How to COUNT rows within EntityFramework without loading contents?
I'm trying to determine how to count the matching rows on a table using the EntityFramework.
The problem is that each row might have many megabytes of data (in a Binary field). Of course the SQL would be something like this:
SELECT COUNT(*) FROM [MyTable] WHERE [fkID] = '1';
I could load all of the rows and then find the Count with:
var owner = context.MyContainer.Where(t => t.ID == '1');
owner.MyTable.Load();
var count = owner.MyTable.Count();
But that is grossly inefficient. Is there a simpler way?
EDIT: Thanks, all. I've moved the DB from a private attached so I can run profiling; this helps but causes confusions I didn't expect.
And my real data is a bit deeper, I'll use Trucks carrying Pallets of Cases of Items -- and I don't want the Truck to leave unless there is at least one Item in it.
My attempts are shown below. The part I don't get is that CASE_2 never access the DB server (MSSQL).
var truck = context.Truck.FirstOrDefault(t => (t.ID == truckID));
if (truck == null)
return "Invalid Truck ID: " + truckID;
var dlist = from t in ve.Truck
where t.ID == truckID
select t.Driver;
if (dlist.Count() == 0)
return "No Driver for this Truck";
var plist = from t in ve.Truck where t.ID == truckID
from r in t.Pallet select r;
if (plist.Count() == 0)
return "No Pallets are in this Truck";
#if CASE_1
/// This works fine (using 'plist'):
var list1 = from r in plist
from c in r.Case
from i in c.Item
select i;
if (list1.Count() == 0)
return "No Items are in the Truck";
#endif
#if CASE_2
/// This never executes any SQL on the server.
var list2 = from r in truck.Pallet
from c in r.Case
from i in c.Item
select i;
bool ok = (list.Count() > 0);
if (!ok)
return "No Items are in the Truck";
#endif
#if CASE_3
/// Forced loading also works, as stated in the OP...
bool ok = false;
foreach (var pallet in truck.Pallet) {
pallet.Case.Load();
foreach (var kase in pallet.Case) {
kase.Item.Load();
var item = kase.Item.FirstOrDefault();
if (item != null) {
ok = true;
break;
}
}
if (ok) break;
}
if (!ok)
return "No Items are in the Truck";
#endif
And the SQL resulting from CASE_1 is piped through sp_executesql, but:
SELECT [Project1].[C1] AS [C1]
FROM ( SELECT cast(1 as bit) AS X ) AS [SingleRowTable1]
LEFT OUTER JOIN (SELECT
[GroupBy1].[A1] AS [C1]
FROM ( SELECT
COUNT(cast(1 as bit)) AS [A1]
FROM [dbo].[PalletTruckMap] AS [Extent1]
INNER JOIN [dbo].[PalletCaseMap] AS [Extent2] ON [Extent1].[PalletID] = [Extent2].[PalletID]
INNER JOIN [dbo].[Item] AS [Extent3] ON [Extent2].[CaseID] = [Extent3].[CaseID]
WHERE [Extent1].[TruckID] = '....'
) AS [GroupBy1] ) AS [Project1] ON 1 = 1
[I don't really have Trucks, Drivers, Pallets, Cases or Items; as you can see from the SQL the Truck-Pallet and Pallet-Case relationships are many-to-many -- although I don't think that matters. My real objects are intangibles and harder to describe, so I changed the names.]
Solution 1:
Query syntax:
var count = (from o in context.MyContainer
where o.ID == '1'
from t in o.MyTable
select t).Count();
Method syntax:
var count = context.MyContainer
.Where(o => o.ID == '1')
.SelectMany(o => o.MyTable)
.Count()
Both generate the same SQL query.
Solution 2:
I think you want something like
var count = context.MyTable.Count(t => t.MyContainer.ID == '1');
(edited to reflect comments)
Solution 3:
As I understand it, the selected answer still loads all of the related tests. According to this msdn blog, there is a better way.
http://blogs.msdn.com/b/adonet/archive/2011/01/31/using-dbcontext-in-ef-feature-ctp5-part-6-loading-related-entities.aspx
Specifically
using (var context = new UnicornsContext())
var princess = context.Princesses.Find(1);
// Count how many unicorns the princess owns
var unicornHaul = context.Entry(princess)
.Collection(p => p.Unicorns)
.Query()
.Count();
}
Solution 4:
This is my code:
IQueryable<AuctionRecord> records = db.AuctionRecord;
var count = records.Count();
Make sure the variable is defined as IQueryable then when you use Count() method, EF will execute something like
select count(*) from ...
Otherwise, if the records is defined as IEnumerable, the sql generated will query the entire table and count rows returned.