I am using Linq to Sql with Predicate Builder and am trying to optimize how much information is retrieved from the database. I would like to select only certain fields to display them in a gridview. When I select only what I want, the search parameters I add (see below) don't work, and neither does PredicateBuilder. Here's what I'm currently doing (that works, but gets EVERYTHING which is way too much info)
' Initial Setup '
Dim db As New MyDataContext()
Dim results = From p In db.Products _
Select p
' Search '
If (testCase) Then
results = results.Where(Function(p) p.SomeAttribute = 123)
End If
If I change that to only select what I need, like this:
Dim results = From p In db.Products _
Select p.Name, p.SomethingElse
then I've noticed if the information is selected (ie I select p.SomeAttribute) then I can search (add the where clause) on that attribute, but if its not, I can't. And with predicate builder it only works if I select the entire item (ie select p). All this should be doing is creating SQL statements which don't have to select the attribute to search by it. How can I get this to work and select only what I need, but search by anything and keep prediate builder working? Any help MUCH APPRECIATED! Thanks
You could try to initially do a "select p" at the beginning, then add all your where clauses, and at the very end, select just what you need from it.
' Initial Setup '
Dim db As New MyDataContext()
Dim results = From p In db.Products _
Select p
' Search '
If (testCase) Then
results = results.Where(Function(p) p.SomeAttribute = 123)
End If
' trim down the columns after you've added the wheres...
Dim results2 = from p in results
Select p.Name, p.SomethingElse
You can't modify the "select list" (this is how I understood your question. I might have misunderstood it) with predicate builder (which builds boolean expressions). You should manually use stuff in System.Linq.Expressions namespace to do that but I suggest using Dynamic LINQ instead.
Sounds like you are doing the where on the projection and not the original Product. Do the projection Select p.Name, p.SomethingElse at the end after all the search criteria has been applied.
Related
i am bit confused by the nature and working of query , I tried to access database which contains each name more than once having same EMPid so when i accessed it in my DROP DOWN LIST then same repetition was in there too so i tried to remove repetition by putting DISTINCT in query but that didn't work but later i modified it another way and that worked but WHY THAT WORKED, I DON'T UNDERSTAND ?
QUERY THAT DIDN'T WORK
var names = (from n in DataContext.EmployeeAtds select n).Distinct();
QUERY THAT WORKED of which i don't know how ?
var names = (from n in DataContext.EmployeeAtds select new {n.EmplID, n.EmplName}).Distinct();
why 2nd worked exactly like i wanted (picking each name 1 time)
i'm using mvc 3 and linq to sql and i am newbie.
Both queries are different. I am explaining you both query in SQL that will help you in understanding both queries.
Your first query is:
var names = (from n in DataContext.EmployeeAtds select n).Distinct();
SQL:-
SELECT DISTINCT [t0].[EmplID], [t0].[EmplName], [t0].[Dept]
FROM [EmployeeAtd] AS [t0]
Your second query is:
(from n in EmployeeAtds select new {n.EmplID, n.EmplName}).Distinct()
SQL:-
SELECT DISTINCT [t0].[EmplID], [t0].[EmplName] FROM [EmployeeAtd] AS
[t0]
Now you can see SQL query for both queries. First query is showing that you are implementing Distinct on all columns of table but in second query you are implementing distinct only on required columns so it is giving you desired result.
As per Scott Allen's Explanation
var names = (from n in DataContext.EmployeeAtds select n).Distinct();
The docs for Distinct are clear – the method uses the default equality comparer to test for equality, and the default comparer sees 4 distinct object references. One way to get around this would be to use the overloaded version of Distinct that accepts a custom IEqualityComparer.
var names = (from n in DataContext.EmployeeAtds select new {n.EmplID, n.EmplName}).Distinct();
Turns out the C# compiler overrides Equals and GetHashCode for anonymous types. The implementation of the two overridden methods uses all the public properties on the type to compute an object's hash code and test for equality. If two objects of the same anonymous type have all the same values for their properties – the objects are equal. This is a safe strategy since anonymously typed objects are essentially immutable (all the properties are read-only).
Try this:
var names = DataContext.EmployeeAtds.Select(x => x.EmplName).Distinct().ToList();
Update:
var names = DataContext.EmployeeAtds
.GroupBy(x => x.EmplID)
.Select(g => new { EmplID = g.Key, EmplName = g.FirstOrDefault().EmplName })
.ToList();
I am having a bit of a difficult time figuring out how accomplish a task outlined in my question header.
Basically, I have a list of 'News' objects defined as:
Dim news_list As List(Of News) = myNamespcae.News.ListNews()
Depending on a condition, I have another 'News' object list as;
Dim news_headlines As List(Of News) = myNamespace.News.getHeadlines()
Then, I have 'spots' again as List(of News) as;
Dim spots = (From n In news_list Take (10) Select n)
I am trying to accomplish;
if news_headlines is not empty,if any news_headlines News object exists in spots, remove it from spots. return filtered spots.
Any guidance will be appreciated..
Thanks.
If you want to pick ten from Spots and then filter out the ones in news_headlines, it would be something like this (warning: LINQ syntax in VB entirely from memory):
Dim spots1 = (From n In news_list Take (10) Select n)
Dim spots2 = (From n in spots1 Where Not news_headlines.Contains(n) Select n)
If you want to filter out the ones in news_headlines and then pick ten, it would be something like this:
Dim spots1 = (From n In news_list Where Not news_headlines.Contains(n) Select n)
Dim spots2 = (From n in spots1 Take (10) select n)
You can of course combine the two queries. Note that I'm assuming news_headlines is not null. I'm also assuming that your news items are either the same object instances or implement IEquatable<T>.
I think there are similar questions in stackoverflow. Anyway, you can possibly use "Intersect" to get the elements that are both in the spots and the headlines and remove them from the spots list.
The code should be trivial but I haven't used VB.net in a while, sorry.
Actually I spend whole day on the EntityFramework for foreign key.
assume we have two table.
Process(app_id,process_id)
LookupProcessId(process_id, process_description)
you can understand two tables with names, first table ,use process_id to indicate every application, and description is in the seoncd table.
Actually i try many times and figure out how to do inquery: it was like
Dim result = (from x in db.Processes where x.LookupProcess is (from m in db.LookupProcessIds where descr = "example" select m).FirstOrDefault() select x).FirstOrDefault()
First I want to ask is there easier way to do it.
Second i want to ask question is about insert
p As New AmpApplication.CUEngData.Process
p.app_id=100
p.LookupProcess = (from m in db.LookupProcessIds where descr = "example" select m).FirstOrDefault()
db.AddToProcesses(p)
db.SaveChanges()
from appearance it looks fine, but it give me error says
Entities in 'AmpCUEngEntities.Processes' participate in the 'FK_Process_LookupProcess' relationship. 0 related 'LookupProcess' were found. 1 'LookupProcess' is expected.
can i ask is that insert wrong? and is that my query correct?
For your first question:
Dim result = (from x in db.Processes
where x.LookupProcess.descr = "example"
select x).FirstOrDefault()
Actually, you missed some concepts from DataEntityModel, and its Framework. To manipulate data, you have to call object from contextual point of view. Those allow you to specify to the ObjectStateManager the state of an DataObject. In your case, if you have depending data from FK, you will have to add/update any linked data from leaf to root.
This example demonstrate simple (no dependances) data manipulation. A select if existing and an insert or update.
If you want more info about ObjectStateManager manipulation go to http://msdn.microsoft.com/en-us/library/bb156104.aspx
Dim context As New Processing_context 'deseign your context (this one is linked to a DB)
Dim pro = (From r In context.PROCESS
Where r.LOOKUPPROCESS.descr = LookupProcess.descr
Select r).FirstOrDefault()
If pro Is Nothing Then 'add a new one
pro = New context.PROCESS With {.AP_ID = "id", .PROCESS_ID = "p_id"}
context.PROCESS.Attach(pro)
context.ObjectStateManager.ChangeObjectState(pro, System.Data.EntityState.Added)
Else
'update data attibutes
pro.AP_ID = "id"
pro.PROCESS_ID = "p_id"
context.ObjectStateManager.ChangeObjectState(pro, System.Data.EntityState.Modified)
'context.PROCESS.Attach(pro)
End If
context.SaveChanges()
I hope this will help. Have a nice day!
For your first question, to expand on what #jeroenh suggested:
Dim result = (from x in db.Processes.Include("LookupProcess")
where x.LookupProcess.descr = "example"
select x).FirstOrDefault()
The addition of the Include statement will hydrate the LookupProcess entities so that you can query them. Without the Include, x.LookupProcess will be null which would likely explain why you got the error you did.
If using the literal string as an argument to Include is not ideal, see Returning from a DbSet 3 tables without the error "cannot be inferred from the query" for an example of doing this using nested entities.
For your second question, this line
p.LookupProcess = (from m in db.LookupProcessIds
where descr = "example" select m).FirstOrDefault()
Could cause you problems later on because if there is no LookupProcessId with a process_description of "example", you are going to get null. From MSDN:
The default value for reference and nullable types is null.
Because of this, if p.LookupProcess is null when you insert the entity, you will get the exception:
Entities in 'AmpCUEngEntities.Processes' participate in the 'FK_Process_LookupProcess' relationship. 0 related 'LookupProcess' were found. 1 'LookupProcess' is expected.
To avoid this kind of problem, you will need to check that p.LookupProcess is not null before it goes in the database.
If Not p.LookupProcess Is Nothing Then
db.AddToProcesses(p)
db.SaveChanges()
End If
i have a datatable which contains "InvalidCodes".
Before uploading the data to database(data is still in datatable), i want to perform linq on the datatable to remove Invalid entries and move them in another datatable
datatable allEntries ( entries yet to be uploaded in database)
datatable InvalidCodes(single column datatable - retrieved from database)
datatable invalidEntries
right now "allEnties" contains valid entries and invalid entries. the linq query on "allEntries" should move the nonexistend code entries to invalidEntries datatable.
plz help me perform this.
below is the query i formed but its not valid
string query = "select [CityCode] from [StateCity] ";
DataTable citylist = getDataTableFromSelect(query);
var result = from myrow in inputDt.AsEnumerable()
where !myrow.Field<string>("CityCode").Contains(from myrow2 in citylist.AsEnumerable() select myrow2.Field<string>("CityCode") )
select myrow;
I'd make a HashSet for the invalid city codes - this will allow the code to quickly/efficiently identify which of the codes are in the invalid set.
e.g. something like:
var invalidCityCodes = from myrow2 in citylist.AsEnumerable()
select myrow2.Field<string>("CityCode");
var invalidCityCodeHashSet = new HashSet<string>(invalideCityCodes);
var result = from myrow in inputDt.AsEnumerable()
where !invalidCityCodeHashSet.Contains(myrow.Field<string>("CityCode"))
select myrow;
You can also take both the results in 2 Different Lists and then you can
use
List1 = List1.RemoveAll(Item=>List2.Contains(Item))
This works fine with me and will work for you also.
I am using a sqlDataReader to get data and set it to session variables. The problem is it doesn't want to work with expressions. I can reference any other column in the table, but not the expressions. The SQL does work. The code is below. Thanks in advance, Anthony
Using myConnectionCheck As New SqlConnection(myConnectionString)
Dim myCommandCheck As New SqlCommand()
myCommandCheck.Connection = myConnectionCheck
myCommandCheck.CommandText = "SELECT Projects.Pro_Ver, Projects.Pro_Name, Projects.TL_Num, Projects.LP_Num, Projects.Dev_Num, Projects.Val_Num, Projects.Completed, Flow.Initiate_Date, Flow.Requirements, Flow.Req_Date, Flow.Dev_Review, Flow.Dev_Review_Date, Flow.Interface, Flow.Interface_Date, Flow.Approval, Flow.Approval_Date, Flow.Test_Plan, Flow.Test_Plan_Date, Flow.Dev_Start, Flow.Dev_Start_Date, Flow.Val_Start, Flow.Val_Start_Date, Flow.Val_Complete, Flow.Val_Complete_Date, Flow.Stage_Production, Flow.Stage_Production_Date, Flow.MKS, Flow.MKS_Date, Flow.DIET, Flow.DIET_Date, Flow.Closed, Flow.Closed_Date, Flow.Dev_End, Flow.Dev_End_Date, Users_1.Email AS Expr1, Users_2.Email AS Expr2, Users_3.Email AS Expr3, Users_4.Email AS Expr4, Users_4.FNAME, Users_3.FNAME AS Expr5, Users_2.FNAME AS Expr6, Users_1.FNAME AS Expr7 FROM Projects INNER JOIN Users AS Users_1 ON Projects.TL_Num = Users_1.PIN INNER JOIN Users AS Users_2 ON Projects.LP_Num = Users_2.PIN INNER JOIN Users AS Users_3 ON Projects.Dev_Num = Users_3.PIN INNER JOIN Users AS Users_4 ON Projects.Val_Num = Users_4.PIN INNER JOIN Flow ON Projects.id = Flow.Flow_Pro_Num WHERE id = "
myCommandCheck.CommandText += QSid
myConnectionCheck.Open()
myCommandCheck.ExecuteNonQuery()
Dim count As Int16 = myCommandCheck.ExecuteScalar
If count = 1 Then
Dim myDataReader As SqlDataReader
myDataReader = myCommandCheck.ExecuteReader()
While myDataReader.Read()
Session("TL_email") = myDataReader("Expr1").ToString()
Session("PE_email") = myDataReader("Expr2").ToString()
Session("DEV_email") = myDataReader("Expr3").ToString()
Session("VAL_email") = myDataReader("Expr4").ToString()
Session("Project_Name") = myDataReader("Pro_Name").ToString()
End While
myDataReader.Close()
End If
End Using
This may be because column names need to be unique for the SqlDataReader to be able to index them using a string name for the column.
A couple of things:
1) You are executing the query 3 times. You can lose the ExecuteNonQuery and ExecuteScalar calls, and replace the while loop with "if myDataReader.Read() / end if" to get the data values for the first resulting record. If no records are found, no session variables are set, just as in your current code.
2) It looks more like the problem lies in your session management (ie getting values from Session) rather than your sql query, which looks OK to me.
Check:
that you have sessionState enabled in your web.config file,
that you don't reset the Session values anywhere, and
that you ask for the same Session field name when you are trying to send the email. (e.g. are you setting Session("DEV_Email") but asking for Session("DEV Email") (space instead of underscore) ?
Sorry everyone. The code works just fine. The sqlDataReader WILL accept expressions as column names.
The reason I was getting an error saying the value of the from and to parameters cannot be null. There was no data in that column for any of the records in my table.