使用 property-ref 映射到非键字段时，延迟加载不适用于多对一关系
我有一个旧数据库，正在使用 NHibernate 进行映射。 关注的对象是帐户和通知对象列表。 对象看起来像：
public class Notification
{
public virtual int Id { get; set; }
public virtual DateTime BatchDate { get; set; }
/* other properties */
public virtual Account Account { get; set; }
}
public class Account
{
public virtual int Id { get; set; }
public virtual string AccountNumber { get; set; }
/* other properties */
}
映射文件看起来像：
<class name="Account" table="Account" dynamic-update="true">
<id name="Id" column="AccountID">
<generator class="native" />
</id>
<property name="AccountNumber" length="15" not-null="true" />
<!-- other properties -->
</class>
<class name="Notification" table="Notification">
<id name="Id" column="Id">
<generator class="native" />
</id>
<!-- other properties -->
<many-to-one name="Account" class="Account" property-ref="AccountNumber" lazy="proxy">
<column name="AcctNum" />
</many-to-one>
但是，当我创建一个标准时，例如
return session.CreateCriteria(typeof(Notification)).List<Notification>();
我收到一个 Select N+1 情况，其中每个帐户都会被加载，即使该帐户从未被引用。 当多对一映射为惰性代理时，为什么所有帐户都会被加载？
I have a legacy database that I am mapping using NHibernate. The objects of concern are an Account and a list of Notification objects. The objects look like:
public class Notification
{
public virtual int Id { get; set; }
public virtual DateTime BatchDate { get; set; }
/* other properties */
public virtual Account Account { get; set; }
}
public class Account
{
public virtual int Id { get; set; }
public virtual string AccountNumber { get; set; }
/* other properties */
}
The mapping files look like:
<class name="Account" table="Account" dynamic-update="true">
<id name="Id" column="AccountID">
<generator class="native" />
</id>
<property name="AccountNumber" length="15" not-null="true" />
<!-- other properties -->
</class>
<class name="Notification" table="Notification">
<id name="Id" column="Id">
<generator class="native" />
</id>
<!-- other properties -->
<many-to-one name="Account" class="Account" property-ref="AccountNumber" lazy="proxy">
<column name="AcctNum" />
</many-to-one>
However, when I create a criteria such as
return session.CreateCriteria(typeof(Notification)).List<Notification>();
I am getting a Select N+1 case where each account is loaded even though the Account is never referenced. Why are all of the accounts getting loaded when the many-to-one is mapped as a lazy proxy?
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该问题是由
property-ref属性引起的。 延迟加载仅在多对一引用使用其他对象的主键时才有效，因为 NHibernate 假定存在外键约束来强制执行此类值的有效性。 对于非主键（由 property-ref 指示），NHibernate 不会做出此假设，因此不会假设相关对象必须存在。 由于它不想为不存在的对象创建代理（即应该为 null 而不是代理），因此它急切地获取远程对象。 当指定not-found="ignore"时，也存在同样的问题，因为这表明外键关系未强制执行，并可能导致空引用。另请参阅：
The issue is caused by the
property-refattribute. Lazy loading only works when themany-to-onereference is using the other object's primary key since NHibernate assumes there's a foreign key constraint enforcing the validity of such a value. With a non-primary key (indicated by the property-ref), NHibernate does not make this assumption and thus does not assume the related object must exist. Since it does not want to create a proxy for an object that does not exist (i.e. should be null instead of a proxy), it eagerly fetches the remote object. This same issue exists whennot-found="ignore"is specified since this indicates that the foreign key relationship is not enforced and may result in a null reference.See also: