FetchMode.SubSelect - in addition to the mapping-level fetch mode of sub select

today there's a fetch="subselect" option that can be set via the hbm.
it cannot be overriden with fetchmode lazy, which makes it hard to use if fetch mode is only neccessary in some situations.

also, the query generated by this looks like this

first query over the father entities - lets call it q1, and lets say its where clause is w1
second query:

from Children c
where c.Father.Id in (select Id from Fathers where w1)

the problem with this (in addition to the not-overridable-with-regular-laziness) is
that w1 might have sub selects and joins, making the second query very inefficient.

NHibernate could in theory take the ids of the fathers from the first query and create the following query

from Children c
where c.Father.Id in (x1,x2,x3...) // x1,x2... the ids returned by the query over the fathers

making this very efficient.

having a FetchMode.SubSelect can help easily get a whole object graph, by setting the subselect fetch mode for a given criteria\hql query\linq query\any other query object for a set (or all) of the associations of the root entity.

It sounds like not very trivial task, but a very helpful one.