.NET核心-实体框架-急于加载Include（）的意外行为

Question

初始查询

如果运行以下查询， CityTranslation.IdFkLanguageNavigation得到以下结果， CityTranslation.IdFkLanguageNavigation将为null 。 另一方面，将填充CityTranslation.IdFkCityNavigation 。 考虑到它们都处于相同的层次结构级别。

    var data = await _context.City
                        .Include(x => x.CityTranslation)
                        .Include(c => c.IdFkCountryNavigation)
                        .ToListAsync();

我试过在.Include(x => x.CityTranslation) ThenInclude()之后使用ThenInclude() ，但是它不允许我使用Language属性。

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变更查询

通过下面的查询，我确实得到了一些意外的结果。 一旦我通过data2调试，突然我的第一个查询结果中的Language navigation属性就被填充了！

仅当我在第二个查询上调用ToList() ，此方法才有效，否则仍然不会被填充。 我必须假设数据是在整个EF上下文中共享的，但是如何在不向数据库发送第二个查询的情况下利用这种行为呢？ 这种行为令我非常震惊，希望您能提供任何参考或解释。

                var data = await _context.City
                    .Include(x => x.CityTranslation)
                    .Include(c => c.IdFkCountryNavigation).ToListAsync();

                var data2 = _context.CityTranslation.Include(c => c.IdFkLanguageNavigation).ToListAsync();

---

上下文

我正在使用Microsoft.EntityFrameworkCore.SqlServer版本1.1.2

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市

public partial class City
{
    public City()
    {
        CityTranslation = new HashSet<CityTranslation>();
    }

    public int IdPkCity { get; set; }
    public string Id { get; set; }
    public string Code { get; set; }
    public string Latitude { get; set; }
    public string Longitude { get; set; }
    public string TimeZone { get; set; }
    public string Uri { get; set; }
    public string Name { get; set; }
    public string Language { get; set; }
    public int? IdFkCountry { get; set; }

    public virtual ICollection<CityTranslation> CityTranslation { get; set; }
    public virtual Country IdFkCountryNavigation { get; set; }

---

城市翻译

public partial class CityTranslation
{
    public int IdPkCityTranslation { get; set; }
    public string Translation { get; set; }
    public int IdFkCity { get; set; }
    public int IdFkLanguage { get; set; }

    public virtual City IdFkCityNavigation { get; set; }
    public virtual Language IdFkLanguageNavigation { get; set; }
}

---

语言

public partial class Language
{
    public Language()
    {
        AirportTranslation = new HashSet<AirportTranslation>();
        CityTranslation = new HashSet<CityTranslation>();
        CountryTranslation = new HashSet<CountryTranslation>();
    }

    public int IdPkLanguage { get; set; }
    public string Name { get; set; }
    public string Code { get; set; }

    public virtual ICollection<AirportTranslation> ATranslation{ get; set; }
    public virtual ICollection<CityTranslation> CTranslation { get; set; }
    public virtual ICollection<CountryTranslation> C2Translation { get; set; }
}

Answer 1

这将起作用，只需粘贴此即可。

// Hits the database only once
var data = await _context.City
    .Include(x => x.CityTranslation)
        .ThenInclude(x => x.IdFkLanguageNavigation)
    .ToListAsync();

现在，关于第二个查询为什么加载第一个查询的“ Language导航属性的原因，您必须研究EntityFramework的整体工作原理，并仔细阅读显式加载。

这是显式加载的示例。

// Hits the database once.
var data = await _context.City
    .Include(x => x.CityTranslation)
    .ToListAsync();

var cityTranslationIds = data.Select(x => x.CityTranslation.IdPkCityTranslation);

// Hits the database the second time.
// Language navigation property will be loaded onto the data variable above
_context.Language
    .Where(x => cityTranslateIds.Contains(x.IdPkLanguage))
    .Load();

// Your second query, what you did here is essentially the same as the above's Load(), 
// but the Load() is better suited for your intention.
var data2 = await _context.CityTranslation
    .Include(c => c.IdFkLanguageNavigation)
    .ToListAsync();

根据情况，您需要在渴望加载和显式加载之间进行选择，以实现最佳性能。