Hopefully this question is not to confusing. Basically I'm looking for pointers on how to OrderByDecending with a date from relational tables. I have constructed a basic method that looks like it could possibly work but I'm getting errors:
DbSortClause expressions must have a type that is order comparable.
Parameter name: key
I understand what this is saying but I'm not entirely sure how to fix using Linq method syntax.
public BusinessEntities.Application GetLastUpdatedAppliction(int userID)
{
return context.tbl_User_To_Application
.Where(x => x.UserID == userID)
.OrderByDescending
(o => o.tbl_Application.tbl_ApplicationChanges
.Where(oo => oo.ApplicationID == o.ApplicationID)
.Select(s => s.ChangeDate))
.ThenByDescending(t => t.DateAdded)
.Select(y => new BusinessEntities.Application
{
ApplicationID = y.tbl_Application.ApplicationID,
ApplicationName = y.tbl_Application.ApplicationName
}).FirstOrDefault();
}
Basically I have a cross reference table that binds a user to a specific application(Website) Then inside I need to nest into two tables to get the latest changes to the Application with a "ChangesDate". So ideally this would return the last updated application. Then obviously populates my DTO.
I'm still trying to get to grips with Linq method syntax so any help would be greatly appreciated!
Regards,
Tez Wingfield
If you want to order by the last application change date:
(...)
.OrderByDescending(o =>
o.tbl_Application.tbl_ApplicationChanges
.Where(ac => ac.ApplicationID == o.ApplicationID)
.OrderByDescending(ac => ac.ChangeDate)
.First()
.Select(ac => ac.ChangeDate)
)
(...)
Related
var reassignForms = _formQueryRepository.WithRelatedEntities()
.Where(x => x.Id == formId && x.SchoolId == schoolId)
.Select(y => y.FormUsers.Where(m => m.CanEdit).Select(z => new ReassignFormDTO
{
Name = z.UserProfile.FullName,
Id = z.UserProfile.Id,
Email = z.UserProfile.Email,
Role = z.UserProfile.UserRole,
}));
I want to convert it to ReassignFormDTO but somehow it is returning an object which I cannot convert. I want this to be direct object not [0].objects.
You can use .FirstOrDefault() then it will return the first entity from the linq query. According to the debug window you have two ReassignFormDTO objects and seems like you would like to get only one.
Please provide more detail, so we can help more. Not sure if my previous answer will help you.
EDIT: I didn't noticed the second select inside you statement. According to the comments .SelectMany(z => z) should help you out.
I'm lost when it comes to to QueryOver in NHibernate, I'm trying to query over a database and retrive 4 values of importans, the rest are unnecessary and take up processing power.
I'm trying this:
var ext = _session.QueryOver<ExternServiceSettings>()
.Where(x => x.ExternService == ExternServiceEnum.Outlook).List();
which works fine but takes too long and returns everything in the database. then I tried:
var ext = _session.QueryOver<ExternServiceSettings>()
.Where(x => x.ExternService == ExternServiceEnum.Outlook)
.List<ExternServiceSettings>()
.Select(y => y.UserName);
However this only return the username and won't let me fetch more than one value...
All help is appreciated!
We should use .SelectList()
Check the example from doc:
var selection =
session.QueryOver<Cat>()
.SelectList(list => list
.Select(c => c.Name)
.SelectAvg(c => c.Age))
.List<object[]>();
see more here:
16.7. Projections
In this query:
public static IEnumerable<IServerOnlineCharacter> GetUpdated()
{
var context = DataContext.GetDataContext();
return context.ServerOnlineCharacters
.OrderBy(p => p.ServerStatus.ServerDateTime)
.GroupBy(p => p.RawName)
.Select(p => p.Last());
}
I had to switch it to this for it to work
public static IEnumerable<IServerOnlineCharacter> GetUpdated()
{
var context = DataContext.GetDataContext();
return context.ServerOnlineCharacters
.OrderByDescending(p => p.ServerStatus.ServerDateTime)
.GroupBy(p => p.RawName)
.Select(p => p.FirstOrDefault());
}
I couldn't even use p.First(), to mirror the first query.
Why are there such basic limitations in what's otherwise such a robust ORM system?
That limitation comes down to the fact that eventually it has to translate that query to SQL and SQL has a SELECT TOP (in T-SQL) but not a SELECT BOTTOM (no such thing).
There is an easy way around it though, just order descending and then do a First(), which is what you did.
EDIT:
Other providers will possibly have different implementations of SELECT TOP 1, on Oracle it would probably be something more like WHERE ROWNUM = 1
EDIT:
Another less efficient alternative - I DO NOT recommend this! - is to call .ToList() on your data before .Last(), which will immediately execute the LINQ To Entities Expression that has been built up to that point, and then your .Last() will work, because at that point the .Last() is effectively executed in the context of a LINQ to Objects Expression instead. (And as you pointed out, it could bring back thousands of records and waste loads of CPU materialising objects that will never get used)
Again, I would not recommend doing this second, but it does help illustrate the difference between where and when the LINQ expression is executed.
Instead of Last(), Try this:
model.OrderByDescending(o => o.Id).FirstOrDefault();
Replace Last() by a Linq selector OrderByDescending(x => x.ID).Take(1).Single()
Something like that would be works if you prefert do it in Linq :
public static IEnumerable<IServerOnlineCharacter> GetUpdated()
{
var context = DataContext.GetDataContext();
return context.ServerOnlineCharacters.OrderBy(p => p.ServerStatus.ServerDateTime).GroupBy(p => p.RawName).Select(p => p.OrderByDescending(x => x.Id).Take(1).Single());
}
Yet another way get last element without OrderByDescending and load all entities:
dbSet
.Where(f => f.Id == dbSet.Max(f2 => f2.Id))
.FirstOrDefault();
That's because LINQ to Entities (and databases in general) does not support all the LINQ methods (see here for details: http://msdn.microsoft.com/en-us/library/bb738550.aspx)
What you need here is to order your data in such a way that the "last" record becomes "first" and then you can use FirstOrDefault. Note that databasese usually don't have such concepts as "first" and "last", it's not like the most recently inserted record will be "last" in the table.
This method can solve your problem
db.databaseTable.OrderByDescending(obj => obj.Id).FirstOrDefault();
Adding a single function AsEnumerable() before Select function worked for me.
Example:
return context.ServerOnlineCharacters
.OrderByDescending(p => p.ServerStatus.ServerDateTime)
.GroupBy(p => p.RawName).AsEnumerable()
.Select(p => p.FirstOrDefault());
Ref:
https://www.codeproject.com/Questions/1005274/LINQ-to-Entities-does-not-recognize-the-method-Sys
In this query:
public static IEnumerable<IServerOnlineCharacter> GetUpdated()
{
var context = DataContext.GetDataContext();
return context.ServerOnlineCharacters
.OrderBy(p => p.ServerStatus.ServerDateTime)
.GroupBy(p => p.RawName)
.Select(p => p.Last());
}
I had to switch it to this for it to work
public static IEnumerable<IServerOnlineCharacter> GetUpdated()
{
var context = DataContext.GetDataContext();
return context.ServerOnlineCharacters
.OrderByDescending(p => p.ServerStatus.ServerDateTime)
.GroupBy(p => p.RawName)
.Select(p => p.FirstOrDefault());
}
I couldn't even use p.First(), to mirror the first query.
Why are there such basic limitations in what's otherwise such a robust ORM system?
That limitation comes down to the fact that eventually it has to translate that query to SQL and SQL has a SELECT TOP (in T-SQL) but not a SELECT BOTTOM (no such thing).
There is an easy way around it though, just order descending and then do a First(), which is what you did.
EDIT:
Other providers will possibly have different implementations of SELECT TOP 1, on Oracle it would probably be something more like WHERE ROWNUM = 1
EDIT:
Another less efficient alternative - I DO NOT recommend this! - is to call .ToList() on your data before .Last(), which will immediately execute the LINQ To Entities Expression that has been built up to that point, and then your .Last() will work, because at that point the .Last() is effectively executed in the context of a LINQ to Objects Expression instead. (And as you pointed out, it could bring back thousands of records and waste loads of CPU materialising objects that will never get used)
Again, I would not recommend doing this second, but it does help illustrate the difference between where and when the LINQ expression is executed.
Instead of Last(), Try this:
model.OrderByDescending(o => o.Id).FirstOrDefault();
Replace Last() by a Linq selector OrderByDescending(x => x.ID).Take(1).Single()
Something like that would be works if you prefert do it in Linq :
public static IEnumerable<IServerOnlineCharacter> GetUpdated()
{
var context = DataContext.GetDataContext();
return context.ServerOnlineCharacters.OrderBy(p => p.ServerStatus.ServerDateTime).GroupBy(p => p.RawName).Select(p => p.OrderByDescending(x => x.Id).Take(1).Single());
}
Yet another way get last element without OrderByDescending and load all entities:
dbSet
.Where(f => f.Id == dbSet.Max(f2 => f2.Id))
.FirstOrDefault();
That's because LINQ to Entities (and databases in general) does not support all the LINQ methods (see here for details: http://msdn.microsoft.com/en-us/library/bb738550.aspx)
What you need here is to order your data in such a way that the "last" record becomes "first" and then you can use FirstOrDefault. Note that databasese usually don't have such concepts as "first" and "last", it's not like the most recently inserted record will be "last" in the table.
This method can solve your problem
db.databaseTable.OrderByDescending(obj => obj.Id).FirstOrDefault();
Adding a single function AsEnumerable() before Select function worked for me.
Example:
return context.ServerOnlineCharacters
.OrderByDescending(p => p.ServerStatus.ServerDateTime)
.GroupBy(p => p.RawName).AsEnumerable()
.Select(p => p.FirstOrDefault());
Ref:
https://www.codeproject.com/Questions/1005274/LINQ-to-Entities-does-not-recognize-the-method-Sys
I have a table and one of the properties of the table is TotalDue.I wish to first order it by TotalDue and then select the "top" record which in this case would be the record with the highest value.
homeVM.LastSaleAmount = (from i in salesService.GetSalesOrderHeaders()
.OrderByDescending(a => a.TotalDue).First();
This is what I've tried so far but I think .First() needs a parameter and I think I need a select as well but not really sure.
You can try with Take method, is like top, but in Linq world.
homeVM.LastSaleAmount = salesService.GetSalesOrderHeaders().OrderByDescending(a => a.TotalDue).Take(1);
https://msdn.microsoft.com/en-us/library/vstudio/bb503062%28v=vs.100%29.aspx
You're mixing method syntax and query syntax, and your use of query syntax isn't necessary and making this harder. Just remove it:
homeVM.LastSaleAmount = salesService.GetSalesOrderHeaders()
.OrderByDescending(a => a.TotalDue)
.Select(a => a.TotalDue)
.First();
You are trying to put an entire entity into LastSaleAmount. use .Select(a => a.TotalDue) like:
homeVM.LastSaleAmount = salesService.GetSalesOrderHeaders()
.OrderByDescending(a => a.TotalDue).Select(a => a.TotalDue).First();