Why does this work (it returns only records with last name of Brown)
getAll = (from a in getAll where a.PatientLastName.StartsWith("brown") select a);
but this doesn't work (it returns all records in the db)?
getAll = (from a in getAll where a.PatientLastName.StartsWith(request.FirstName) select a);
Where request.FirstName is a string field and is equal to "brown".
Related
After running the query below and hovering over "usersToWork" when debugging, I can view all of the properties of the single entry that I get returned to me in addition to the other tables that have relations to this value. What I need to display to the user is the "Lines.Id" (Lines being the table and Id being the column in the Lines table) value, however that value gets lost from the SelectMany() statements. Is there anyway to select that "Lines.Id" value to include in the final value that I get from all of my joins? In the code below, I commented out what I want but I can't place that there otherwise I get error on the first SelectMany statement saying 'int' does not contain a definition for 'Shifts' and no extension method 'Shifts' accepting a first argument of type 'int' could be found.'
Correct me if I'm wrong but SelectMany() selects all of the columns from what you want to join on. In this case, in the first SelectMany() I get only values from the "Shifts" table and in the second SelectMany() I get only values from the "Users" table. Why is this different from the SQL join? When joining in SQL you can get every column as you join them together, SelectMany() yields only the values of the second table that you are joining on. Is it even possible to get that value in the "Lines" table or will I have to do another query? Any help would be great.
int idEnteredByUser = 123;
var usersToWork = entityDataModel.Lines
//....NOT IN MY CODE NOW....
// .Select(line => line.Id)//THIS IS WHAT I NEED.
// .Select(line => line.Description, line.Id//OR THIS TO RETURN TWO VALUES IF POSSIBLE
//This is my current code, I need to include on of the select lines above.
.SelectMany(line => line.Shifts) //Join lines on shifts.
.Where(shift => shift.EndTime >= DateTime.Now) //Join restricted times.
.SelectMany(user => user.Users) //Join the restricted shift times on users.
.Where(user => user.UserId == idEnteredByUser ); //Only look for the specific user
This works much easier using LINQ query syntax.
I'm assuming that you made a typo in your posted code and that user is a property of shift.
var idEnteredByUser = 123;
var usersToWork =
from line in entityDataModel.Lines
from shift in line.Shifts
where shift.EndTime >= DateTime.Now
from user in shift.Users
where user.UserId == idEnteredByUser
select new
{
Description = line.Description,
Id = line.Id
};
What is the difference between the query in this post:
Save LINQ Query As Variable To Call Another LINQ Query
var parentLoc = (from a in db.PartsLocations
where a.LocationName == aContainer
select a.ParentLocation);
var locations = (from b in db.PartsLocations
where b.LocationID == parentLoc
select b).ToList();
to the following example from this post.
Dim persVogel = From p In db.People
Where p.LastName = "Vogel"
Select p
Dim persVogelPHVIS = From pp In persVogel
Where pp.Company.Name = "PHVIS"
Select pp
Both of these have declared 2 queries and use the first query variable into second query.
What is the reason to use Single() in the first example but was not used in visualstudiomagazine.com article? Thanks
The two samples are fundamentally different.
First Sample
I think you pasted this one incorrectly...from the referenced question, the query should be:
var parentLoc = (from a in db.PartsLocations
where a.LocationName == aContainer
select a.ParentLocation).Single();
var locations = (from b in db.PartsLocations
where b.LocationID == parentLoc
select b).ToList();
(I am going to assume that LocationID and ParentLocation are typed as int.)
In this sample parentLoc is an int - a single instance of a ParentLocation value, obtained from the PartsLocations table. So what you get is an int.
The second linq statement sources its records also from the PartsLocations table. It uses parentLoc to identify records within that table (e.g. where b.LocationID == parentLoc). What you get at the end is a set of PartsLocations records.
The .Single() call is made because you want to compare the result to LocationID in the second statement, and cannot compare int to IEnumerable<int>.
Second Sample
Dim persVogel = From p In db.People
Where p.LastName = "Vogel"
Select p
Dim persVogelPHVIS = From pp In persVogel
Where pp.Company.Name = "PHVIS"
Select pp
In the second sample, persVogel is a subset of records from the People table (specifically, the subset of people with LastName == "Vogel") - so what you get is a set of People records.
The second linq statement is based on this subset of records (From pp In persVogel) and further filters them down to records where pp.Company.Name = "PHVIS". What you get is still a set of People records.
These two statements could easily be compressed into one single statement:
Dim persVogelPHVIS = From p In db.People
Where p.LastName = "Vogel"
AndAlso p.Company.Name = "PHVIS"
Select p
You will still get a set of People records at the end.
i have following query to select the last record in the database
using (Entities ent = new Entities())
{
Loaction last = (from x in ent.Locations select x).Last();
Location add = new Location();
add.id = last.id+1;
//some more stuff
}
the following error is returned when calling the method containing these lines via "Direct event" on ext.net:
LINQ to Entities does not recognize the method 'Prototype.DataAccess.Location Last[Location]
(System.Linq.IQueryable`1[Prototype.DataAccess.Location])' method,
and this method cannot be translated into a store expression.
table structure is following:
int ident IDENTITY NOT NULL,
int id NOT NULL,
varchar(50) name NULL,
//some other varchar fields
Last is not supported by Linq to Entities. Do OrderByDescending (usually by ID or some date column) and select first. Something like:
Location last = (from l in ent.Locations
orderby l.ID descending
select l).First();
Or
Location last = ent.Locations.OrderByDescending(l => l.ID).First();
See MSDN article Supported and Unsupported LINQ Methods (LINQ to Entities) for reference.
I am trying to return the results of a query into a List object, however the following code, as I normally use, does not work. Still relatively new to Linq, can someone explain the correct syntax/what's going on? This will work if I change the data type of productTraining to var...
List<AgentProductTraining> productTraining = new List<AgentProductTraining>();
productTraining = from records in db.CourseToProduct
where records.CourseCode == course.CourseCode
select records;
Select() and Where() will return IQueryable<T>, not List<T>. You've got to convert it to a List<T> - which actually executes the query (instead of just preparing it).
You just need to call ToList() at the end of the query. For example:
// There's no need to declare the variable separately...
List<AgentProductTraining> productTraining = (from records in db.CourseToProduct
where records.CourseCode == course.CourseCode
select records).ToList();
Personally I wouldn't use a query expression though, when all you're doing is a single Where clause:
// Changed to var just for convenience - the type is still List<AgentProductTraining>
var productTraining = db.CourseToProduct
.Where(records => records.CourseCode == course.CourseCode)
.ToList();
I couldn't get last articles of every writers in this statement.
List<Editor> lstEditors = dataContext.GetTable<Editor>().Where(t => t.M_Active).Select(t => t).ToList();
var lstArticles = from article in DAO.context.GetTable<Article>().ToList()
join editor in lstEditors on article.RefEditorId equals editor.EditorId
select
new
{
article.M_ArticleId,
article.M_Subject,
article.M_Text,
editor.M_EditorId,
editor.M_Member.M_EditorPicture,
M_NameSurname = editor.M_Member.M_Fname + " " + editor.M_Member.M_Lname
};
Be careful, your query is fetching all the contents of both the Editor and the Yazi tables and then performs Linq-to-Objects on it.
I'm not sure what you ask exactly either, do you want to obtain the list of all writers (editors) along with the last article of each one of these writers?
Do you want to get the writers that did not write any articles yet also?
Edit:
explanation of methods causing an immediate query
Any time you call one of the methods listed below on an IQueryable object (tables or other queries), it performs the actual query to SQL server:
ToList(), ToArray(), ToLookup(), ToDictionay()
Count(), Sum(), Avg(), Aggregate(), Min(), Max()
First(), FirstOrDefault(), Last(), LastOrDefault()
getting last article written by each writer
//create a subquery that returns an editor and its last article date
var editorLastArticleDates =
from article in DAO.context.GetTable<Article>()
group article by article.RefEditor into g
let lastArticleDate= g.Max(x => x.Date)
select new
{
Editor = g.Key,
LastArticleDate = lastArticleDate,
};
//Note: We did not do a ToList() here so the query is not executed
// The editorLastArticleDates object is a IQueryable<>
var query =
from article in DAO.context.GetTable<Article>()
join editorLastArticleDate in editorLastArticleDates
on new { article.Editor, article.Date } // 1
equals new { editorLastArticleDate.Editor, // 2
Date = editorLastArticleDate.LastArticleDate } // 3
select new
{
article.M_ArticleId,
article.M_Subject,
article.M_Text,
article.RefEditor.M_EditorId,
article.RefEditor.M_Member.M_EditorPicture,
M_NameSurname = article.RefEditor.M_Member.M_Fname + " "
+ article.RefEditor.M_Member.M_Lname,
};
//Note: We did not do a ToList() yet so the query is not executed
// The query object is a IQueryable<>
Console.WriteLine(query.ToString()); //Displays SQL query on the console
var results = query.ToList(); // SQL query is executed on this line.
In the code above, I left some remarks on things I had problems with:
When using join, the section between new and equals access only variables declared before the join keyword while the section after the equals keyword has access to the variable defined between join and in.
When writing your join condition, make sure you use equals and not ==.
When using new { XXX, YYY } syntax in your join condition, you declare anonymous types. If the property names are not identical on both sides, it will not compile. In order to have identical property names in this sample, I added the Date = before my value.
By the way, you should use LinqPad to test your queries, it is really a nice tool.