Develop Reference

Use LINQ to get only the most recent JOINed item for each element

I have a LINQ query:
Elements.Join(ElementStates,
element => element.ElementID,
elementState => elementState.ElementID,
(element , elementState ) => new { element, elementState })
OK, so each Element has an ElementState associated to it. However there can be multiple states for each element for historical purposes, marked by a DateModified column. In this query, I would like to return only the most recent ElementState for each Element.
Is such a thing possible, using LINQ?
EDIT:
Credit to Gilad Green for their helpful answer.
I have converted it to Method syntax for anyone else who would like to see this in the future:
Elements.GroupJoin(ElementStates,
element => element.ElementID,
elementState => elementState.ElementID,
(element, elementState) =>
new { element, elementState = elementState.OrderByDescending(y => y.DateModified).FirstOrDefault() });

You can use GroupJoin instead of Join and then retrieve the first record after ordering the group by the DateModified:
var result = from e in Elements
join es in ElementStates on e.ElementID equals es.ElementID into esj
select new {
Element = e,
State = esj.OrderByDescending(i => i.DateModified).FirstOrDefault()
};
The same can be implemented with method syntax instead of query syntax but in my opinion this is more readable
For the difference between simply joining and group joining: Linq to Entities join vs groupjoin

QueryOver: select columns from subquery

How can I select / project values from a subquery from a different table into my main query?
I have an NH-model like this:
[Serializable]
public class MyModel
{
public virtual int Id {get; set;}
//more mapped values
....
//unmapped values
public virtual string ValueFromOtherTable {get;set;}
}
And I want to fill ValueFromOtherTable with a left join like this:
Select mt.*, ..., ot.ValueFromOtherTable from MyModelTable mt left
join OtherTable ot ON (somecondition)
where MyModelTable is the table mapped to MyModel-class. I want to fill ValueFromOtherTable (no NH-mapping) by selecting all values from mt (to fill the NH-mapped columns) and then by using OtherTable I want to fill ValueFromOtherTable.
I can´t join both tables via QueryOver as there exists no direct parent-child correlation in the model, so JoinAlias or JoinQueryOver won´t work. My MainQueryOver queries MyModelTable.
ALTERNATIVE:
The alternative is to first get all values from MyModelTable and then using the properties there to query OtherTable. However this will result in an SELECT N+1 problem (for each model from MyModel select some OtherTable...) and also makes the code very complicated.
Is there a good way to solve this problem or is the only way to fill MyModel by using described alternative
?

One way would be to use Projections, Subquery and DTO. So let's say, that we have DTO (almost the same as MyModel, but with new extern property ... e.g. Count). Then we can do it like this:
MyModel main = null;
MyModelDTO dto = null;
// the main query
var query = session.QueryOver<MyModel>(() => main);
// the subquery used for projection
var subquery = QueryOver.Of<OtherModel>()
// select something, e.g. count of the ID
.SelectList(selectGroup => selectGroup.SelectCount(o => o.ID))
// some condition
// kind of JOIN inside of the subquery
.Where(o => o.xxx == main.yyy); // just example
// now select the properties from main MyModel and one from the subquery
query.SelectList(sl => sl
.SelectSubQuery(subquery)
.WithAlias(() => dto.Count)
.Select(() => main.ID)
.WithAlias(() => dto .ID)
....
);
// we have to use transformer
query.TransformUsing(Transformers.AliasToBean<MyModelDTO >())
// and we can get a list of DTO
var list = query.List<MyModelDTO>();

Using LINQ to select desired results between two related IEnumerable query objects

I think this is kind of a basic question but I'm getting confused. I have two objects, Orders and OrderTags. In the database, Orders has no relation to OrderTags, but OrderTags has a FK relation to Orders.
So I capture both objects in my context like so:
orders = context.Orders;
tags = context.OrderTags.Where(tag=> tag.ID = myID);
Now I want to reduce the orders list to only be equal to the orders that exist in my tags list. Here is my best pseudocode of what I want to do:
orders = orders.Where(every order id exists somewhere in the tags list of order ids)
For clarification, each Tag object has a TagID and an OrderID. So I only want the orders that correspond to the tags I have looked up. Can anyone assist me with the syntax so I can get what I'm looking for?

Using a LINQ query:
var results = (from o in context.Orders
join t in context.Tags on o.OrderId equals t.OrderId
where t.ID == myID
select o ).ToList();

Using LINQ query:
orders = orders.Where(order => tags.Contains(tag => tag.ID == order.OrderID)).ToList();

Using a LINQ query with lambda expressions:
orders.RemoveAll(x => !tags.ConvertAll(y => y.tagId).Contains(x.tagID));

Something like this should work.
orders = orders.Where(o=>tags.Contains(t=>o.ID == t.OrderID));
You could also just perform a join.

Linq lambda expression many to many table select

I have three tables, which two of them are in many to many relationship.
Picture:
This is the data in middle mm table:
Edit:
Got until here, I get proper 4 rows back, but they are all the same result(I know I need 4 rows back, but there are different results)
return this._mediaBugEntityDB.LotteryOffers
.Find(lotteryOfferId).LotteryDrawDates
.Join(this._mediaBugEntityDB.Lotteries, ldd => ldd.LotteryId, lot => lot.Id, (ldd, lot) =>
new Lottery
{
Name = lot.Name,
CreatedBy = lot.CreatedBy,
ModifiedOn = lot.ModifiedOn
}).AsQueryable();
My question is, how can I retrieve all the Lotteries via many to many table WHERE I have LotteryOfferId given only?
What I want to achieve is to get data from lottery table by LotteryDrawDateId.
First I use LotteryOfferId to get DrawDates from middle table, and by middle table I get drawDateIds to use them in LotteryDrawDate table. From that table I need to retreive Lottey table by LotteryId in LotteryDrawDate table.
I gain this by normal SQL(LotteryOffersLotteryDrawDates is middle table in DB, not seen in model):
select
Name, Lotteries.CreatedBy, Lotteries.ModifiedOn, count(Lotteries.Id)
as TotalDrawDates from Lotteries join LotteryDrawDates on Lotteries.Id
= LotteryDrawDates.LotteryId join LotteryOffersLotteryDrawDates on LotteryDrawDates.Id =
LotteryOffersLotteryDrawDates.LotteryDrawDate_Id
where LotteryOffersLotteryDrawDates.LotteryOffer_Id = 19 group by
Name, Lotteries.CreatedBy, Lotteries.ModifiedOn
But Linq is different story :P
I would like to do this with lambda expressions.
Thanks

db.LotteryOffer.Where(lo => lo.Id == <lotteryOfferId>)
.SelectMany(lo => lo.LotteryDrawDates)
.Select( ldd => ldd.Lottery )
.GroupBy( l => new { l.Name, l.CreatedBy, l.ModifiedOn } )
.Select( g => new
{
g.Key.Name,
g.Key.CreatedBy,
g.Key.ModifiedOn,
TotalDrawDates = g.Count()
} );

You can do this:
var query = from lo in this._mediaBugEntityDB.LotteryOffers
where lo.lotteryOfferId == lotteryOfferId
from ld in lo.LotteryDrawDates
group ld by ld.Lottery into grp
select grp.Key;
I do this in query syntax, because (in my opinion) it is easier to see what happens. The main point is the grouping by Lottery, because you get a number of LotteryDrawDates any of which can have the same Lottery.
If you want to display the counts of LotteryDrawDates per Lottery it's better to take a different approach:
from lot in this._mediaBugEntityDB.Lotteries.Include(x => x.LotteryDrawDates)
where lot.LotteryDrawDates
.Any(ld => ld.LotteryDrawDates
.Any(lo => lo.lotteryOfferId == lotteryOfferId))
select lot
Now you get Lottery objects with their LotteryDrawDates collections loaded, so afterwards you can access lottery.LotteryDrawDates.Count() without lazy loading exceptions.

How can I query this hierarchical data using LINQ?

I have 3 kinds of objects: Agency, BusinessUnit and Client (each with their own respective table)
In terms of hierarchy, Agencies own BusinessUnits, and BusinessUnits own Clients.
I have 3 C# POCO Objects to represent them (I usually select new {} into them, rather than use the LINQ generated classes):
public class Agency
{
public IEnumerable<BusinessUnit> BusinessUnits { get; set; }
}
public class BusinessUnit
{
public IEnumerable<Client> Clients { get; set; }
}
public class Client
{
public int NumberOfAccounts { get; set; }
public Decimal AmountOfPlacement { get; set; }
public Decimal AvgBalance { get; set; }
public Double NeuPlacementScore { get; set; }
}
You can see that Agencies contain a list of BusinessUnits, and BusinessUnits contain a list of Clients.
I also have a mapping table called BAC_Map in the database which says which owns which, and it looks something like this:
How can I construct a query, so I can query for and return a list of Agencies? Meaning that, I want each Agency to have its list of BusinessUnit objects set, and I want the list of BusinessObjects to have its list of Clients set.
I can do basic LINQ queries, but this is a little over my head concerning the Map table and the multiple? queries.
How could I construct a method like GetAllAgencies() which would query, for not only all agencies, but populate its BusinessUnits that Agency owns, and the Clients those BusinessUnits own?
Edit: Any tips or info is appreciated. Do I need to do joins? Does this need to be multiple queries to return an Agency list, with its submembers populated?

If you drop all four tables (Agency, BusinessUnit, Client, Map) on the linq to sql designer, and draw relationships from Map to the other three, there will be some useful properties on Map.
//construct a query to fetch the row/column shaped results.
var query =
from m in db.map
//where m.... ?
let a = m.Agency
let b = m.BusinessUnit
let c = m.Client
// where something about a or b or c ?
select new {
AgencyID = a.AgencyID,
AgencyName = a.Name,
BusinessUnitID = b.BusinessUnitID,
ClientID = c.ClientID,
NumberOfAccounts = c.NumberOfAccounts,
Score = c.Score
};
//hit the database
var rawRecords = query.ToList();
//shape the results further into a hierarchy.
List<Agency> results = rawRecords
.GroupBy(x => x.AgencyID)
.Select(g => new Agency()
{
Name = g.First().AgencyName,
BusinessUnits = g
.GroupBy(y => y.BusinessUnitID)
.Select(g2 => new BusinessUnit()
{
Clients = g2
.Select(z => new Client()
{
NumberOfAccounts = z.NumberOfAccounts,
Score = z.Score
})
})
})
.ToList();
If approriate filters are supplied (see the commented out where clauses), then only the needed portions of the tables will be pulled into memory. This is standard SQL joining at work here.

I created your tables in a SQL Server database, and tried to recreate your scenario in LinqPad. I ended up with the following LINQ statements, which basically result in the same structure of your POCO classes:
var map = from bac in BAC_Maps
join a in Agencies on bac.Agency_ID equals a.Agency_ID
join b in BusinessUnits on bac.Business_Unit_ID equals b.Business_Unit_ID
join c in Clients on bac.Client_ID equals c.Client_ID
select new
{
AgencyID = a.Agency_ID,
BusinessUnitID = b.Business_Unit_ID,
Client = c
};
var results = from m in map.ToList()
group m by m.AgencyID into g
select new
{
BusinessUnits = from m2 in g
group m2 by m2.BusinessUnitID into g2
select new
{
Clients = from m3 in g2
select m3.Client
}
};
results.Dump();
Note that I called map.ToList() in the second query. This actually resulted in a single, efficient query. My initial attempt did not include .ToList(), and resulted in nine separate queries to produce the same results. The query generated by the .ToList() version is as follows:
SELECT [t1].[Agency_ID] AS [AgencyID], [t2].[Business_Unit_ID] AS [BusinessUnitID], [t3].[Client_ID], [t3].[NumberOfAccounts], [t3].[AmountOfPlacement], [t3].[AvgBalance], [t3].[NeuPlacementScore]
FROM [BAC_Map] AS [t0]
INNER JOIN [Agencies] AS [t1] ON [t0].[Agency_ID] = [t1].[Agency_ID]
INNER JOIN [BusinessUnits] AS [t2] ON [t0].[Business_Unit_ID] = [t2].[Business_Unit_ID]
INNER JOIN [Clients] AS [t3] ON [t0].[Client_ID] = [t3].[Client_ID]
Here is a screenshot of the results:
alt text http://img411.imageshack.us/img411/5003/agencybusinessunitclien.png

If you are doing this with direct LINQ to SQL, there is no way to do this without some kind of recursion, whether you do it yourself or you hide it behind an extension method. Recursive SQL is very bad (many round trips, many single queries).
There are two options here. One is to pull the entire table(s) with the hierarchy into memory and use LINQ to Objects on it. Leave the "details" tables in SQL. If you have less than several thousand entities, this is probably the most efficient way to go. You can keep a single copy of the table(s) in cache and refresh them when necessary. When you need to fetch more detailed data from the DB for a single record, you can reattach that entity from your cached hierarchy to a new DataContext and fetch it.
The other option is to use a more complex relationship model in your database. Storing parent only by nature demands recursion, but you can use the adjacency list model to construct a single query which can span many levels of inheritance. This will mean your LINQ to SQL queries become less intuitive (querying against Entity.Right and Entity.Left isn't quite as pretty as Parent or Children...) but you can do in one query what might take hundreds or thousands in the literal recursive approach.