如何使用 Entity Framework Core fluent API 配置一对多关系

Question

我正在学习 Entity Framework Core。我有两个简单的类：Country 和 Currency。货币应与国家（法国、德国、意大利等）具有一对多的关系。它们都共享欧元货币。

我只想使用 fluent API，没有数据注释。目标是使用 LINQ 获取存储在 MYSQL 数据库中的 Country 对象的货币名称。

我定义了我的类：

public class Country
{
    public string CountryID {get; set;} // ISO3166, "US" for United States
    public string CurrencyID {get; set;} // ISO4217 "USD" for US Dollar
    public string Name {get; set;}

    public virtual Currency Currency {get; set;}

    public Country() {};
}

public class Currency
{
    public string CurrencyID {get; set;} // ISO4217
    public string Name {get; set;}

    public virtual ICollection<Country> Countries {get; set;}

    public Currency(){}; 
}

我在 DBContext 中定义了我的 DBSets 对象

public DbSet<Country> Countries {get; set;}

public DbSet<Currency> Currencies {get; set;}

我使用 fluent API 来定义我的数据：

// Country

modelBuilder.Entity<Country>().HasKey(c => new { c.CountryID });
modelBuilder.Entity<Country>().Property(c => c.CountryID).IsRequired();
modelBuilder.Entity<Country>().Property(c => c.CountryID).HasMaxLength(2);
modelBuilder.Entity<Country>().Property(c => c.Name).IsRequired();
modelBuilder.Entity<Country>().Property(c => c.Name).HasMaxLength(50);
modelBuilder.Entity<Country>().Property(c => c.CurrencyID).IsRequired();    //Foreign Key
modelBuilder.Entity<Country>().Property(c => c.CurrencyID).HasMaxLength(3); //Foreign Key

// Currency

modelBuilder.Entity<Currency>().HasKey(c => new { c.CurrencyID });
modelBuilder.Entity<Currency>().Property(c => c.CurrencyID).IsRequired();  
modelBuilder.Entity<Currency>().Property(c => c.CurrencyID).HasMaxLength(3);
modelBuilder.Entity<Currency>().Property(c => c.Name).IsRequired();
modelBuilder.Entity<Currency>().Property(c => c.Name).HasMaxLength(70);

然后我创建了一些默认数据：

using (var dbContext = ContextFactory.Create(connectionString))
{
            {
    dbContext.Add(new Country(iso3166Code: "FR", name: "France", iso4217Code: "EUR"));
    dbContext.Add(new Country(iso3166Code: "DE", name: "Germany", iso4217Code: "EUR"));
    dbContext.Add(new Country(iso3166Code: "IT", name: "Italy", iso4217Code: "EUR"));
    dbContext.Add(new Country(iso3166Code: "US", name: "United States of America", iso4217Code: "USD"));
    dbContext.Add(new Currency(iso4217Code: "EUR", name: "Euro"));
    dbContext.Add(new Currency(iso4217Code: "USD", name: "US Dollar"));

    dbContext.SaveChanges();
}

我确定数据存储在 MYSQL 数据库中并且服务器正在运行。我可以连接到数据库，但是当我尝试时：

var italyCurrencyName = context.Countries.First(c => c.CountryID == "IT").Currency.Name;
WriteLine($"Italy currency is: {italyCurrencyName}");

我得到一个例外：Object reference not set to an instance of an object。原因是 Country 对象的 Currency 导航属性似乎为空。

以下代码获取正确的 Country 对象，“EUR”设置为CurrencyID（外键），但Currency 属性为null。

var italy = context.Countries.First(c => c.CountryID == "IT"); 

我试图指定这样的关系但没有成功：

modelBuilder.Entity<Country>().HasOne(e => e.Currency).WithMany(e => e.Countries).HasForeignKey(e => e.CurrencyID);
modelBuilder.Entity<Currency>().HasMany(e => e.Countries);

我错过了什么？

更新：

只有在强制加载货币 DbSet 时才能获取数据：

var italy = context.Countries.First(c => c.CountryID == "IT");
var euro = context.Currencies.First(c => c.CurrencyID == "EUR");
WriteLine($"Italy currency is: {italyCurrencyName}");

Answer 1

您获得空值的原因是 Entity Framework Core 尚不支持“延迟加载”：

https://docs.microsoft.com/en-us/ef/core/querying/related-data

要获取相关数据，您应该使用“eager loading”：

var italy = context.Countries.Include(c=>c.Currency).First(c => c.CountryID == "IT");