EF代码优先:定义外键

我看过其他一些答案和很多文章，但这一简单的部分仍然让我难以捉摸。我首先使用EF代码4.1，但如果它更容易的话，我很乐意转到一个新的版本。我有一个这样的主要事实表：

namespace Spend.Models
{
    public class ExpenseItem
    {
        [Key]
        public String UniqueID_ERLineID { get; set; }
        public String ERNum { get; set; }
        public String ItemNum { get; set; }
        public String Parent_Expense_Item { get; set; }
        public String Card_Number { get; set; }
...

以及与ExpenseItems:有多对一关系的几个表

public class ExpenseItemAccounting
{
    [Key]
    public String UniqueID_Accounting { get; set; }
    public String ERLineID { get; set; }
    public String ERNum { get; set; }
    public String ItemNum { get; set; }

正如我们所看到的，第二个表中的ERLineID连接到第一个表中UniqueID_ERLineID，所以我通常依赖的"约定"不起作用。因此，我需要使用一个虚拟ICollection，但我希望它将这些字段指定为链接。如有任何关于如何做到这一点的帮助，我们将不胜感激。

PS。我目前无法重命名DB字段。

@卢克：

我应用了你提到的更改，它们是有意义的。然而，我得到以下错误：

System.Data.Entity.ModelConfiguration.ModelValidationException occurred
  Message=One or more validation errors were detected during model generation:
    System.Data.Edm.EdmAssociationType: : Multiplicity conflicts with the referential constraint in Role 'ExpenseItemAccounting_ExpenseItem_Target' in relationship 'ExpenseItemAccounting_ExpenseItem'. Because all of the properties in the Dependent Role are non-nullable, multiplicity of the Principal Role must be '1'.
    System.Data.Edm.EdmAssociationEnd: : Multiplicity is not valid in Role 'ExpenseItemAccounting_ExpenseItem_Source' in relationship 'ExpenseItemAccounting_ExpenseItem'. Because the Dependent Role refers to the key properties, the upper bound of the multiplicity of the Dependent Role must be �1�.
  Source=EntityFramework
  StackTrace:
       at System.Data.Entity.ModelConfiguration.Edm.EdmModelExtensions.ValidateAndSerializeCsdl(EdmModel model, XmlWriter writer)
       at System.Data.Entity.DbModelBuilder.Build(DbProviderManifest providerManifest, DbProviderInfo providerInfo)
       at System.Data.Entity.DbModelBuilder.Build(DbConnection providerConnection)
       at System.Data.Entity.Internal.LazyInternalContext.CreateModel(LazyInternalContext internalContext)
       at System.Data.Entity.Internal.RetryLazy`2.GetValue(TInput input)
       at System.Data.Entity.Internal.LazyInternalContext.InitializeContext()
       at System.Data.Entity.Internal.InternalContext.GetEntitySetAndBaseTypeForType(Type entityType)
       at System.Data.Entity.Internal.Linq.InternalSet`1.Initialize()
       at System.Data.Entity.Internal.Linq.InternalSet`1.get_InternalContext()
       at System.Data.Entity.Infrastructure.DbQuery`1.System.Linq.IQueryable.get_Provider()
       at System.Linq.Queryable.SelectMany[TSource,TCollection,TResult](IQueryable`1 source, Expression`1 collectionSelector, Expression`1 resultSelector)
       at EmailClient.Prog.getData() in C:'MF'Dropbox'Dev_LN_Projects'04_QA'EmailClient'EmailClient'Prog.cs:line 172
  InnerException: 

当我尝试以下linq查询时出现了这种情况：

        var geee = (from e in db.ExpenseItems
                        from f in db.ExpenseItemFbtItems
                        where
                        e.Item_Transaction_Date.Value.Year == 2011 &&
                        e.Item_Transaction_Date.Value.Month == 8
                        select new { A = e.UniqueID_ERLineID, B = f.ERLineID.First() });

实际上，我希望能够说e.ExpenseItemAccounting.ItemNum或类似的话——我需要在ExpenseItem定义中添加一些东西来实现这一点吗？

我的模型设置如下。基地。OnModelCreating是通过intellisense出现的，我尝试过使用/不使用它，得到了相同的结果：

public class SpendDB : DbContext
{
    public DbSet<ExpenseAttachment> ExpenseAttachments {get; set; }
    public DbSet<ExpenseComment> ExpenseComments {get; set; }
    public DbSet<ExpenseItemAccounting> ExpenseAccountings {get; set; }
    public DbSet<ExpenseItemFbtItem> ExpenseItemFbtItems {get; set; }
    public DbSet<ExpenseItem> ExpenseItems {get; set; }
    public DbSet<ExpenseItemViolation> ExpenseItemViolations {get; set; }
    protected override void OnModelCreating(DbModelBuilder modelBuilder)
    {
        base.OnModelCreating(modelBuilder);
        modelBuilder.Entity<ExpenseItemAccounting>().HasOptional(e => e.ExpenseItem).WithMany().HasForeignKey(e => e.UniqueID_Accounting);
    }
}

也许我需要将虚拟ICollection放入ExpenseItem定义中？或者它可能是另一种方式——比如modelBuilder.Entity有可选的ExpenseItemAccounting？这对我来说听起来更直观，但我（显然）不太擅长这一点，所以对此持怀疑态度！

再次感谢

public class ExpenseItemAccounting
{
    [Key]
    public String UniqueID_Accounting { get; set; }
    public ExpenseItem ExpenseItem{get;set;}
    public String ERLineID { get; set; }
    public String ERNum { get; set; }
    public String ItemNum { get; set; }
}

modelBuilder.Entity<ExpenseItemAccounting>()
    .HasOptional(e => e.ExpenseItem).WithMany()
    .HasForeignKey(e => e.UniqueID_Accounting );

public class ExpenseItem
{
        [Key]
        public String UniqueID_ERLineID { get; set; }
        public String ERNum { get; set; }
        public String ItemNum { get; set; }
        public String Parent_Expense_Item { get; set; }
        public String Card_Number { get; set; }
        public ICollection<ExpenseItemAccounting> ExpenseItemAccountings{ get; set; }
}

 modelBuilder.Entity<ExpenseItemAccounting>()
     .HasOptional(e => e.ExpenseItem).WithMany(e=> e.ExpenseItems)
     .HasForeignKey(e => e.UniqueID_Accounting );

 public class ExpenseItem
    {
            [Key]
            public String UniqueID_ERLineID { get; set; }
            public String ERNum { get; set; }
            public String ItemNum { get; set; }
            public String Card_Number { get; set; }
            public ExpenseItemAccounting Parent_Expense_Item { get; set; }
    }

modelBuilder.Entity<ExpenseItemAccounting>()
    .HasOptional(e => e.ExpenseItem)
    .WithOptionalDependent(e=>e.Parent_Expense_Item);