Develop Reference

Iterate over an IQueryable without calling ToList()

I have a DB used for a production line. It has an Orders table, and Ordertracker table, an Item table, and an Itemtracker table.
Both Orders and Items have many-to-many relationships with status. The tracker tables resolves these relationships in such a way that an item can have multiple entries in the tracker - each with a particular status.
I tried to upload a picture of the tables to make things clearer but alas, I don't have enough points yet :C
I need to find items whose last status in the Itemtracker table meets a condition, either '3' or '0'.
I then need to get the first one of these items.
The steps I am using to accomplish this are as follows:
Get all the Orders which have a certain status.
Get all the Items in that Order.
Get all the Items whose last status was = 0 or 3.
Get the first of these items.
My code is as follows:
public ITEM GetFirstItemFailedOrNotInProductionFromCurrentOrder()
{
var firstOrder = GetFirstOrderInProductionAndNotCompleted();
var items = ERPContext.ITEM.Where(i => i.OrderID == firstOrder.OrderID) as IQueryable<ITEM>;
if (CheckStatusOfItems(items) != null)
{
var nextItem = CheckStatusOfItems(items);
return nextItem ;
}
else
{
return null;
}
}
private ITEM CheckStatusOfItems(IQueryable<ITEM> items)
{
List<ITEM> listOfItemsToProduce = new List<ITEM>();
foreach (ITEM item in items.ToList())
{
var lastStatusOfItem = ERPContext.ITEMTRACKER.Where(it => it.ItemID == item.ItemID)
.OrderByDescending(it => it.ItemTrackerID).FirstOrDefault();
if (lastStatusOfItem.ItemStatus == (int)ItemStatus.Failed || lastStatusOfItem.ItemStatus == (int)ItemStatus.Confirmed)
{
listOfItemsToProduce.Add(item);
}
}
return listOfItemsToProduce.FirstOrDefault();
}
Now, this all works fine and returns what I need but I'm aware that this might not be the best approach. As it is now my IQueryable collection of items will never contain more than 6 items - but if it could grow larger, then calling ToList() on the IQueryable and iterating over the results in-memory would probably not be a good idea.
Is there a better way to iterate through the IQueryable items to fetch out the items that have a certain status as their latest status without calling ToList() and foreaching through the results?
Any advice would be much appreciated.

Using LINQ query syntax, you can build declaratively a single query pretty much the same way you wrote the imperative iteration. foreach translates to from, var to let and if to where:
private ITEM CheckStatusOfItems(IQueryable<ITEM> items)
{
var query =
from item in items
let lastStatusOfItem = ERPContext.ITEMTRACKER
.Where(it => it.ItemID == item.ItemID)
.OrderByDescending(it => it.ItemTrackerID)
.FirstOrDefault()
where (lastStatusOfItem.ItemStatus == (int)ItemStatus.Failed || lastStatusOfItem.ItemStatus == (int)ItemStatus.Confirmed)
select item;
return query.FirstOrDefault();
}
or alternatively using from instead of let and Take(1) instead of FirstOrDefault():
private ITEM CheckStatusOfItems(IQueryable<ITEM> items)
{
var query =
from item in items
from lastStatusOfItem in ERPContext.ITEMTRACKER
.Where(it => it.ItemID == item.ItemID)
.OrderByDescending(it => it.ItemTrackerID)
.Take(1)
where (lastStatusOfItem.ItemStatus == (int)ItemStatus.Failed || lastStatusOfItem.ItemStatus == (int)ItemStatus.Confirmed)
select item;
return query.FirstOrDefault();
}

Using LINQ's .Any() on a DataTable

I have a datatable loaded up with some records and I am then pulling a query from another file and want to check if the ID that I pull in this query exists in my datatable.
foreach (var item in records)
{
bool hasit = dt.AsEnumerable().Any(p => p.Field<string>(0) == item.ID);
if (!hasit)
{
//Logic
}
}
I'm using that .Any() function and expecting it to return true if there is an ID in the first field of the datatable that matches the id in the records collection. It continually returns false though, am I missing something? Is there a better way to do this?

I'm using that .Any() function and expecting it to return true if there is an ID in the first field of the datatable that matches the id in the records collection. It continually returns false
When one uses == it compares object references. I recommend you instead use Equals which will just compare the values. Hence change your statement to
dt.AsEnumerable().Any(p => p.Field<string>(0).Equals(item.ID))
Which will achieve what you are expecting.

The method
.Any(p => p.Field(0) == item.ID)
will return true IF ANY element is found. Your posted code specifies that the next thing you do is ask
if (!hasit)
{
//Logic
}
which means if(NOT any has it)... which is producing the incorrect behavior. Change it to:
if (hasit)
{
//Logic
}
and you'll get the desired results.
Edit: kudos to Cuong Le for the observation.

I would try breaking it up to see if I could find the error:
foreach (var item in records)
{
var enumer = dt.AsEnumerable(); // <-- Check this to make sure it has elements
var filtered = enumer.Any(p => p.Field<string>(0) == item.ID); // <- Check to make sure it has elements
}

Poorly performing query needs rewriting

I have a piece of code that's performing badly, and need to rewite it to introduce a proper where clause before starting the .ToList however, that's where I'm getting stuck.
Currently the code looks lke this (roughly, I've taken some of the search criteria out to make it easier to display)
var Widgets = from b in _caspEntities.Widgets.Include("WidgetRegionLogs")
.Include("WidgetStatusLogs").Include("WidgetVoltageTests")
select b;
IEnumerable<Widget> results = Widgets.ToList();
if (comboBoxRegion.SelectedValue.ToString() != "0")
{
results = from b in results
where b.CurrentRegionLog != null && b.CurrentRegionLog.RegionId == int.Parse(comboBoxRegion.SelectedValue.ToString())
select b;
}
if (comboBoxStatus.SelectedValue != null)
{
results = from b in results
where b.CurrentStatusLog != null && b.CurrentStatusLog.StatusId == comboBoxStatus.SelectedValue.ToString()
select b;
}
if (txtCode.Text.Trim().Length > 0)
{
results = from b in results
where b.CodeNumber == txtCode.Text.Trim()
select b;
}
dataGridViewWidget.DataSource = results.ToList();
I can write the SQL easily enough, essentially the model is simple, I have a Widget it has a RegionLog and a StatusLog, both of which store a history. The current region and status are retrieved from this by grouping by WidgetID and selecting the most recent Date Updated (and then going off to Region and Status tables to get the actual value).
So, I need to translate this into LINQ, but to be honest I don't have a clue but am ken and willing to learn. In my head, I think I need to add some better where clauses, and then do the Widget.toList after I have applied the where clauses. I'm struggling with the CurrentRegionLog and CurrentStatusLog concepts as they are not populated until I run the IEnumerable.
If anyone can give some pointers, I'd be grateful,
Thanks
Edit - Added
public BatteryRegionLog CurrentRegionLog
{
get { return _currentRegionLog; }
}
private BatteryRegionLog _currentRegionLog
{
get
{
if (this.BatteryRegionLogs.Count > 0)
{
BatteryRegionLog log = this.BatteryRegionLogs.OrderByDescending(item => item.LastModifiedDate).First();
return log;
}
else
{
return null;
}
}
}

You can compose the query like this:
if (comboBoxRegion.SelectedValue.ToString() != "0")
{
var id = int.Parse(comboBoxRegion.SelectedValue.ToString()
Widgets = from b in Widgets
let currentRegionLog =
b.BatteryRegionLogs
.OrderByDescending(item => item.LastModifiedDate)
.FirstOrDefault()
where currentRegionLog.RegionId == id)
select b;
}
... // Same for the other criteria.
dataGridViewWidget.DataSource = Widgets.ToList();
The whole query is not executed before you do ToList(). As everything is translated to SQL you don't need the null check b.CurrentRegionLog != null. SQL will evaluate b.CurrentRegionLog.RegionId == id just fine when there is no CurrentRegionLog.
Edit
Since CurrentRegionLog is a calculated property of your Widget class it cannot be translated into SQL. I made an effort to incorporate the code of calculated property into the query in a way that only the basic navigation property is used, so EF can translate it to SQL again.

try remove this line:
IEnumerable<Widget> results = Widgets.ToList();
and just use the Widgets variable you get in at the top
The .ToList() goes to the database and materialiaze all the data into entities.
if you don't call the .ToList() the query is still "open" for a where clause

EF - Can not delete an object

Why does this code not work? It inserts an object but does not delete it
public int Toggle(RequestArchive RequestArchiveObj)
{
var ra = DataContext.RequestArchives.Where(rec => rec.UserId == RequestArchiveObj.UserId && rec.RequestId == RequestArchiveObj.RequestId);
if(ra.Count() > 0)
{
foreach (var item in ra)
{
DataContext.DeleteObject(item);
}
}
else
{
DataContext.AddToRequestArchives(RequestArchiveObj);
}
DataContext.SaveChanges();
return RequestArchiveObj.Id;
}

There's a potentially dangerous issue with your code and your problem might relate to that:
If you loop through your query object (the object returned by DataContext.RequestArchives.Where()) without executing it, you're going to have a roundtrip to the database for every single item in the loop. This is called the N+1 selects problem.
You can mitigate this using the ToList() method:
var ra = DataContext.RequestArchives
.Where(rec =>
rec.UserId == RequestArchiveObj.UserId &&
rec.RequestId == RequestArchiveObj.RequestId)
.ToList(); // this executes the query
// ...
foreach (var item in ra) // without ToList() this will query every item by itself
{
DataContext.DeleteObject(item); // and this might collide with the query
}
I'm not sure about this, but maybe the deletion problem occurs because you try to delete objects while still querying them through the foreach loop. If that is the case, it should work once you're using ToList() as recommended above.

linq statuses must return distinct

I have a DB table that looks similar to this.
ID | Status | Type
etc...etc
I am using linq to try and discern distinct Statuses from this collection like so
results = ctx.Status.Distinct(new StatusComparer()).ToList();
but this returns all statuses, I used the following Link to construct the Comparer below, I found that suggestion in another StackOverflow Post
public class StatusComparer : IEqualityComparer<Status>
{
public bool Equals(Status x, Status y)
{
// Check whether the compared objects reference the same data.
if (ReferenceEquals(x, y))
{
return true;
}
// Check whether any of the compared objects is null.
if (ReferenceEquals(x, null) || ReferenceEquals(y, null))
{
return false;
}
// Check whether the status' properties are equal.
return x.StatusDescription == y.StatusDescription && x.Type == y.Type && x.StatusID == y.StatusID;
}
public int GetHashCode(Status status)
{
// Get hash code for the Name field if it is not null.
var hashStatusId = status.StatusID.GetHashCode();
// Get hash code for the Code field.
var hashStatusDescription = status.StatusDescription.GetHashCode();
var hashStatusType = status.Type.GetHashCode();
// Calculate the hash code for the product.
return hashStatusId ^ hashStatusDescription ^ hashStatusType;
}
}
}
My problem is as follows early on we had a system that worked fine, so well in fact they wanted another system using the same Database so we plumbed it in. The search has an advanced options with several filters one of them being Status but as you can see from the above (loose) DB structure statuses have different types but similar text. I need to be able to select via Linq the whole status by the distinct text. all help would be greatly appreciated.
have also tried
results = (from s in context.Status group s by s.StatusDescription into g select g.First()).ToList();
this also failed with a System.NotSupportedException

To select all distinct statuses:
ctx.Status.Select(s => new { s.StatusDescription, s.Type }).Distinct();