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Multiple joins with multiple on statements using Linq Lambda expressions [duplicate]

Suppose I have a list of {City, State}. It originally came from the database, and I have LocationID, but by now I loaded it into memory. Suppose I also have a table of fast food restaurants that has City and State as part of the record. I need to get a list of establishments that match city and state.
NOTE: I try to describe a simplified scenario; my business domain is completely different.
I came up with the following LINQ solution:
var establishments = from r in restaurants
from l in locations
where l.LocationId == id &&
l.City == r.City &&
l.State == r.State
select r
and I feel there must be something better. For starters, I already have City/State in memory - so to go back to the database only to have a join seems very inefficient. I am looking for some way to say {r.City, r.State} match Any(MyList) where MyList is my collection of City/State.
UPDATE
I tried to update based on suggestion below:
List<CityState> myCityStates = ...;
var establishments =
from r in restaurants
join l in myCityStates
on new { r.City, r.State } equals new { l.City, l.State } into gls
select r;
and I got the following compile error:
Error CS1941 The type of one of the expressions in the join clause is incorrect. Type inference failed in the call to 'Join'.
UPDATE 2
Compiler didn't like anonymous class in the join. I made it explicit and it stopped complaining. I'll see if it actually works in the morning...

It seems to me that you need this:
var establishments =
from r in restaurants
join l in locations.Where(x => x.LocationId == id)
on new { r.City, r.State } equals new { l.City, l.State } into gls
select r;

Well, there isn't a lot more that you can do, as long as you rely on a table lookup, the only thing you can do to speed up things is to put an index on City and State.
The linq statement has to translate into a valid SQL Statement, where "Any" would translate to something like :
SELECT * FROM Restaurants where City in ('...all cities')
I dont know if other ORM's give better performance for these types of scenarios that EF, but it might be worth investigating. EF has never had a rumor for being fast on reads.
Edit: You can also do this:
List<string> names = new List { "John", "Max", "Pete" };
bool has = customers.Any(cus => names.Contains(cus.FirstName));
this will produce the necessary IN('value1', 'value2' ...) functionality that you were looking for

Convert IQueryable to IOrderedQueryable

I have 2 tables : Items and ItemMetrics. Items contains my items and ItemMetrics contains statistics about those items.
I'm trying to sort a list of Items by the corresponding statistic Weight.
I got it to work using a subquery, but I thought it was a little inefficent and could be optimized...
items.OrderBy(item => (from metric in Context.ItemMetrics where item.ItemId == metric.ItemId select metric.Weight).FirstOrDefault());
...So I am trying to accomplish the same thing using a join...
from item in items join metric in ItemMetrics on item.ItemId equals metric.ItemId orderby metric.Weight select item;
This is working fine in LINQPad. I need to return an IOrderedQueryable for a supplmentary method to do some paging using Skip and Take, but the compiler is telling me the result of this query is an IQueryable.
The only way I can think of explictly implying IOrderedQueryable is wrapping the query in
OrderBy(x => 0)
Is there a better way?

The problem is that an IOrderedQueryable needs to be able to apply a secondary ordering - and by the time you've projected the item/metric pair to just an item, you've lost the primary ordering.
One approach would be to defer the projection until later:
var ordered = items.Join(ItemMetrics,
item => item.ItemId,
metric => metric.ItemId,
(item, metric) => new { item, metric })
.OrderBy(pair => pair.Metric.Weight);
var paged = ApplyPaging(ordered, pageConfiguration); // Whatever
var query = paged.Select(pair => pair.Item);
That's assuming ApplyPaging (or whatever you're using) returns an IQueryable or an IOrderedQueryable.

Join vs Navigation property for sub lists in Entity Framework

I have a sql statement like this:
DECLARE #destinations table(destinationId int)
INSERT INTO #destinations
VALUES (414),(416)
SELECT *
FROM GroupOrder grp (NOLOCK)
JOIN DestinationGroupItem destItem (NOLOCK)
ON destItem.GroupOrderId = grp.GroupOrderId
JOIN #destinations dests
ON destItem.DestinationId = dests.destinationId
WHERE OrderId = 5662
I am using entity framework and I am having a hard time getting this query into Linq. (The only reason I wrote the query above was to help me conceptualize what I was looking for.)
I have an IQueryable of GroupOrder entities and a List of integers that are my destinations.
After looking at this I realize that I can probably just do two joins (like my SQL query) and get to what I want.
But it seems a bit odd to do that because a GroupOrder object already has a list of DestinationGroupItem objects on it.
I am a bit confused how to use the Navigation property on the GroupOrder when I have an IQueryable listing of GroupOrders.
Also, if possible, I would like to do this in one trip to the database. (I think I could do a few foreach loops to get this done, but it would not be as efficient as a single IQueryable run to the database.)
NOTE: I prefer fluent linq syntax over the query linq syntax. But beggars can't be choosers so I will take whatever I can get.

If you already have the DestinationGroupItem as a Navigation-property, then you already have your SQL-JOIN equivalent - example. Load the related entities with Include. Use List's Contains extension method to see if the desired DestinationId(s) is(are) hit:
var destinations = new List<int> { 414, 416 };
var query = from order in GroupOrder.Include(o => o.DestinationGroupItem) // this is the join via the navigation property
where order.OrderId == 5662 && destinations.Contain(order.DestinationGroupItem.DestinationId)
select order;
// OR
var query = dataContext.GroupOrder
.Include(o => o.DestinationGroupItem)
.Where(order => order.OrderId == 5662 && destinations.Contain(order.DestinationGroupItem.DestinationId));

Linking Multiple Tables in LINQ to SQL

I would like to get the list of albums (Distinct) which was sung by the artistId=1
I am very new to LINQ to SQL and do not know how to join multiple tables. Please see the database diagram below:
alt text http://a.imageshack.us/img155/8572/13690801.jpg
SingBy is the middle table between Track and Artist.
How could I achieve this?

var albums = from singer in artist
from sb in singby
from t in track
from a in album
where singer.artistId == 1 &&
sb.artistId == 1 &&
sb.trackId == t.trackId &&
a.albumId == track.albumId
select a;
I'm sure there must be a better way. You should look into creating Navigation Properties on your entities. Navigation Properties are like foreign keys.
Edit - corrected to get albums, not artists.

Now, I wrote the codes like the following and it works.
var albums = (from a in db.artists
where a.artistId == 1
join sb in db.singbies on a equals sb.artist
join t in db.tracks on sb.track equals t
join al in db.albums on t.album equals al
select al).Distinct();
return albums.ToList() as List<album>;
I tested the Chad's version and it works too. I would like to know which way is better and good for query optimization? Thanks all.

If you have all the foreign key relationship defined, you should be able to issue call like below:
dc.GetTable<Album>().Where(a => a.Track.Singby.ArtistId == 1).ToList();
This is relying on Linq to perform lazy load for Track and Singby automatically when required. Obviously this is not optimal to use when you have a large set of data in the db and performance is critical. You can chain the query with GroupBy or Distinct operation to return only the distinct set such as
dc.GetTable<Album>().Where(a => a.Track.Singby.ArtistId == 1).Distinct().ToList();

I would like to get the list of albums
(Distinct) which was sung by the
artistId=1
DBDataContext = new DBDataContext();
album[] = db.artists.Where(a => a.artistId == 1) /* Your artist */
.SelectMany(a => a.singbies) /* Check if `singby` converted to `singbies` */
.Select(sb => sb.track) /* The tracks */
.Select(t => t.album) /* The albums */
.GroupBy(al => al.albumId) /* Group by id */ /* "Distinct" for objects */
.Select(alG => alG.First()) /* Select first of each group */
.ToArray();

IEnumerable<Album> query =
from album in myDC.Albums
let artists =
from track in album.Tracks
from singBy in track.SingBys
select singBy.Artist
where artists.Any(artist => artist.ArtistId == 1)
select album;

List<int> Ids = dc.Albums.Where(a => a.Track.Singby.ArtistId == 1).Select(a=> a.albumId).Distinct().ToList();
List<Album> distinctAlbums = dc.Albums.Where(a => distinctAlbumIds.Contains(a.albumId)).ToList();
Hey TTCG, above is the simplest way to do it. This is because doing a Distinct on a List of objects won't do it based on the albumId.
Either you do it in two steps as above, or, you write your own Album Comparer which specifies uniqueness based on AlbumId and pass it to the Distinct call on a List.
NOTE:
The above will only work if you've defined the constraints in your DBML, but better still in your DB.
For best practices, always define your relationships IN THE DATABASE when using Linq to SQL, as Linq to SQL is not like EF, or NHibernate, in that is does not "abstract" your db, it simply reflects it. It's a tool for Data Driven Design, not Domain Driven, so define the relationships in the db.

LINQ: Doing an order by!

i have some Linq to Entity code like so:
var tablearows = Context.TableB.Include("TableA").Where(c => c.TableBID == 1).Select(c => c.TableA).ToList();
So i'm returning the results of TableA with TableB.TableBID = 1
That's all good
Now how can I sort TableA by one of its column? There is a many to many relation ship between the two tables
I tried various ways with no look, for example
var tablearows = Context.TableB.Include("TableA").Where(c => c.TableBID == 1).Select(c => c.TableA).OrderBy(p => p.ColumnToSort).ToList();
In the above case when i type "p." i don't have access to the columns from TableA, presumably because it's a collection of TableA objects, not a single row

How about using SelectMany instead of Select :
var tablearows = Context.TableB.Include("TableB")
.Where(c => c.TableBID == 1)
.SelectMany(c => c.TableA)
.OrderBy(p => p.ColumnToSort)
.ToList();
EDIT :
The expression below returns collection of TableAs -every element of the collection is an instance of TableA collection not TableA instance- (that's why you can't get the properties of the TableA) :
var tablearows = Context.TableB.Include("TableB")
.Where(c => c.TableBID == 1)
.Select(c => c.TableA);
If we turn the Select to SelectMany, we get the result as one concatenated collection that includes elements :
var tablearows = Context.TableB.Include("TableB")
.Where(c => c.TableBID == 1)
.SelectMany(c => c.TableA);

Okay, so now I've taken on board that there's a many to many relationship, I think Canavar is right - you want a SelectMany.
Again, that's easier to see in a query expression:
var tableARows = from rowB in Context.TableB.Include("TableA")
where rowB.TableBID == 1
from rowA in rowB.TableA
orderby rowA.ColumnToSort
select rowA;
The reason it didn't work is that you've got a different result type. Previously, you were getting a type like:
List<EntitySet<TableA>>
(I don't know the exact type as I'm not a LINQ to Entities guy, but it would be something like that.)
Now we've flattened all those TableA rows into a single list:
List<TableA>
Now you can't order a sequence of sets by a single column within a row - but you can order a sequence of rows by a column. So basically your intuition in the question was right when you said "presumably because it's a collection of TableA objects, not a single row" - but it wasn't quite clear what you mean by "it".
Now, is that flattening actually appropriate for you? It means you no longer know which B contributed any particular A. Is there only actually one B involved here, so it doesn't matter? If so, there's another option which may even perform better (I really don't know, but you might like to look at the SQL generated in each case and profile it):
var tableARows = Context.TableB.Include("TableA")
.Where(b => b.TableBID == 1)
.Single()
.TableA.OrderBy(a => a.ColumnToSort)
.ToList();
Note that this will fail (or at least would in LINQ to Objects; I don't know exactly what will happen in entities) if there isn't a row in table B with an ID of 1. Basically it selects the single row, then selects all As associated with that row, and orders them.