I'm converting old code to use LINQ. The old code looked like this:
// Get Courses
sqlQuery = #"SELECT Comment.Comment, Status.StatusId, Comment.DiscussionBoardId, DiscussionBoard.CourseId, Comment.CommentID
FROM Status INNER JOIN Comment ON Status.StatusId = Comment.StatusId INNER JOIN
DiscussionBoard ON Comment.DiscussionBoardId = DiscussionBoard.DiscussionBoardId
WHERE (DiscussionBoard.CourseID = 'CourseID')";
var comments = new List<Comment>(dataContext.ExecuteQuery<Comment>(sqlQuery));
I've converted the above SQL to LINQ:
var db = new CMSDataContext();
var query = from c in db.Comments
join s in db.Status on c.StatusId equals s.StatusId
join d in db.DiscussionBoards on c.DiscussionBoardId equals d.DiscussionBoardId
where d.CourseId == "CourseID"
select new
{
d.ItemType,
c.Comment1,
s.Status1,
c.DiscussionBoardId,
d.CourseId,
c.CommentID
};
The problem I've having, though, is with trying to get the results of the query into the List. Can someone offer me some pointers?
Thanks!
Try adding the ToList() method at the end of the query.
Enclose the whole query in parentheses and add .ToList() at the end.
Or add another line:
var list = query.ToList();
How about the ToList method: query.ToList() ?
You'll need to do two things.
First, change your select to create a new instance of Comment instead of an anonymous type.
Second, either wrap the whole query in a call to ToList() or store the results in a temporary variable and call ToList() on that variable to get the List<Comment> as a result.
Either (A) wrap the entire call with Enumerable.ToList(<your query>), (B) surround the entire query with parentheses and call the ToList extension method at the end, or (C) call query.ToList() as a separate statement.
Related
Apparently I'm missing something with how LINQ to entities works. Hopefully one of you all can educate me.
Please try the below locally and let me know if you are seeing the same results. Something is really strange here...
Lets look at a very simple LINQ expression using navigation properties.
This was generated in LinqPad in a C# statement.
var result = (from ge in group_execution
where ge.automation_sequences.project.client_id == 1 && ge.parent_group_exec_id != null
select new
{
ge.id,
ge.parent_group_exec_id,
ge.automation_sequences.project.client_id
});
result.Dump();
OR, we can use joins...which will lead to the same bad results, but lets continue...
var result = (from ge in group_execution
join aseq in automation_sequences on ge.automation_sequence_id equals aseq.id
join p in project on aseq.project_id equals p.id
where p.client_id == 1 && ge.parent_group_exec_id != null
select new
{
ge.id,
ge.parent_group_exec_id,
p.client_id
});
result.Dump();
These very simple LINQ expressions generate the following SQL:
SELECT
[Filter1].[id1] AS [id],
[Filter1].[parent_group_exec_id] AS [parent_group_exec_id],
[Extent5].[client_id] AS [client_id]
FROM (SELECT [Extent1].[id] AS [id1], [Extent1].[automation_sequence_id] AS [automation_sequence_id], [Extent1].[parent_group_exec_id] AS [parent_group_exec_id]
FROM [dbo].[group_execution] AS [Extent1]
INNER JOIN [dbo].[automation_sequences] AS [Extent2] ON [Extent1].[automation_sequence_id] = [Extent2].[id]
INNER JOIN [dbo].[project] AS [Extent3] ON [Extent2].[project_id] = [Extent3].[id]
WHERE ([Extent1].[parent_group_exec_id] IS NOT NULL) AND (1 = [Extent3].[client_id]) ) AS [Filter1]
LEFT OUTER JOIN [dbo].[automation_sequences] AS [Extent4] ON [Filter1].[automation_sequence_id] = [Extent4].[id]
LEFT OUTER JOIN [dbo].[project] AS [Extent5] ON [Extent4].[project_id] = [Extent5].[id]
This baffles me. For the life of me I can't understand why LINQ is doing this. It's horrible, just look at the execution plan:
Now lets manually clean this up in SSMS and view the correct SQL and execution plan:
Much better, but how do we get LINQ to act this way?
Is anyone else seeing this? Has anyone else ever saw this and corrected it and if so how?
Thanks for looking into this.
UPDATE, attempting Chris Schaller fix:
var result = (from ge in group_execution
select new
{
ge.id,
ge.parent_group_exec_id,
ge.automation_sequences.project.client_id
}).Where(x=>x.client_id == 1 && x.parent_group_exec_id != null);
result.Dump();
Just so you all know I'm monitoring the SQL through SQL Server Profiler. If anyone knows of any issues doing it this way let me know.
UPDATE, a fix for JOINS, but not nav properties, and a cause, but why?
Here's your solution:
var result = (from ge in group_execution.Where(x=>x.parent_group_exec_id != null)
join aseq in automation_sequences on ge.automation_sequence_id equals aseq.id
join p in project on aseq.project_id equals p.id
where p.client_id == 1// && ge.parent_group_exec_id != null
select new
{
ge.id,
ge.parent_group_exec_id,
p.client_id
});
result.Dump();
Null checks shouldn't cause the framework to mess up like this. Why should I have to write it this way? This just seems like a defect to me in the framework. It will make my dynamic expressions a little bit more difficult to write, but maybe I can find a way.
Navigation Properties still mess up...so I'm still really sad. Picture below:
var result = (from ge in group_execution.Where(x=>x.parent_group_exec_id != null)
where ge.automation_sequences.project.client_id == 1// && ge.parent_group_exec_id != null
select new
{
ge.id,
ge.parent_group_exec_id,
ge.automation_sequences.project.client_id
});
result.Dump();
move your where clause to after you have defined the structure of the select statement
var result = (from ge in group_execution
select new
{
ge.id,
ge.parent_group_exec_id,
ge.automation_sequences.project.client_id
}).Where(x => x.client_id == 1 && x.parent_group_exec_id != null)
result.Dump();
Remember that Linq-to-entities flattens the results of queries to execute as SQL and then hydrates the object graph from those results.
When your query uses navigation properties or joins the query parser has to allow for zero results from those sub queries (Extents) to make sure that all columns that are required in the output and any interim processing are represented. By explicitly specifying a filter on a table for != null early in the query the parser knows that there is no further possibility that the field and any relationships linked by that field will be null, until then the parser prepares the query as if the joins will return null results
It is worth checking, but i wonder if UseDatabaseNullSemantics has anything to do with this?
Try:
dbContext.Configuration.UseDatabaseNullSemantics = false
In Linq, we can specify Where clauses as often as we like, improve the resulting SQL we should filter early in the query and granularly.
The parser engine is optimized to follow and implement your query sequentially, and generate good SQL at the end of it. Don't try to write linq-to-entities the same way that you structure your SQL, I know it's counter intuitive because the syntax is similar
A good technique is to assume that before each clause all the records from the previous statements have been loaded into memory, and that the next operation will affect all of those records. So you want to reduce the records before each additional operation by specifying a filter before moving on to the next clause
In general, if you have a filter condition based on the root table, apply this to the query before define all other joins and filters and even selects, you will get much cleaner sql.
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 saw this post Linq int to string, and tried it
var personalInfoQuery = from t in crnnsupContext.Tombstones
join i in crnnsupContext.InitialEducations on t.InitialEducation equals SqlFunctions.StringConvert((double)i.InitalEducationID)
where t.RegNumber == 25952
select new CPersonalInfo
{
Tombstone = t,
InitialEducation = i
};
in the database t.InitialEducation is char, i.InitalEducationID is int, but the retrieved result is null. I am pretty sure the value is not empty in the SQL server. So I think the problem is SqlFunctions.StringConvert((double)i.InitalEducationID)
when i remove the join statement, it got this person's information.
Does anyone know why. thanks
Finally find the reason!!
t.InitialEducation is nvarchar(1) in the database, i.InitalEducationID is int, after I modified to SqlFunctions.StringConvert((double)i.InitialEducationID, 1) it works!
"1" is the length of the returned string, the default length is 10, I guess there are some extra space.
Since you're using the SqlFunctions.StringConvert method, I'm assuming you're using EF as the underlying LINQ provider, no?
From the information given, it would appear that you're looking to do a 1 to many join on those properties. The code you've written could coalesce (not enough info on the context to be certain) as an INNER JOIN in SQL, so to force the LEFT JOIN behavior, you can add a .DefaultIfEmpty() call:
crnnsupContext.InitialEducations.DefaultIfEmpty()
I have a custom table with a list of sales orders I want to post picking lists for.
How can I pass them all at once to the SalesFormLetter object to pick them in a group?
I see SalesFormLetter_PickingList\newJournalList is being called, and I was wondering if there was a way I could just pass a simple RecordSortedList in of the sales orders I wanted to pick. That list is of the wrong table though...so that wouldn't work. It looks like I can somehow pass a query but I'm not exactly sure how to do that. Here is the basic code I'm using to post the picking lists:
salesFormLetter = SalesFormLetter::construct(DocumentStatus::PickingList);
salesFormLetter.update(SalesTable::find(_salesId), today(), SalesUpdate::All, AccountOrder::None, NoYes::No, NoYes::Yes);
This involves setting up a query to select your sales orders then calling the chooseLines to select the orders.
by: Jubal1234Posted on 2010-07-27 at 04:13:28ID: 33296972
Found the solution:
SalesFormLetter salesFormLetter;
QueryRun queryRun;
Query query;
str strSalesTable = "V683904, V683905, V683906";
;
salesFormLetter = SalesFormLetter::construct(DocumentStatus::PackingSlip);
query = new Query(QueryStr(SalesUpdate));
query.dataSourceTable(tablenum(SalesTable)).addRange(fieldnum(SalesTable, SalesId)).value(strSalesTable);
queryRun = new QueryRun(query);
salesFormLetter.chooseLinesQuery(queryRun);
salesFormLetter.transDate(systemdateget());
salesFormLetter.specQty(SalesUpdate::All);
salesFormLetter.printFormLetter(false);
salesFormLetter.createParmUpdate();
salesFormLetter.chooseLines(null,true);
salesFormLetter.reArrangeNow(true);
salesFormLetter.run();
Case closed
I'm using the Massive Query method to write a simple join query against an Oracle database. This is my code with the query simplified even further by taking out some columns:
dynamic logTable = new DynamicModel("mydatabase", "table1");
var sb = new StringBuilder();
sb.Append("select CONTACT_ID from table1 inner join table2 on table1.ID = table2.ID ");
sb.Append("where table1.ID=:0");
dynamic dbResult = logTable.Query(sb.ToString(), id);
The following code gives me an error: 'object' does not contain a definition for 'CONTACT_ID'
string id = dbResult.CONTACT_ID.ToString();
If I take the exact query and run it through sqldeveloper, I get back the expected results. If I try to Query through Massive without a join, I get back an object I can work with.
Any ideas?
My mistake! I was expecting my query to return only one record, but forgot that Query returns IEnumerable. Solution is to take First() or loop over the results.