    [ForeignKey("TeamB")]
    public string TeamBID { get; set; }
    public Team TeamB { get; set; }
}

public class Team
{
    [Key]
    public string TeamID { get; set; }
    public string TeamName { get; set; }
    public string Captain { get; set; }

    //Instead of these two
    //[InverseProperty("TeamA")]
    //public virtual ICollection<Tournament> TeamA { get; set; }

    //[InverseProperty("TeamB")]
    //public virtual ICollection<Tournament> TeamB { get; set; }

    //Let's just include the tournaments this team participated in
    public virtual ICollection<Tournament> Tournaments { get; set; }
}

现在，您可以使用Tournaments拨打Include()。

public async Task<IActionResult> TheAction(string id)
{
    Team team = await _context.Teams.SingleOrDefaultAsync(x => x.TeamID == id).Include(t => t.Tournaments);

    //You should be able to eager load the teams at this point since you don't use virtual, put a break point to check. If not, you can load them using .ThenInclude()

    TeamVM model = new TeamVM
    {
        ID = team.TeamID,
        Name = team.TeamName,
        Captain = team.Captain,
        Tournaments = team.Tournaments.Select(tournament => new TournamentVM() 
        {
            ID = tournament.TournamentID,
            Date = tournament.TournamentDate,
            Place = tournament.Place,
            Competitor = (tournament.TeamA == team) ? tournament.TeamB.Name : tournament.TeamA.TeamName
            //pick the other team as competitor
        }).OrderBy(x => x.Date)
    };
    return View(model);
}

我自己没有测试过代码，所以欢迎您将其视为伪代码。让我知道结果。

Answer 2

雅虎！

最后，我确实找到了解决方案。

哇！这个ASP核心世界太棒了！

感谢@Mithgroth的建议，但不能偏离数据库模型。但是，如果可以建议任何进一步的干净版本，我将非常感谢，只要数据库结构保持不变。

无论如何，我在24小时后完成的解决方案是更新的控制器操作，如下所示，其余部分继续保持不变：

public async Task<IActionResult> TheAction(string id){
Team team = await _context.Teams.Where(x => x.TeamID == id).Include(x => x.TeamA).Include(x => x.TeamB).SingleOrDefaultAsync();

var gamesList = (from x in _context.Tournaments
                    .Where(x => x.TeamAID == id || x.TeamBID == id)
                    select new TournamentVM
                    {
                        ID = x.TournamentID,
                        Date = x.TournamentDate,
                        Place = x.Place,
                        Competitor = x.TeamAID == id ? x.TeamB.TeamName : x.TeamA.TeamName
                    });

TeamVM model = new TeamVM
{
    ID = team.TeamID,
    Name = team.TeamName,
    Captain = team.Captain,
    Tournaments = from x in gamesList
        .Select(x => new TournamentVM
        {
            ID = x.ID,
            Date = x.Date,
            Place = x.Place,
            Competitor = x.Competitor
        }).OrderBy(x => x.Date) select x
};
return View("TheAxion", model);}

： - ）

以特殊格式显示相关数据

2 个答案: