nhibernate: select parents and newest child

问题

I have the following entities

public class Vehicle
{
    public virtual string InternalNumber { get; set; }
    public virtual IList<VehicleDriverHistory> DriverHistory { get; set; }
}

public class VehicleDriverHistory : HistoryEntity
{
    public virtual Vehicle Vehicle { get; set; }
    public virtual DriverPerson Driver { get; set; }
    public virtual DateTime Date { get; set; }
}

Now I need to select every Vehicle and, if present, the newest VehicleDriverHistory-Entry according to "Date". But I'm kinda struggling, I have no problem doing it in sql, but failed in nhibernate so far.

What I have so far:

VehicleDriverHistory vdHistory = null;
VehicleDriverHistory vdHistory2 = null;
q.Left.JoinAlias(x => x.DriverHistory, () => vdHistory);
q.Left.JoinAlias(x => x.DriverHistory, () => vdHistory2).Where(x => vdHistory2.Date > vdHistory.Date);
q.Where(x => vdHistory2.Id == null);

This doesn't work and it was just my attempt to "translate" the sql query (which yields the correct data) to nhibernate.

So, how would I select parents and the newest child (or none if none is present)

Thanks

UPDATE

With Firos help I ended up with the following:

VehicleDriverHistory historyAlias = null;
var maxDateQuery = QueryOver.Of<VehicleDriverHistory>()
                   .Where(h => h.Vehicle == historyAlias.Vehicle)
                   .Select(Projections.Max<VehicleDriverHistory>(h => h.Date));

var vehiclesWithEntry = DataSession.Current.QueryOver(() => historyAlias)
                        .WithSubquery.WhereProperty(h => h.Date).Eq(maxDateQuery)
                        .Fetch(h => h.Vehicle).Eager
                        .Future();

Vehicle VehicleAlias = null;
var vehiclesWithoutEntry = DataSession.Current.QueryOver<Vehicle>(() => VehicleAlias)
                           .WithSubquery
                           .WhereNotExists(QueryOver.Of<VehicleDriverHistory>()
                           .Where(x => x.Vehicle.Id == VehicleAlias.Id).Select(x => x.Id))
                           .Future();

return vehiclesWithEntry.Select(h => new PairDTO { Vehicle = h.Vehicle, NewestHistoryEntry = h }).Concat(vehiclesWithoutEntry.Select(v => new PairDTO { Vehicle = v })).ToList();

I had to replace the Any() statement in vehclesWithoutEntry with a subquery-clause because it raised an exception.

回答1:

some other idea using Futures to make only one roundtrip

VehicleDriverHistory historyAlias = null;
var maxDateQuery = QueryOver.Of<VehicleDriverHistory>()
    .Where(h => h.Vehicle == historyAlias.Vehicle)
    .Select(Projections.Max<VehicleDriverHistory>(h => h.Date));

var vehiclesWithEntry = session.QueryOver(() => historyAlias)
    .WithSubquery.WhereProperty(h => h.Date).Eq(maxDateQuery)
    .Fetch(h => h.Vehicle).Eager
    .Select(h => new PairDTO { Vehicle = h.Vehicle, NewestHistoryEntry = h })
    .Future<PairDTO>();

var vehiclesWithoutEntry = session.QueryOver<Vehicle>()
    .Where(v => !v.DriverHistory.Any())
    .Select(v => new PairDTO{ Vehicle = v })
    .Future<PairDTO>();

return vehiclesWithoutEntry.Concat(vehiclesWithEntry); // .ToList() if immediate executing is required

Update: i can not reproduce the exception but you could try this

VehicleDriverHistory historyAlias = null;
var maxDateQuery = QueryOver.Of<VehicleDriverHistory>()
    .Where(h => h.Vehicle == historyAlias.Vehicle)
    .Select(Projections.Max<VehicleDriverHistory>(h => h.Date));

var vehiclesWithEntry = session.QueryOver(() => historyAlias)
    .WithSubquery.WhereProperty(h => h.Date).Eq(maxDateQuery)
    .Fetch(h => h.Vehicle).Eager
    .Future();

var vehiclesWithoutEntry = session.QueryOver<Vehicle>()
    .Where(v => !v.DriverHistory.Any())
    .Select(v => new PairDTO{ Vehicle = v })
    .Future<PairDTO>();

return vehiclesWithoutEntry.Select(h => new PairDTO { Vehicle = h.Vehicle, NewestHistoryEntry = h }).Concat(vehiclesWithEntry);

来源：https://stackoverflow.com/questions/11241560/nhibernate-select-parents-and-newest-child

标签

nhibernate

parent-child

queryover