有条件地获取关联实体避免 SELECT N+1 问题

Question

Suppose I have the following domain:

public class Company
{
    public long Id;
}

public class EmployeeDetailsInCompany
{
    public long Id;
    public Company Company;
}

public class Employee
{
    public long Id;
    public List<EmployeeDetailsInCompany> EmployeeDetailsInCompany;
}

public class Position
{
    public long Id;
    public Employee Employee;
    public Company Company;
}

(Please note that this is a very simplified version to make the problem easier to understand.)

Basically I want to access the EmployeeDetailsInCompany of the Employee I have in a Position entity. Currently I have to loop through the list I have on Employee and find the EmployeeDetailsInCompany whose Company.Id match that of my Company entity. This is because a Position is only valid for a certain Employee on a certain Company , so that's where the EmployeeDetailsInCompany gets in.

The best option here would be to have an association between Position and EmployeeDetailsInCompany , but that's not possible because the data on that table can change (eg a company might decide to wipe out all of its employees details and load new data again).

Is it possible to rewrite those associations so I don't fall into SELECT N+1 problems (which is happening now that I have to loop through the list)? How could I change it to make it easier to locate the details I want without much hassle?

I have considered creating a EmployeeDetailsInCompany on Position that would be mapped using a formula, but that would give me a readonly column and I'm trying to avoid HQL.

Answer 1

Read the chapter on improving performance in the reference documentation if you haven't done so. I think especially the section on batch fetching is relevant. It means that NHibernate can fetch several potentially interesting database rows in one go, when you ask for one of them.

Another option is to write a specialized query that would generate SQL to let the DBMS do the work.