如何在 NHibernate 中与 null 建立一对一的关系？答案

【问题标题】：How to do a one-to-one relationship with null in NHibernate?如何在 NHibernate 中与 null 建立一对一的关系？
【发布时间】：2013-03-04 04:02:22
【问题描述】：

我有两个具有一对一关系的 NHibernate 实体。我们称他们为狗和主人。

public class Dog 
{
    public virtual int Id { get;set; }
    public virtual Owner Owner { get; set; }
}

public class Owner 
{
    public virtual int Id { get; set; }
    public virtual Dog Dog { get; set; }
}

Dog 可能有一个或零个所有者。 Fluent/NHibernate 映射如下：

public class DogMap : ClassMap<Dog> 
{
    public DogMap() 
    {
        Table("Dogs");

        Id(x => x.Id);

        HasOne( x=> x.Owner)
            .Fetch.Join()
            .Not.LazyLoad();
    }
}

public class OwnerMap : ClassMap<Owner> 
{
    public OwnerMap() 
    {
        Table("Owners");

        // Owners share the same primary-key as dogs
        Id(x => x.Id).GeneratedBy.Foreign("Dog"); 

        References( x => x.Dog)
            .Unique()
            .Not.LazyLoad();
   }
}

现在我只想选择 没有主人的狗。

我假设查询是：

Owner owner = null;
var ownerlessDogs = session
    .QueryOver<Dog>()
    .Left.JoinAlias(x => x.Owner, () => owner)
    // Restrict on alias
    .WhereRestrictionOn(() => owner).IsNull
    .List();

但正如您可能已经猜到的那样，这是行不通的。它会抛出“对象引用未设置为 ...”。

如果我尝试，

var ownerlessDogs = session
    .QueryOver<Dog>()
    .Left.JoinAlias(x => x.Owner, () => owner)
    // Restrict on property of root object
    .WhereRestrictionOn(x => x.Owner).IsNull
    .List();

它生成基本上生成

SELECT {{relevant columns}}
FROM Dogs dogs
LEFT OUTER JOIN OWNERS owners
WHERE dogs.Id IS NULL

几乎正确，但它是过滤狗的主键，而不是狗的外键。

【问题讨论】：

标签： nhibernate fluent-nhibernate nhibernate-mapping nhibernate-criteria

【解决方案1】：

在an old post in the Hibernate forum 的帮助下，我找到了解决方案：

public class DogMap : ClassMap<Dog> {

    public DogMap() {
        Table("Dogs");
        Id(x => x.Id);

        HasOne( x=> x.Owner)
          .Fetch.Join();
    }
}

public class OwnerMap : ClassMap<Owner> {

   public OwnerMap() {

   Table("Owners");

   //owners share the same primary-key as dogs
   Id(x => x.Id).GeneratedBy.Foreign("Dog"); 

   //both sides should be mapped as HasOne
   HasOne( x => x.Dog)
     .Constrained()
     .Fetch.Join();
   }
}

工作查询是

var ownerlessDogs = session
  .QueryOver<Dog>()
  .Left.JoinAlias(x => x.Owner, () => owner)
  //this is the trick, restrict on Id
  .WhereRestrictionOn( x => x.Owner.Id ).IsNull
  .List();

【讨论】：

显然要等两天过去了。