Well, im doing a linq query to get a list of results with the same column, and then i need to replace that column value with a new one.
First Code:
var db = GetContext();
var result = from f in GetContext().ProjectStateHistories
where f.ProjectId.Equals(oldProjectId)
select f;
foreach (var item in result)
{
var projectStateHistoryUpdate = db.ProjectStateHistories.Find(item.Id);
projectStateHistoryUpdate.ProjectId = newProjectId;
db.Entry(projectStateHistoryUpdate).State = EntityState.Modified;
}
db.SaveChanges();
I searched for some answers, and i found that i can use Select, and make a new object (Linq replace null/empty value with another value)
Second Code:
var result = (from f in GetContext().ProjectStateHistories
where f.ProjectId.Equals(oldProjectId)
select f).Select(d=> new { Id = d.Id, EventName = d.EventName, LogUser = d.LogUser, ProjectId = newProjectId, TimeStamp = d.TimeStamp });
And even, Third Code:
var db = GetContext();
var result = (from f in db.ProjectStateHistories
where f.ProjectId.Equals(oldProjectId)
select f).Select(d=> new { ProjectId = newProjectId});
But only the First Code works.
I wanted to ask what i am doing wrong, since i think it is better to change the value with a query, instead of using a foreach.
See code below:
var db = GetContext();
(from f in db.ProjectStateHistories
where f.ProjectId.Equals(oldProjectId)
select f)
.ToList()
.ForEach(i => i.ProjectId = newProjectId);
db.SaveChanges();
Alternatively:
var db = GetContext();
db.ProjectStateHistories
.Where(f => f.ProjectId.Equals(oldProjectId))
.ToList()
.ForEach(f => f.ProjectId = newProjectId);
db.SaveChanges();
The shortest way I know of to replace your code is this:
var db = getcontext();
db.ProjectStateHistories
.Where(f => f.ProjectId.Equals(oldProjectId))
.ToList()
.ForEach(f => f.ProjectId = newProjectId);
db.SaveChanges();
Other answers can be found here
I've just had a thought that could help you, I am just free coding here!
If you just put the for each as part of the select, and then save your changes will that work?
foreach (var source in db.ProjectStateHistories.Where(x => x.ProjectId == oldProjectId))
{
source.ProjectId= newProjectId;
db.Entry(source).State = EntityState.Modified;
}
db.SaveChanges();
I think this is a more efficient way of doing it.
Also the .Select() method is only really useful if you need to Project to a view Model, it won't change the variables in the database, just show them in the newly declared object.
Thanks,
Phill
Related
Is any way to select all fields in a query but modifying one field like this?
var notes = from n in myContext.Notes
select new
{
... // all fiedls
date = n.date.ToString("MM/YYYY") // but one field edited
}
this is the query I want but less verbose when I have several properties.
var note = await _dbContext.ClientChartNotes
.Select(s => new
{
s.ClientChartNoteId,
s.ClientId,
s.ChartNoteType,
s.Title,
s.Note,
ChartNoteDate = s.ChartNoteDate.ToString("MM/dd/yyyy")
})
.FirstOrDefaultAsync(s => s.ClientChartNoteId.Equals(id));
Lambda preferred.
Thanks
Edit: to include original query.
I recommend selecting the Note itself and the additional field.
var notes = from n in myContext.Notes
select new
{
Note= n
NewDate = n.date.ToString("MM/YYYY")
}
So your notes will have all the Original Note and the additional properties you added in the result.
Your query using lambda:
var fetchedNote = await myDbContext.Notes // get the collection of all Notes
.Where(note => note.ClientChartNoteId == Id) // take only those notes that ...
.Select(note => new // from every remaining note, make one new object
{ // with only the properties you plan to use
Title = note.Title, // some are original values
...
Date = note.Data.ToString(...), // some are calculated values
})
.FirstOrDefaultAsync();
I am trying to link up the RestaurantId in the RestaurantReservationEventsTbl with the RestaurantID in the RestaurantTbl to display reservations that are only made for the currently logged in restaurant.
I am receiving the following error in my code operator == cannot be applied to operands of type int and iqueryable int
Here is what I am doing in my home controller
var RestaurantIDRestaurantTbl = from r in db.Restaurants select r.RestaurantID;
//var listOfRestaurantsReservations = db.RestaurantReservationEvents.ToList();
var listOfRestaurantsReservations = db.RestaurantReservationEvents.Where(x => x.RestaurantID == RestaurantIDRestaurantTbl).ToList();
//return View(restaurants.Where(x => x.RestaurantEmailAddress == UserEmail).ToList());
//create partial view called _RestaurantReservation
return PartialView("_RestaurantReservations", listOfRestaurantsReservations);
You have to change your code to materialize the restaurantIds like this:
var RestaurantIDRestaurantTbl = (from r in db.Restaurants
select r.RestaurantID).ToList();
Then you may change the code as below for the comparison to work:
var listOfRestaurantsReservations = db.RestaurantReservationEvents.Where(x => RestaurantIDRestaurantTbl.Contains(x.RestaurantID)).ToList();
Anyway this is not the best solution. I will write another example for you, just try this example if it is working or not and let me know for the result.
I would considering changing the code as below to be much more efficient:
var listOfRestaurantsReservations = (from r in db.Restaurants
join e in db.RestaurantReservationEvents
on r.RestaurantID equals e.RestaurantID
//where r.RestaurantID == something //if where condition needed
select e).ToList();
If your tables are not connected with foreignkeys please consider to read this documentation here to make a better structure of the tables since they are related to each-other.
If your tables are related as in documentation article you might have something like that:
var RestaurantIDRestaurantTbl = db.Restaurants.SingleOrDefault(x => x.RestaurantID == something);
if(RestaurantIDRestaurantTbl != null)
{
var listOfRestaurantsReservations = RestaurantIDRestaurantTbl.RestaurantReservationEvents.ToList();
}
{
// This will give you a list of IDs
var RestaurantIDRestaurantTbl = db.Restaurants
.Select(p => p.RestaurantID)
.ToList();
// Using .Any() is a better choice instead of .Contains()
// .Contains is used to check if a list contains an item while .Any will look for an item in a list with a specific ID
var listOfRestaurantsReservations = db.RestaurantReservationEvents
.Where(p => RestaurantIDRestaurantTbl.Any(r => r.pRestaurantID == p))
.ToList();
}
I have a query that looks like this:
var caseList = (from x in context.Cases
where allowedCaseIds.Contains(x => x.CaseId)
select new Case {
CaseId = x.CaseId,
NotifierId = x.NotifierId,
Notifier = x.NotifierId.HasValue ? new Notifier { Name = x.Notifier.Name } : null // This line throws exception
}).ToList();
A Case class can have 0..1 Notifier
The query above will result in the following System.NotSupportedException:
Unable to create a null constant value of type 'Models.Notifier'. Only entity types, enumeration types or primitive types are supported
in this context.
At the moment the only workaround I found is to loop the query result afterwards and manually populate Notifierlike this:
foreach (var c in caseList.Where(x => x.NotifierId.HasValue)
{
c.Notifier = (from x in context.Notifiers
where x.CaseId == c.CaseId
select new Notifier {
Name = x.Name
}).FirstOrDefault();
}
But I really don't want to do this because in my actual scenario it would generate hundreds of additional queries.
Is there any possible solution for a situation like this?.
I think you need to do that in two steps. First you can fetch only the data what you need with an anonymous type in a single query:
var caseList = (from x in context.Cases
where allowedCaseIds.Contains(x => x.CaseId)
select new {
CaseId = x.CaseId,
NotifierId = x.NotifierId,
NotifierName = x.Notifier.Name
}).ToList();
After that, you can work in memory:
List<Case> cases = new List<Case>();
foreach (var c in caseList)
{
var case = new Case();
case.CaseId = c.CaseId;
case.NotifierId = c.NotifierId;
case.NotifierName = c.NotifierId.HasValue ? c.NotifierName : null;
cases.Add(case);
}
You could try writing your query as a chain of function calls rather than a query expression, then put an .AsEnumerable() in between:
var caseList = context.Clases
.Where(x => allowedCaseIds.Contains(x.CaseId))
.AsEnumerable() // Switch context
.Select(x => new Case() {
CaseId = x.CaseId,
NotifierId = x.NotifierId,
Notifier = x.NotifierId.HasValue
? new Notifier() { Name = x.Notifier.Name }
: null
})
.ToList();
This will cause EF to generate an SQL query only up to the point where you put the .AsEnumerable(), further down the road, LINQ to Objects will do all the work. This has the advantage that you can use code that cannot be translated to SQL and should not require a lot of changes to your existing code base (unless you're using a lot of let expressions...)
Here's my code:
var myStrings = (from x in db1.MyStrings.Where(x => homeStrings.Contains(x.Content))
join y in db2.MyStaticStringTranslations on x.Id equals y.id
select new MyStringModel()
{
Id = x.Id,
Original = x.Content,
Translation = y.translation
}).ToList();
And I get the error that the specified LINQ expression contains references to queries that are associated with different contexts. I know that the problem is that I try to access tables from both db1 and db2, but how do I fix this?
MyStrings is a small table
Load filtered MyStrings in memory, then join with MyStaticStringTranslations using LINQ:
// Read the small table into memory, and make a dictionary from it.
// The last step will use this dictionary for joining.
var byId = db1.MyStrings
.Where(x => homeStrings.Contains(x.Content))
.ToDictionary(s => s.Id);
// Extract the keys. We will need them to filter the big table
var ids = byId.Keys.ToList();
// Bring in only the relevant records
var myStrings = db2.MyStaticStringTranslations
.Where(y => ids.Contains(y.id))
.AsEnumerable() // Make sure the joining is done in memory
.Select(y => new {
Id = y.id
// Use y.id to look up the content from the dictionary
, Original = byId[y.id].Content
, Translation = y.translation
});
You are right that db1 and db2 can't be used in the same Linq expression. x and y have to be joined in this process and not by a Linq provider. Try this:
var x = db1.MyStrings.Where(xx => homeStrings.Contains(xx.Content)).ToEnumerable();
var y = db2.MyStaticStringTranslations.ToEnumerable();
var myStrings = (from a in x
join b in y on x.Id equals y.id
select new MyStringModel()
{
Id = x.Id,
Original = x.Content,
Translation = y.translation
}).ToList();
Refer to this answer for more details: The specified LINQ expression contains references to queries that are associated with different contexts
dasblinkenlight's answer has a better overall approach than this. In this answer I'm trying to minimize the diff against your original code.
I also faced the same problem:
"The specified LINQ expression contains references to queries that are associated with different contexts."
This is because it's not able to connect to two context at a time so i find the solution as below.
Here in this example I want to list the lottery cards with the owner name but the Table having the owner name is in another Database.So I made two context DB1Context and DB2Context.and write the code as follows:
var query = from lc in db1.LotteryCardMaster
from om in db2.OwnerMaster
where lc.IsActive == 1
select new
{
lc.CashCardID,
lc.CashCardNO,
om.PersonnelName,
lc.Status
};
AB.LottryList = new List<LotteryCardMaster>();
foreach (var result in query)
{
AB.LottryList.Add(new LotteryCardMaster()
{
CashCardID = result.CashCardID,
CashCardNO = result.CashCardNO,
PersonnelName =result.PersonnelName,
Status = result.Status
});
}
but this gives me the above error so i found the other way to perform joining on two tables from diffrent database.and that way is as below.
var query = from lc in db1.LotteryCardMaster
where lc.IsActive == 1
select new
{
lc.CashCardID,
lc.CashCardNO,
om.PersonnelName,
lc.Status
};
AB.LottryList = new List<LotteryCardMaster>();
foreach (var result in query)
{
AB.LottryList.Add(new LotteryCardMaster()
{
CashCardID = result.CashCardID,
CashCardNO = result.CashCardNO,
PersonnelName =db2.OwnerMaster.FirstOrDefault(x=>x.OwnerID== result.OwnerID).OwnerName,
Status = result.Status
});
}
I have a database table that contains an nvarchar column like this:
1|12.6|18|19
I have a Business Object that has a Decimal[] property.
My LINQ Query looks like this:
var temp = from r in db.SomeTable select new BusinessObject {
// Other BusinessObject Properties snipped as they are straight 1:1
MeterValues = r.MeterValues.Split('|').Select(Decimal.Parse).ToArray()
};
var result = temp.ToArray();
This throws an NotSupportedException: Method 'System.String[] Split(Char[])' has no supported translation to SQL.
That kinda sucks :) Is there any way I can do this without having to add a string property to the business object or selecting an anonymous type and then iterating through it?
My current "solution" is:
var temp = from r in db.SomeTable select new {
mv = r.MeterValues,
bo = new BusinessObject { // all the other fields }
};
var result = new List<BusinessObject>();
foreach(var t in temp) {
var bo = t.bo;
bo.MeterValues = t.mv.Split('|').Select(Decimal.Parse).ToArray();
result.Add(bo);
}
return result.ToArray(); // The Method returns BusinessObject[]
That's kinda ugly though, with that temporary list.
I've tried adding a let mv = r.MeterValues.Split('|').Select(Decimal.Parse).ToArray() but that essentially leads to the same NotSupportedException.
This is .net 3.5SP1 if that matters.
You need to force the select clause to run on the client by calling .AsEnumerable() first:
var result = db.SomeTable.AsEnumerable().Select(r => new BusinessObject {
...
MeterValues = r.MeterValues.Split('|').Select(Decimal.Parse).ToArray()
}).ToList();
You can't use split, but in this scenario you can do the following:
// Database value is 1|12.6|18|19
string valueToFind = "19";
var temp = from r in db.SomeTable.Where(r => ("|" + r.MeterValues + "|").Contains("|" + valueToFind + "|"));
This code adds outer pipes (|) to the database value on the fly inside the query so you can do start, middle, and end value matches on the string.
For example, the above code looks for "|19|" inside "|1|12.6|18|19|", which is found and valid. This will work for any other valueToFind.
You don't need to use a temporary list:
var query = from r in db.SomeTable
select new
{
r.Id,
r.Name,
r.MeterValues,
...
};
var temp = from x in query.AsEnumerable()
select new BusinessObject
{
Id = x.Id,
Name = x.Name,
MeterValues = x.mv.Split('|').Select(Decimal.Parse).ToArray(),
...
};
return temp.ToArray();
Unfortunately its the IQueryable you are using (Linq to SQL) that is not supporting the Split function.
You are really only left with the IEnumerable (Linq to Objects) support for it in this case. You second code snippet is what you need to do, or something like...
var temp = (from r in db.SomeTable select new {
mv = r.MeterValues,
bo = new BusinessObject { // all the other fields }
}).AsEnumerable().Select(blah, blah) ;