I want to call controller method for data in that there can be multiple data output from different table so it depends on user selection from which table they wanna to take output and so i was thinking to get output using single linq just by passed name of table according to user selection.
query = (from Ledger in fen.TblLedgers
join acc in fen.Accounts
on Ledger.Code equals acc.AccId
where acc.DrCr == Fin.DRCR && Ledger.Grp == Fin.LedGrp && acc.Vdate <= DateTime.Today && Ledger.isActive == "Y" group acc by new { acc.AccId } into g select g.Sum(acc => acc.Amt)).Sum() ;
If you want to do this, one approach would be to use lambdas and implement something like the below.
Long and short of it - you'd need to leverage the DbContext.Set < T > method DbContext.Set method (MSDN) or the generic one (MSDN)
Vary T (or the type parameter) by the users choice, and then lambda from there.
However, in order to do this, all T's would need to implement the same interface and you'd need to cast the elements to that interface before using them. But, you'd have to do this if you want any of this stuff to work at all...)
(Code below written freehand, don't complain if there's the odd mistake!)
So - with classes:
public interface IAnimal
{
....
}
public class Dog :IAnimal
{
....
}
public class Cat :IAnimal
{
....
}
public class Cow :IAnimal
{
...
}
You could use something like:
var userSelection = "Dog";
Type chosenType = chooseTypeAccordingToUserSelection(userSelection);
var animalDBSet = GetOpenDBcontext().Set(chosenType).Cast<IAnimal>();
var insects = animalDBSet.Where(a=>a.NumberOfLegs>4);
HTH!
Related
I have the following error:
LINQ to Entities does not recognize the method
'System.Collections.Generic.List`1 [System.Int32] get_st_past_enrollment_success()'
method, and this method cannot be translated into a store expression.
This is caused by the following linq
IEnumerable<subject> _subjects = (from subject in context.subjects
where
subject.enrollments.Count() < subject.sj_max_enrollment
&& subject.sj_availability == true
&& !this.get_st_past_enrollment_success().Contains(subject.sj_subject_id)
select subject);
get_st_past_enrollment_success() returns a List:
public List<int> get_st_past_enrollment_success()
{
return this.enrollments.Where(e => e.em_enrolled == false && e.em_result >= 50).Select(e => e.em_subject_id).ToList();
}
What am i doing wrong here?
Your query itself contains the method call - and Entity Framework doesn't know what to do with that. Try extracting the list fetch to before the query:
var enrollments = get_st_past_enrollment_success();
var _subjects = from subject in context.subjects
where subject.enrollments.Count() < subject.sj_max_enrollment
&& subject.sj_availability
&& !enrollments.Contains(subject.sj_subject_id)
select subject;
Also note that get_st_past_enrollment_success violates .NET naming conventions - that won't affect whether the code works, but it'll look odd to other developers who are used to the normal conventions.
I have a very basic sql view which joins 3 tables: users, pictures, and tags.
How would one create the query below in a way that it won't list the same pictures more than once? In other words, I want to Group By pictures (I think) and return get the first insance of each.
I think this is very similar to the post Linq Query Group By and Selecting First Items, but I cannot figure out how to apply it in this case where the query is instantiating MyImageClass.
validPicSummaries = (from x in db.PicsTagsUsers
where x.enabled == 1
select new MyImageClass {
PicName = x.picname,
Username= x.Username,
Tag = x.tag }).Take(50);
To exclude duplicates, you can use the Distinct LINQ method:
validPicSummaries =
(from x in db.PicsTagsUsers
where x.tag == searchterm && x.enabled == 1
select new MyImageClass
{
PicName = x.picname,
Username= x.Username,
Tag = x.tag
})
.Distinct()
.Take(50);
You will need to make sure that the objects are comparable so that two MyImageClass objects that have the same PicName, Username, and Tag are considered equal (or however you wish to consider two of them as being equal).
You can write a small class that implements IEqualityComparer<T> if you would like to have a custom comparer for just this case. Ex:
private class MyImageClassComparer : IEqualityComparer<MyImageClass>
{
public bool Equals(MyImageClass pMyImage1, MyImageClass pMyImage2)
{
// some test of the two objects to determine
// whether they should be considered equal
return pMyImage1.PicName == pMyImage2.PicName
&& pMyImage1.Username == pMyImage2.Username
&& pMyImage1.Tag == pMyImage2.Tag;
}
public int GetHashCode(MyImageClass pMyImageClass)
{
// the GetHashCode function seems to be what is used by LINQ
// to determine equality. from examples, it seems the way
// to combine hashcodes is to XOR them:
return pMyImageClass.PicName.GetHashCode()
^ pMyImageClass.UserName.GetHashCode()
^ pMyImageClass.Tag.GetHashCode();
}
}
Then when you call distinct:
...
.Distinct(new MyImageClassComparer())
.Take(50);
I've read ALL of:
Unable to create a constant value of type 'System.Object' in Entity Framework
Entity Framework - "Unable to create a constant value of type 'Closure type'..." error
Entity Framework - Union causes "Unable to create a constant value of type.."
Only primitive types ('such as Int32, String, and Guid') are supported in this context
and searched a bit more, but still no solution. I've seen that this happens on EF 3.5 and in 4.0 the Contains method should be supported, but I'm in EF 4 but I'm getting this error. I have a photo gallery, where albums can have any number of different photos, and each photo can belong to any number of albums. So this is a many-to-many relationship.
I've got a VisibleObjects property which is used by about 100 other methods that work well, but I'm pasting it anyway: (I'm definitely sure the problem is not caused by something here)
static IQueryable<GlobalObject> VisibleObjects
{
get
{
return from obj in db.GlobalObjectSet where obj.IsVisible && !obj.SiteUser.IsDeactivated orderby obj.ID descending select obj;
}
}
I've tried several different queries:
I have a VisiblePhotos property:
This wasn't working:
static IQueryable<Photo> VisiblePhotos(this Album a)
{
return from p in VisibleObjects.OfType<Photo>() where a.Photos.Contains(p) select p;
}
Changed to this:
static IQueryable<Photo> VisiblePhotos(this Album a)
{
return from p in VisibleObjects.OfType<Photo>() where a.Photos.Any(other => p.ID == other.ID) select p;
}
Still didn't work.
Here is the calling method:
public static List<Photo> GetLatestPhotosByAlbum(Album alb, int count = 3)
{
lock (sync)
{
return alb.VisiblePhotos().OrderByDescending(p => p.ID).Take(count).ToList();
}
}
Wasn't working, changed to this:
public static List<Photo> GetLatestPhotosByAlbum(Album alb, int count = 3)
{
lock (sync)
{
return (from p in VisibleObjects.OfType<Photo>()
where alb.Photos.Any(ph => ph.ID == ph.ID)
select p).ToList();
}
}
Still isn't working. Complaining about not being able to create a constant of my Photo object type, which is an Entity object with an ID property, if it helps. I am not sure of the real cause of the error and I don't have any other ideas of queries in my mind. I think the method name is pretty self explanatory: I'm trying to get the photos in a given album. Loading album entries into memory is not a solution, the query should run on database, not memory. I need an explanation of this exception, why it is occuring here, and how can I get my query to work.
It will not work because you want to use local Album in linq-to-entities query. You must either use navigation property on p to get its album:
var query = from p in VisibleObjects.OfType<Photo>()
where p.Album.Id == alb.Id
select p;
or you must build complex query with some join between photos and albums. You cannot pass local object and any its relation to the query. Only simple properties can be passed.
I think that EF is trying to convert where a.Photos.Contains(p) into SQL like WHERE p IN (a.Photos), but it doesn't know how to express a.Photos in SQL. The SQL you want probably looks like WHERE p.Id IN (1, 2, 3), so you could try doing that in C#:
static IQueryable<Photo> VisiblePhotos(this Album a)
{
var photoIds = a.Photos.Select(p => p.Id).ToArray();
return from p in VisibleObjects.OfType<Photo>() where photoIds.Contains(p.Id) select p;
}
I ran into a similar problem, and instead of IQueryable, I tried using List, and it worked. May be of some help.
I tried another way around and it worked:
static IQueryable<Photo> VisiblePhotos(this Album a)
{
return from p in VisibleObjects.OfType<Photo>()
where p.Albums.Any(alb => a.ID == alb.ID)
select p;
}
Quite weird to see this works but the other one not. But I'm still wondering why Contains is not working.
I know this as asked many times but cannot see something that works.
I am reading a csv file and then I have to remove duplicate lines based on one of the columns "CustomerID".
Basically the CSV file can have multiple lines with the same customerID.
I need to remove the duplicates.
//DOES NOT WORK
var finalCustomerList = csvCustomerList.Distinct().ToList();
I have also tried this extension method //DOES NOT WORK
public static IEnumerable<t> RemoveDuplicates<t>(this IEnumerable<t> items)
{
return new HashSet<t>(items);
}
What works for me is
I Read the CSV file into a csvCustomerList
Loop through csvCustomerList and check if a
customerExists If it doesnt I add
it.
foreach (var csvCustomer in csvCustomerList)
{
var Customer = new customer();
customer.CustomerID = csvCustomer.CustomerID;
customer.Name = csvCustomer.Name;
//etc.....
var exists = finalCustomerList.Exists(x => x.CustomerID == csvCustomer.CustomerID);
if (!exists)
{
finalCustomerList.Add(customer);
}
}
Is there a better way of doing this?
For Distinct to work with non standard equality checks, you need to make your class customer implement IEquatable<T>. In the Equals method, simply compare the customer ids and nothing else.
As an alternative, you can use the overload of Distinct that requires an IEqualityComparer<T> and create a class that implements that interface for customer. Like that, you don't need to change the customer class.
Or you can use Morelinq as suggested by another answer.
For a simple solution, check out Morelinq by Jon Skeet and others.
It has a DistinctBy operator where you can perform a distinct operation by any field. So you could do something like:
var finalCustomerList = csvCustomerList.DistinctBy(c => c.customerID).ToList();
Let's say I have an array, and I want to do a LINQ query against a varchar that returns any records that have an element of the array anywhere in the varchar.
Something like this would be sweet.
string[] industries = { "airline", "railroad" }
var query = from c in contacts where c.industry.LikeAnyElement(industries) select c
Any ideas?
This is actually an example I use in my "Express Yourself" presentation, for something that is hard to do in regular LINQ; As far as I know, the easiest way to do this is by writing the predicate manually. I use the example below (note it would work equally for StartsWith etc):
using (var ctx = new NorthwindDataContext())
{
ctx.Log = Console.Out;
var data = ctx.Customers.WhereTrueForAny(
s => cust => cust.CompanyName.Contains(s),
"a", "de", "s").ToArray();
}
// ...
public static class QueryableExt
{
public static IQueryable<TSource> WhereTrueForAny<TSource, TValue>(
this IQueryable<TSource> source,
Func<TValue, Expression<Func<TSource, bool>>> selector,
params TValue[] values)
{
return source.Where(BuildTrueForAny(selector, values));
}
public static Expression<Func<TSource, bool>> BuildTrueForAny<TSource, TValue>(
Func<TValue, Expression<Func<TSource, bool>>> selector,
params TValue[] values)
{
if (selector == null) throw new ArgumentNullException("selector");
if (values == null) throw new ArgumentNullException("values");
if (values.Length == 0) return x => true;
if (values.Length == 1) return selector(values[0]);
var param = Expression.Parameter(typeof(TSource), "x");
Expression body = Expression.Invoke(selector(values[0]), param);
for (int i = 1; i < values.Length; i++)
{
body = Expression.OrElse(body,
Expression.Invoke(selector(values[i]), param));
}
return Expression.Lambda<Func<TSource, bool>>(body, param);
}
}
from c in contracts
where industries.Any(i => i == c.industry)
select c;
something like that. use the any method on the collection.
IEnumerable.Contains() translates to SQL IN as in:
WHERE 'american airlines' IN ('airline', 'railroad') -- FALSE
String.Contains() which translates to SQL LIKE %...% as in:
WHERE 'american airlines' LIKE '%airline%' -- TRUE
If you want the contacts where the contact's industry is LIKE (contains) any of the given industries, you want to combine both Any() and String.Contains() into something like this:
string[] industries = { "airline", "railroad" };
var query = from c in contacts
where industries.Any(i => c.Industry.Contains(i))
select c;
However, combining both Any() and String.Contains() like this is NOT supported in LINQ to SQL. If the set of given industries is small, you can try something like:
where c.Industry.Contains("airline") ||
c.Industry.Contains("railroad") || ...
Or (although normally not recommended) if the set of contacts is small enough, you could bring them all from the DB and apply the filter with LINQ to Objects by using contacts.AsEnumerable() or contacts.ToList() as the source of the query above:
var query = from c in contacts.AsEnumerable()
where industries.Any(i => c.Industry.Contains(i))
select c;
it will work if you build up the query as follows:
var query = from c in contacts.AsEnumerable()
select c;
query = query.Where(c=> (c.Industry.Contains("airline")) || (c.Industry.Contains("railroad")));
you just need to programmatically generate the string above if the parameters airline and railroad are user inputs. This was in fact a little more complicated than I was expecting. See article - http://www.albahari.com/nutshell/predicatebuilder.aspx
Unfortunately, LIKE is not supported in LINQ to SQL as per here:
http://msdn.microsoft.com/en-us/library/bb882677.aspx
To get around this, you will have to write a stored procedure which will accept the parameters you want to use in the like statement(s) and then call that from LINQ to SQL.
It should be noted that a few of the answers suggest using Contains. This won't work because it looks to see that the entire string matches the array element. What is being looked for is for the array element to be contained in the field itself, something like:
industry LIKE '%<element>%'
As Clark has mentioned in a comment, you could use a call to IndexOf on each element (which should translate to a SQL call):
string[] industries = { "airline", "railroad" }
var query =
from c in contacts
where
c.industry.IndexOf(industries[0]) != -1 ||
c.industry.IndexOf(industries[1]) != -1
If you know the length of the array and the number of elements, then you could hard-code this. If you don't, then you will have to create the Expression instance based on the array and the field you are looking at.