ActiveRecord的ProjectionQuery

Castle.ActiveRecord.Queries.ProjectionQuery<CheckOutInfo> pq = new Castle.ActiveRecord.Queries.ProjectionQuery<RelationInfo>(
    criteria, Projections.ProjectionList()
    .Add(Projections.Sum("Quantity"))
    .Add(Projections.GroupProperty("ClassA"))
    .Add(Projections.GroupProperty("ClassB"))
);

需要作分组以及统计操作时，需要用到ProjectionQuery.