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EF Core Reuse subquery in different queries

I have a problem trying to reuse some subqueries. I have the following situation:
var rooms = dbContext.Rooms.Select(r => new
{
RoomId = r.Id,
Zones = r.Zones.Select(zr => zr.Zone),
Name = r.Name,
Levels = r.Levels.Select(lr => lr.Level),
IdealSetpoint = (double?)r.Group.Setpoints.First(sp => sp.ClimaticZoneId == dbContext.ClimaticZonesLogs.OrderByDescending(cz => cz.Timestamp).First().ClimaticZoneId).Setpoint??int.MinValue,
Devices = r.Devices.Select(rd => rd.Device)
}).ToList();
var tagsTypes = rooms.Select(r => r.Devices.Select(d => GetSetpointTagTypeId(d.DeviceTypeId))).ToList().SelectMany(x => x).Distinct().ToList();
predicate = predicate.And(pv => tagsTypes.Contains(pv.TagSettings.TagTypeId) &&
pv.ClimaticZoneId == dbContext.ClimaticZonesLogs.OrderByDescending(cz => cz.Timestamp).First().ClimaticZoneId);
var setpoints = valuesSubquery.Include(t=>t.TagSettings).Where(predicate).ToList();
This works fine, and generates the exact queries as wanted. The problem is that I want to have this subquery dbContext.ClimaticZonesLogs.OrderByDescending(cz => cz.Timestamp).First().ClimaticZoneId to be taken from a method and not repeat it every time I need it.
I've tested it with the database, where I have values in the corresponding tables, and I've tested the query with the database without any data in the corresponding tables. It works fine with no problems or exceptions.
But when I try to extract the repeating subquery in a separate method and execute it against empty database tables (no data) the .First() statement throws error. Here is the code:
protected long GetClimaticZoneId()
{
return dbContext.ClimaticZonesLogs.OrderByDescending(cz => cz.Timestamp).First().ClimaticZoneId;
}
and the query generation:
var rooms = dbContext.Rooms.Select(r => new
{
RoomId = r.Id,
Zones = r.Zones.Select(zr => zr.Zone),
Name = r.Name,
Levels = r.Levels.Select(lr => lr.Level),
IdealSetpoint = (double?)r.Group.Setpoints.First(sp => sp.ClimaticZoneId == GetClimaticZoneId()).Setpoint??int.MinValue,
Devices = r.Devices.Select(rd => rd.Device)
}).ToList();
var tagsTypes = rooms.Select(r => r.Devices.Select(d => GetSetpointTagTypeId(d.DeviceTypeId))).ToList().SelectMany(x => x).Distinct().ToList();
predicate = predicate.And(pv => tagsTypes.Contains(pv.TagSettings.TagTypeId) &&
pv.ClimaticZoneId == GetClimaticZoneId());
var setpoints = valuesSubquery.Include(t=>t.TagSettings).Where(predicate).ToList();
After execution I get InvalidOperationException "Sequence do not contain any elements" exception in the GetClimaticZoneId method:
I'm sure that I'm not doing something right.
Please help!
Regards,
Julian

As #Gert Arnold suggested, I used the GetClimaticZoneId() method to make a separate call to the database, get the Id and use it in the other queries. I gust modified the query to not generate exception when there is no data in the corresponding table:
protected long GetClimaticZoneId()
{
return dbContext.ClimaticZonesLogs.OrderByDescending(cz => cz.Timestamp).FirstOrDefault()?.ClimaticZoneId??0;
}

Joining table to a list using Entity Framework

I have the following Entity Framework function that it joining a table to a list. Each item in serviceSuburbList contains two ints, ServiceId and SuburbId.
public List<SearchResults> GetSearchResultsList(List<ServiceSuburbPair> serviceSuburbList)
{
var srtList = new List<SearchResults>();
srtList = DataContext.Set<SearchResults>()
.AsEnumerable()
.Where(x => serviceSuburbList.Any(m => m.ServiceId == x.ServiceId &&
m.SuburbId == x.SuburbId))
.ToList();
return srtList;
}
Obviously that AsEnumerable is killing my performance. I'm unsure of another way to do this. Basically, I have my SearchResults table and I want to find records that match serviceSuburbList.

If serviceSuburbList's length is not big, you can make several Unions:
var table = DataContext.Set<SearchResults>();
IQuerable<SearchResults> query = null;
foreach(var y in serviceSuburbList)
{
var temp = table.Where(x => x.ServiceId == y.ServiceId && x.SuburbId == y.SuburbId);
query = query == null ? temp : query.Union(temp);
}
var srtList = query.ToList();
Another solution - to use Z.EntityFramework.Plus.EF6 library:
var srtList = serviceSuburbList.Select(y =>
ctx.Customer.DeferredFirstOrDefault(
x => x.ServiceId == y.ServiceId && x.SuburbId == y.SuburbId
).FutureValue()
).ToList().Select(x => x.Value).Where(x => x != null).ToList();
//all queries together as a batch will be sent to database
//when first time .Value property will be requested

GroupBy Mongodb with Driver C#

I'm making a stadistics module and I need to do a GroupBy Operation with a Count to take the total visitors by country. I'm using MongoRepository (Link to Library)
I have this code in C# working fine, but I don't like the solution:
var repositoryIpFrom = new MongoRepository<IpFrom>();
var countries = repositoryIpFrom.Where(s => s.id == id).Select(s => s.Country).Distinct();
foreach (var country in countries)
{
statisticsCountryDto.Add(new StatisticsCountryDto()
{
Country = country,
Count = repositoryIpFrom.Count(s => s.id == id && s.Country == country)
});
}
return statisticsCountryDto;
And this one with Linq (but it's not working... the error says GroupBy is not supported)
var tst = repositoryIpFrom.Where(s=>s.id == id).GroupBy(s => s.Country).Select(n => new
{
Country = n.Key,
Count = n.Count()
});
Can I have any options to make the GroupBy and not make a lot of queries?
Thanks!!

Since you are not filtering through the query, you can resolve the query by inserting a AsEnumerable operation and then perform the grouping operations on the local data after the MongoDb driver is no longer involved.
var tst = repositoryIpFrom
.Where(s=>s.id == id)
.AsEnumerable()
.GroupBy(s => s.Country)
.Select(n => new
{
Country = n.Key,
Count = n.Count()
});

SQL Query to NHibernate with HAVING COUNT

I am trying to recreate the following query in NHibernate:
SELECT DISTINCT
orderid ,
tasktype
FROM "Task"
WHERE orderid IN ( SELECT orderid
FROM "Task"
GROUP BY orderid
HAVING COUNT(orderid) > 1 )
ORDER BY orderid
In NH, I need a QueryOver that returns a list of task types based on the order id. Basically, I am iterating over each task and for each task that occurs more than once, (because of a different task type), i need to add all those tasks into a list that gets returned to the client. This is what I have tried so far with NH.
var taskList = new List<Task>();
PendingTasks = session.QueryOver<Model.Task>()
.WhereRestrictionOn(c => c.OrderId).IsIn(taskList)
.SelectList
(list => list
.SelectGroup(b => b.OrderId)
.Select(b => b.TaskType)
)
.Where(Restrictions.Eq(Projections.Count<Model.Task>(x => x.OrderId), taskList.Count() > 1))
.TransformUsing((Transformers.AliasToBean<TaskType>()))
.List<TaskType>()
I have just started NH, and found some examples on here regarding the use of grouping and having. The property from the model I am returning to the client is here with TaskType being a simple enum.
public List<TaskType> PendingTasks { get; set; }
It seems to me so far, that the QueryOver is trying to return an IList, against my target type of List, however there is no .ToList(), so I do not know what this will return. Any help matching the sql query above is helpful.
UPDATE: Entire Method:
private static readonly string[] TaskTypeKeys = Enum.GetNames(typeof(TaskType));
var tasksByType = new List<TaskGroup>();
Task taskObject = null;
QueryOver subQuery = QueryOver.Of<Task>()
.Select(
Projections.GroupProperty(
Projections.Property<Task>(t => t.OrderId)
)
)
.Where(Restrictions.Gt(Projections.Count<Task>(t => t.OrderId), 1));
foreach (var type in TaskTypeKeys)
{
TaskType typeEnum;
Enum.TryParse(type, out typeEnum);
var tasks = session.QueryOver<Model.Task>()
.Where(
task =>
task.TaskType == typeEnum &&
task.Completed == false &&
task.DueDate <= DateTime.Today
)
.OrderBy(t => t.DueDate).Asc
.List<Model.Task>()
.Select(t => new Task()
{
Id = t.Id,
OrderId = t.OrderId,
CustomerId = t.CustomerId,
CustomerName = t.CustomerName,
GroupName = t.GroupName,
TripDate = t.TripDate,
TaskType = TaskTypeTitles[t.TaskType.ToString()],
DueDate = t.DueDate,
Completed = t.Completed,
IsActiveTask = t.IsActiveTask,
PendingTasks = session.QueryOver<Task>(() => taskObject)
// the WHERE clause: OrderId IN (subquery)
.WithSubquery
.WhereProperty(() => taskObject.OrderId)
ERROR-------> .In(subQuery)
// the rest of your SELECT/projections and transformation
.SelectList(list => list
.SelectGroup(b => b.OrderId)
.Select(b => b.TaskType)
)
.TransformUsing((Transformers.AliasToBean<TaskType>()))
.List<TaskType>()
}
).ToList();
tasksByType.Add(new TaskGroup()
{
Title = TaskTypeTitles[type.ToString()],
Content = tasks,
RemainingCount = tasks.Count(),
OverdueCount =
tasks.Count(
task =>
task.DueDate < DateTime.Today),
});
};
return tasksByType;
The type arguments for method 'NHibernate.Criterion.Lambda.QueryOverSubqueryPropertyBuilderBase,Api.Task,Api.Task>.In(NHibernate.Criterion.QueryOve‌​r)' cannot be inferred from the usage. Try specifying the type arguments explicitly.

The subquery syntax will help us in this case. First of all, let's declare the inner select this way:
QueryOver<Task> subQuery = QueryOver.Of<Task>()
.Select(
Projections.GroupProperty(
Projections.Property<Task>(t => t.OrderId)
)
)
.Where(Restrictions.Gt(Projections.Count<Task>(t => t.OrderId), 1))
;
This will produce the:
(SELECT this_0_.OrderId as y0_
FROM [Task] this_0_
GROUP BY this_0_.OrderId
HAVING count(this_0_.OrderId) > 1)
Now we can use it as a subquery in the outer SELECT:
Task task = null;
var PendingTasks =
session.QueryOver<Task>(() => task)
// the WHERE clause: OrderId IN (subquery)
.WithSubquery
.WhereProperty(() => task.OrderId)
.In(subQuery)
// the rest of your SELECT/projections and transformation
.SelectList(list => list
.SelectGroup(b => b.OrderId)
.Select(b => b.TaskType)
)
.TransformUsing((Transformers.AliasToBean<TaskType>()))
.List<TaskType>()
;
And this will create the rest, with included subquery

Entity framework use already selected value saved in new variable later in select sentance

I wrote some entity framework select:
var query = context.MyTable
.Select(a => new
{
count = a.OtherTable.Where(b => b.id == id).Sum(c => c.value),
total = a.OtherTable2.Where(d => d.id == id) * count ...
});
I have always select total:
var query = context.MyTable
.Select(a => new
{
count = a.OtherTable.Where(b => b.id == id).Sum(c => c.value),
total = a.OtherTable2.Where(d => d.id == id) * a.OtherTable.Where(b => b.id == id).Sum(c => c.value)
});
Is it possible to select it like in my first example, because I have already retrieved the value (and how to do that) or should I select it again?

One possible approach is to use two successive selects:
var query = context.MyTable
.Select(a => new
{
count = a.OtherTable.Where(b => b.id == id).Sum(c => c.value),
total = a.OtherTable2.Where(d => d.id == id)
})
.Select(x => new
{
count = x.count,
total = x.total * x.count
};

You would simple do
var listFromDatabase = context.MyTable;
var query1 = listFromDatabase.Select(a => // do something );
var query2 = listFromDatabase.Select(a => // do something );
Although to be fair, Select requires you to return some information, and you aren't, you're somewhere getting count & total and setting their values. If you want to do that, i would advise:
var listFromDatabase = context.MyTable.ToList();
listFromDatabase.ForEach(x =>
{
count = do_some_counting;
total = do_some_totalling;
});
Note, the ToList() function stops it from being IQueryable and transforms it to a solid list, also the List object allows the Linq ForEach.
If you're going to do complex stuff inside the Select I would always do:
context.MyTable.AsEnumerable()
Because that way you're not trying to still Query from the database.
So to recap: for the top part, my point is get all the table contents into variables, use ToList() to get actual results (do a workload). Second if trying to do it from a straight Query use AsEnumerable to allow more complex functions to be used inside the Select