如何更新导航属性-不修改任何其他字段

当涉及到fk时，我遇到了使用EF创建更新请求的问题。我想修改现有实体中的导航字段，而不加载它之前。因此，实体没有在上下文中预加载，也没有被代理;

对于这个例子，我有一个简单的1-*关系，它给了我这两个实体:

public partial class NameMap
{
    public NameMap()
    {
        this.SurnameMaps = new HashSet<SurnameMap>();
    }
    public int Id { get; set; }
    public string Name { get; set; }
    public string MainLang { get; set; }
    public virtual ICollection<SurnameMap> SurnameMaps { get; set; }
    public virtual TitleMap TitleMap { get; set; }
}
public partial class TitleMap
{
    public TitleMap()
    {
        this.NameMaps = new HashSet<NameMap>();
    }
    public int Id { get; set; }
    public string Title { get; set; }
    public virtual ICollection<NameMap> NameMaps { get; set; }
}

当我修改一些标量属性时，我做以下操作:

public void PartiallyChangeEntity()
{
    var nmToModify = new NameMap()
    {
        Id = 2, //item i want to change
        Name = "This is a test", //the prop i want to change
        MainLang = String.Empty //must be init and not null - but i come back to this point later (1)
    };
    _context.NameMaps.Attach(nmToModify);
    _context.Entry(nmToModify).Property(a => a.Name).IsModified = true;
    _context.SaveChanges();
}

和EF只修改表中的Name。(1)没有修改MainLang字段，但如果值被设置为null，则验证失败(似乎MainLang字段在db中不可为空的事实意味着在计算验证时它不能为空)

现在，我想修改一个名称的标题。和以前一样，我试过:

public void PartiallyChangeEntityConstraint()
{
    var nmToModify = new NameMap()
    {
        Id = 2,
        MainLang = String.Empty //same as (1)
    };
    _context.NameMaps.Attach(nmToModify);
    var title = new TitleMap {Id = 3}
    _context.NameMaps.Attach(title);
    _context.Entry(title).Collection(a => a.NameMaps).CurrentValue.Add(nmToModify);
    _context.Entry(nmToModify).Reference(a => a.TitleMap).CurrentValue = tm;
    _context.SaveChanges();
}

此方法失败并抛出此错误:

Test Name:  PartiallyChangeEntityConstraint
Test FullName:  EFTests.UnitTest1.PartiallyChangeEntityConstraint
Test Source:    d:'TFS'EFTests'EFTests'UnitTest1.cs : line 100
Test Outcome:   Failed
Test Duration:  0:00:02.0409469
Result Message: 
Test method EFTests.UnitTest1.PartiallyChangeEntityConstraint threw exception: 
System.Data.Entity.Infrastructure.DbUpdateException: An error occurred while saving entities that do not expose foreign key properties for their relationships. The EntityEntries property will return null because a single entity cannot be identified as the source of the exception. Handling of exceptions while saving can be made easier by exposing foreign key properties in your entity types. See the InnerException for details. ---> System.Data.Entity.Core.UpdateException: A relationship from the 'TitleMapNameMap' AssociationSet is in the 'Added' state. Given multiplicity constraints, a corresponding 'NameMap' must also in the 'Added' state.
Result StackTrace:  
at System.Data.Entity.Core.Mapping.Update.Internal.UpdateTranslator.RelationshipConstraintValidator.ValidateConstraints()
   at System.Data.Entity.Core.Mapping.Update.Internal.UpdateTranslator.ProduceCommands()
   at System.Data.Entity.Core.Mapping.Update.Internal.UpdateTranslator.Update()
   at System.Data.Entity.Core.EntityClient.Internal.EntityAdapter.<Update>b__2(UpdateTranslator ut)
   at System.Data.Entity.Core.EntityClient.Internal.EntityAdapter.Update[T](T noChangesResult, Func`2 updateFunction)
   at System.Data.Entity.Core.EntityClient.Internal.EntityAdapter.Update()
   at System.Data.Entity.Core.Objects.ObjectContext.<SaveChangesToStore>b__35()
   at System.Data.Entity.Core.Objects.ObjectContext.ExecuteInTransaction[T](Func`1 func, IDbExecutionStrategy executionStrategy, Boolean startLocalTransaction, Boolean releaseConnectionOnSuccess)
   at System.Data.Entity.Core.Objects.ObjectContext.SaveChangesToStore(SaveOptions options, IDbExecutionStrategy executionStrategy, Boolean startLocalTransaction)
   at System.Data.Entity.Core.Objects.ObjectContext.<>c__DisplayClass2a.<SaveChangesInternal>b__27()
   at System.Data.Entity.SqlServer.DefaultSqlExecutionStrategy.Execute[TResult](Func`1 operation)
   at System.Data.Entity.Core.Objects.ObjectContext.SaveChangesInternal(SaveOptions options, Boolean executeInExistingTransaction)
   at System.Data.Entity.Core.Objects.ObjectContext.SaveChanges(SaveOptions options)
   at System.Data.Entity.Internal.InternalContext.SaveChanges()
 --- End of inner exception stack trace ---
    at System.Data.Entity.Internal.InternalContext.SaveChanges()
   at System.Data.Entity.Internal.LazyInternalContext.SaveChanges()
   at System.Data.Entity.DbContext.SaveChanges()
   at EFTests.UnitTest1.PartiallyChangeEntityConstraint() in d:'TFS'EFTests'EFTests'UnitTest1.cs:line 103

我尝试了很多想法，比如:

_context.Entry(nmToModify).Property(a => a.TitleMap).IsModified = true;

但是TitleMap不是Ef的属性，所以他不想要它;听起来很公平。

我的主要问题是:当涉及到导航属性时，如何解决部分更新?

注意:如果可能的话，我不想在NameMap中暴露TitleMapId。

附加问题:是否有一种方法可以不初始化非空字段，正如我在(1)

谢谢你的帮助。