如何使用EF核心從相關表中獲取數據？

Question

我正在從事網絡項目，我使用代碼優先方法在機器模型和實驗模型之間創建了一對多關系。 這是代碼：

   public class Experiment
{

    public int Id { get; set; }
    public string Name { get; set; }
    public DateTime CreationDateTime { get; set; }
    public string Path { get; set; }
    public string Description { get; set; }

    [ForeignKey("Machine")]
    public int MachineId { get; set; }
    public Machine Machine { get; set; }
}


 public class Machine
{
    public int Id { get; set; }
    public string Name { get; set; }
    public string Type { get; set; }
    public string Description { get; set; }

    public ICollection<Experiment> Experiments { get; set; }
}

我可以通過 userId 過濾所有的實驗：

  public async Task<IList<Experiment>> GetExperimentsAsync(int userId)
    {
        var experiments = await _experimentsDbContext.Experiments
                                .Where(x => x.UserId == userId)
                                .ToListAsync();

        return experiments; 
    }

但是我收到machine = null，我需要做的是按userId過濾實驗列表並包含相關的機器對象，但我不能這樣做。

Answer 1

您需要使用 .Include() 方法明確指定要加載相關數據。

請查看MS 文檔

例子：

public async Task<IList<Experiment>> GetExperimentsAsync(int userId)
{
    var experiments = await _experimentsDbContext.Experiments
                            .Where(x => x.UserId == userId)
                            .Include(e => e.Machine)
                            .ToListAsync();

    return experiments; 
}

Answer 2

試試下面的代碼：

public async Task<IList<Experiment>> GetExperimentsAsync(int userId)
{
    var experiments = await _experimentsDbContext.Experiments
                            .Include(experiment => experiment.Machine)
                            .Where(x => x.UserId == userId)
                            .ToListAsync();
    return experiments; 
}