NHibernate: how to express a specific "group by" query with criteria

Question

Question:

What are the criteria/projections that can generate a following query?

SELECT SUBSTRING(Name, 0, 1) FROM Person GROUP BY SUBSTRING(Name, 0, 1)

(Obviously this one is easier with DISTINCT, but I'll need counts later, when I fix this one).

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My approaches:

My main problem here is with constants, because if I use

Projections.GroupProperty(Projections.SqlFunction(
   "SUBSTRING",
   NHibernateUtil.String,
   Projections.GroupProperty("Name"),
   Projections.Constant(0),
   Projections.Constant(1)
))

I get

SELECT SUBSTRING(Name, 0, 1) FROM Person GROUP BY SUBSTRING(Name, , )

which is kind of obvious from NH source code, but useless. And if I do

Projections.GroupProperty(Projections.SqlFunction(
   "SUBSTRING",
   NHibernateUtil.String,
   Projections.GroupProperty("Name"),
   Projections.GroupProperty(Projections.Constant(0)),
   Projections.GroupProperty(Projections.Constant(1))
))

then I get

SELECT SUBSTRING(Name, @p0, @p1) FROM Person GROUP BY SUBSTRING(Name, ?, ?)

where question marks seem to be some unresolved parameters, but I have no idea why.

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More details:

I just found

AbstractEntityJoinWalker.InitProjection(
    SqlString projectionString,
    SqlString whereString,
    SqlString orderByString,
    string /* WTF? */ groupByString,
    SqlString havingString,
    LockMode lockMode
)

The type of groupByString looks extremely suspicious.