NHibernate QueryOver with Fetch resulting multiple sql queries and db hits

问题

I\'m trying to select an entity and fetch a related list:

    Session.QueryOver<UserRole>()
           .Fetch(x => x.UsersInRole).Eager
           .List();

Which results in a lot of database hits. The first one is something like:

 SELECT ... FROM UserRoles
 left outer join UsersInRoles on ...

And hundreds more seperate queries which looks something like:

 SELECT ... FROM UsersInRoles
 left outer join UserRoles on ...
 WHERE UserRoles.UserId=?

The mapping is as following:

public class UserRoleMap : ClassMap<UserRole>
{
    public UserRoleMap()
    {
        Id(x => x.Id);
        Map(x => x.RoleName);
        HasManyToMany(x => x.UsersInRole)
        .Inverse()
        .LazyLoad()
        .Table(\"UsersInRoles\");
    }
}

回答1:

I would say, that this behaviour is what we should expect. Let's have a scenario, in which we have in the system 2 users and 2 roles

User1 - Role1 // has only Role1
User2 - Role1 // now we see that Role2 has more then User1
User2 - Role2

Let's say, that the first query, will retrieve only User1 and its many-to-many relation Role1. What we have in the ISession at the moment is only User1, so the set of Users for Role1 is incomplete (we cannot reuse objects loaded into ISession at the moment). But how should one know where we are? That all data loaded for Role1 is or is not in the session?

New query, loading the data for Role1 must be issued. And this way, we can at the end have dosens of these queries...

What I see as the best solution (I am using it in almost all scenarios) is the batch-size setting: 19.1.5. Using batch fetching

HasManyToMany(x => x.UsersInRole)
  ...
  .BatchSize(25)

Mark all your collection maps with .BatchSize(25) and do that even for the Class map as well. That will cause more then 1 SQL Script, but at the end not more then 1 + (2-4) dependent on the batch-size and page size.

来源：https://stackoverflow.com/questions/19284228/nhibernate-queryover-with-fetch-resulting-multiple-sql-queries-and-db-hits

标签

sql

nhibernate

fetch

queryover