Linq 最有效的实体框架代码第一种使用特定子实体展平/投影父实体的方法

我有一个具有核心成员的父实体

小部件

，以及多个具有语言翻译成员的

WidgetTranslation

子实体，即

说明

文本，可提供英语、法语、德语等

e、 g

给定

languageId

我从

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
  });

但我觉得这样做效率很低，而且无法在

widgetranslation

上扩展更多属性

---

谢谢

我会将

名称

、

说明

和

摘要

移动到您的数据的嵌套类中

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，您可以投影到该类中，并且只需要一次

第一次

，这将在SQL中只产生一个

TOP（1）

子查询，而不是三个子查询：

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()
});

您必须在此处使用

First或default

，LINQ到实体投影中不支持

First

如果不希望使用该嵌套类型，可以先将其投影为匿名类型，然后再转换为最终类，但代码会稍长一些：

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
});

---

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
                  });

---

我在这里假设，对于给定语言中的每个小部件，您只有一个翻译。

谢谢。我很少看到回答者投票支持其他人的答案。谢谢Jim，生成的EF SQL更干净。@JimWooley您提到要使用SelectMany，但您的示例仅适用于SelectMany-我遗漏了什么吗？在封面下，来自。。。语法从转换为

。选择many

。

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
});

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
                  });