NHibernate. Load Entity with 2 query and merge - nhibernate

Question

I've got complex entity with a lot of children collection of objects, which are complex too:
public class Order : AdvancedBaseOrder, ICheckable
{
public virtual ICollection<RouteUnit> RouteUnits
public virtual ICollection<Invoice> Invoices
public virtual ICollection<Call> Calls;
public virtual ICollection<Payment> Payments;
......
}
My payment class aggregates a lot of other objects
public class Payment: ICheckable
{
public virtual A A;
public virtual B B;
public virtual C C;
public virtual D D;
......
}
I want get order with 2 queries:
load order entity without payments (FetchMode.Lazy)
load payments by order with joining its objects
combine\merger order with its payments
I don't want lazy load payments by nhib, cause I would like override fetch strategies for payment's objects.
So my question is how can i merge combine two result of queries in one aggregate
. Thanks

Answer 1

In this case, batch-size="25" setting could do the job for you. Read more in the documentation: 19.1.5. Using batch fetching.
batch size coulde be applied on a class or collection maping:
Payment class
<class name="Payment" batch-size="10">...</class>
Collection of Payments
<class name="Order">
<set name="Payments" batch-size="3">
...
</set>
</class>
How the batching works in a nutshell: NHibernates loads the set of all Orders. Then based on a batch-size setting (e.g. 25) creates few calls to Payments filtered by IDs of just loaded Orders:
WHERE OrderId in (#o1, #o2, #O3... #o25)
The merge will be done for you inside NHibernate session. From my experience this is most powerful mapping... Lazy & Batching.