NHibernate一对一关系

Question

我有以下Domain Model(s):

public class WriteOffApprovalUser
{
    public virtual string UserName { get; set; }
    public virtual Employee Employee { get; set; }
}
public class Employee
{
    public virtual string EmployeeID { get; set; }
    public virtual string EmployeeStatusCode { get; set; }
    public virtual string FirstName { get; set; }
    public virtual string LastName { get; set; }
    public virtual string PreferredName { get; set; }
    public virtual string JobTitle { get; set; }
    public virtual string Division { get; set; }
    public virtual string Department { get; set; }
    public virtual string Location { get; set; }
    public virtual string City { get; set; }
    public virtual string DeskLocation { get; set; }
    public virtual string MailID { get; set; }
    public virtual string Phone { get; set; }
    public virtual string Fax { get; set; }
    public virtual string SecCode { get; set; }
    public virtual string UserId { get; set; }
    public virtual string SupervisorID { get; set; }
}

这些是我的Fluent Mappings

public class WriteOffApprovalUserMap : ClassMap<WriteOffApprovalUser>
{
    public WriteOffApprovalUserMap()
    {
        //Schema("LEGAL");
        Table("WRITEOFF_APPROVAL_USER");
        Id(x => x.UserName).Column("USER_NAME");
        HasOne(x => x.Employee).PropertyRef("UserId");
    }
}
public class EmployeeMap : ClassMap<Employee>
{
    public EmployeeMap()
    {
        // Table Name
        //Schema("ADP_FEED_OWNER");
        Table("ADP_EMPLOYEE");
        // Primary Key
        Id(x => x.EmployeeID).Column("EMPLID");
        // Mappings
        Map(x => x.UserId).Column("USER_ID");
        Map(x => x.FirstName).Column("FIRST_NAME");
        Map(x => x.LastName).Column("LAST_NAME");
        Map(x => x.PreferredName).Column("PREFERRED_NAME");
    }
}

这是我的查询:

var results = new Repository<WriteOffApprovalUser>(session)
                    .Query()
                    .ToList();

这是它正在生成的SQL，而我期望的是一个JOIN。

select writeoffap0_.USER_NAME as USER1_1_ from WRITEOFF_APPROVAL_USER writeoffap0_
SELECT employee0_.EMPLID as EMPLID0_0_, employee0_.USER_ID as USER2_0_0_, employee0_.FIRST_NAME as FIRST3_0_0_, employee0_.LAST_NAME as LAST4_0_0_, employee0_.PREFERRED_NAME as PREFERRED5_0_0_ FROM ADP_EMPLOYEE employee0_ WHERE employee0_.EMPLID=:p0;
SELECT employee0_.EMPLID as EMPLID0_0_, employee0_.USER_ID as USER2_0_0_, employee0_.FIRST_NAME as FIRST3_0_0_, employee0_.LAST_NAME as LAST4_0_0_, employee0_.PREFERRED_NAME as PREFERRED5_0_0_ FROM ADP_EMPLOYEE employee0_ WHERE employee0_.EMPLID=:p0;
SELECT employee0_.EMPLID as EMPLID0_0_, employee0_.USER_ID as USER2_0_0_, employee0_.FIRST_NAME as FIRST3_0_0_, employee0_.LAST_NAME as LAST4_0_0_, employee0_.PREFERRED_NAME as PREFERRED5_0_0_ FROM ADP_EMPLOYEE employee0_ WHERE employee0_.EMPLID=:p0;
SELECT employee0_.EMPLID as EMPLID0_0_, employee0_.USER_ID as USER2_0_0_, employee0_.FIRST_NAME as FIRST3_0_0_, employee0_.LAST_NAME as LAST4_0_0_, employee0_.PREFERRED_NAME as PREFERRED5_0_0_ FROM ADP_EMPLOYEE employee0_ WHERE employee0_.EMPLID=:p0;

现在数据库中有四行，并且返回了正确的数据，但是我不希望五个单独的SQL语句来完成此操作。

Answer 1

您需要急于加载/获取Employee实体，以避免您所看到的通常被称为SELECT N+1问题的行为。要做到这一点，您有两个选项:

选项1。映射中的主动加载意味着当您查询WriteOffApprovalUser实体时，它将始终对Employee表执行JOIN。注意:这可能听起来像你想要的，但要小心，因为你将迫使所有与此实体合作的开发人员一直坚持此设计决策，直到时间结束。您必须问自己，我是否想要查询WriteOffApprovalUser表，而不对Employee表执行JOIN ?如果答案是肯定的，那么不要在映射文件中强制进行即时加载。

要自动获取Employee，更改映射中的HasOne代码如下所示:

HasOne(x => x.Employee).PropertyRef("UserId").Not.LazyLoad().Fetch.Join();

选项2。在查询中执行即时加载。我注意到你正在使用某种T模式的存储库，所以你可能不得不修改它来处理即时加载。典型的急切加载使用NHibernate的内置LINQ Query<T>类。Linq命名空间看起来像这样:

var results = new session.Query<WriteOffApprovalUser>()
                    .Fetch( x => x.Employee ) // This will tell NHibernate to perform a JOIN to the Employee table
                    .ToList();