Question

我有一个包含核心成员和多个Widget子级的父实体WidgetTranslation，其中包含语言翻译成员，即Description文本，提供英语，法语，德语等。

e.g。

public class Widget
{
  public int Id { get; set; }
  public string Code { get; set; }
  public virtual ICollection<WidgetTranslation> WidgetTranslations { get; set; }
}

public class WidgetTranslation
{
  public int WidgetId { get; set; } 
  public virtual Widget Widget { get; set; }

  public int LanguageId { get; set; }
  public virtual Language Language { get; set; }

  public string Name { get; set; }
  public string Description { get; set; }
  public string Summary { get; set; }
}

查询窗口小部件集合的最有效方法是什么，展平给定的LanguageId＆amp;投射到TranslatedWidget DTO

public class TranslatedWidget
{
  public int Id { get; set; }
  public string Code { get; set; }
  public int LanguageId { get; set; }
  public virtual Language Language { get; set; }
  public string Name { get; set; }
  public string Description { get; set; }
  public string Summary { get; set; }
}

鉴于languageId我已经开始

了

DbSet.Select(w => new TranslatedWidget
  {
    Id = w.Id,
    Code = w.Code,
    LanguageId = w.LanguageId,
    Name = w.WidgetTranslations.First(wt=>wt.LanguageId == languageId).Name,
    Description = w.WidgetTranslations.First(wt=>wt.LanguageId == languageId).Description,
    Summary = w.WidgetTranslations.First(wt=>wt.LanguageId == languageId).Summary
  });

但是我觉得这样效率很低，并且无法扩展到WidgetTranslation上的更多属性。

由于

Answer 1

使用SelectMany通过单个连接展平结构：

var widgetQuery = from w in dbSet.Widgets
                  from wt in w.WidgetTranslations
                  where wt.Language == languageId
                  select new TranslatedWidget
                  {
                     Id = w.Id,
                     Code = w.Code,
                     LanguageId = w.LanguageId,
                     Name = wt.Name,
                     Description = wt.Description,
                     Summary = wt.Summary
                  });

我假设您在给定语言中每个小部件只有一个翻译。

Answer 2

我会将Name，Description和Summary移动到您的DTO的嵌套类中...

public class TranslatedWidgetTranslation
{
    public string Name { get; set; }
    public string Description { get; set; }
    public string Summary { get; set; }
}

public class TranslatedWidget
{
    public int Id { get; set; }
    public string Code { get; set; }
    public int LanguageId { get; set; }
    public TranslatedWidgetTranslation Translation { get; set; }
}

然后你可以投射到那个类，只需要First一次，这将导致SQL中只有一个TOP(1)子查询而不是三个：

DbSet.Select(w => new TranslatedWidget
{
    Id = w.Id,
    Code = w.Code,
    LanguageId = languageId,
    Translation = w.WidgetTranslations
        .Where(wt => wt.LanguageId == languageId)
        .Select(wt => new TranslatedWidgetTranslation
        {
            Name = wt.Name,
            Description = wt.Description,
            Summary = wt.Summary
        })
        .FirstOrDefault()
});

您必须在此使用FirstOrDefault，LINQ-to-Entities投影中不支持First。

如果你不想要那个嵌套类型，你可以首先投射到匿名类型然后转换成你的最终类，但代码会更长一些：

DbSet.Select(w => new
{
    Id = w.Id,
    Code = w.Code,
    LanguageId = languageId,
    Translation = w.WidgetTranslations
        .Where(wt => wt.LanguageId == languageId)
        .Select(wt => new
        {
            Name = wt.Name,
            Description = wt.Description,
            Summary = wt.Summary
        })
        .FirstOrDefault()
})
.AsEnumerable()
.Select(x => new TranslatedWidget
{
    Id = x.Id,
    Code = x.Code,
    LanguageId = x.LanguageId,
    Name = x.Translation != null ? x.Translation.Name : null,
    Description = x.Translation != null ? x.Translation.Description : null,
    Summary = x.Translation != null ? x.Translation.Summary : null
});

最有效的实体框架代码使用特定子项展平/投影父实体的第一种方法

2 个答案: