I have a linq to sql database. Very simplified we have 3 tables, Projects and Users. There is a joining table called User_Projects which joins them together.
I already have a working method of getting IEnumberable<Project> for a given user.
from up in User_Projects
select up.Project;
Now I want to get the projects the user isn't involved with. I figured the except method of IEnumerable would be pretty good here:
return db.Projects.Except(GetProjects());
That compiles, however I get a runtime error: "Local sequence cannot be used in LINQ to SQL implementation of query operators except the Contains() operator."
Is there any way to get around this?
Update:
A few views but no answers :p
I have tried this:
IEnumerable<Project> allProjects = db.Projects;
IEnumerable<Project> userProjects = GetProjects();
return allProjects.Except(GetProjects());
I know it's essentially the same as the original statement - but now i dont get a runtime error. Unfortunately, it doesn't really do the except part and just returns all the projects, for some reason
Linq to Sql doesn't understand how to work with an arbitrary in-memory sequence of objects. You need to express this in relational terms, which works on IDs:
var userProjectIds =
from project in GetProjects()
select project.ProjectId;
var nonUserProjects =
from project in db.Projects
where !userProjectIds.Contains(project.ProjectId)
select project;
You could try something simple like
User u = SomeUser;
from up in User_Projects
where up.User != u
select up.Project;
try this:
var userProjects = GetProjects();
return db.Projects.Except(userProjects.ToArray());
The ToArray should force evaluation of the sequence (if I'm understanding the issue right) and allow the operation to succeed.
Related
When I trying to do this
//data.Photos it's IEnumerable<Photo>. Comparer worked by Id.
List<Photo> inDb = db.Photos
.Intersect(data.Photos, new PhotoComparer())
.ToList();
I get an exception:
NotSupportedException: Could not parse expression
'value(Microsoft.EntityFrameworkCore.Query.Internal.EntityQueryable`1[ReportViewer.Models.DbContexts.Photo]).Intersect(__p_0, __p_1)'
This overload of the method #x27;System.Linq.Queryable.Intersect' is currently not supported.
// This works
List<Photo> inDb = db.Photos
.ToList()
.Intersect(data.Photos, new PhotoComparer())
.ToList();
// But will it take a long time - or not ?
What did I need to use Intersect with IQueryable and IEnumerable collection?
Due to the "custom comparer", although it's functionality might be trivial, the framework is currently not able to translate your statement to SQL (which I suspect you are using).
Next, it seems that you have a in memory collection, on which you want to perform this intersect.
So if you're wondering about speed, in order to get it working you'll need to send your data to the database server, and based on the Id's retrieve your data.
So basically, you are looking for a way to perform an inner join, which would be the SQL equivalent of the intersect.
Which you could do with the flowing linq query:
//disclaimer: from the top of my head
var list= from dbPhoto in db.Photos
join dataPhoto in data.Photos on dbPhoto.Id equals dataPhoto.Id
select dbPhoto;
This will not work though, since as far as I know EF isn't able to perform an join against an in-memory dataset.
So, alternatively you could:
fetch the data as IEnumerable (but yes, you'll be retrieving the whole set first)
use a Contains, be carefull though, if you're not using primitive types this can translate to a bunch of SQL OR statements
But basically it depends on the amount of data you're querying. You might want to reconsider your setup and try to be able to query the data based on some ownership, like user or other means.
I am trying to implement some search functionality within our app and have a situation where a User can select multiple Topics from a list and we want to return all activities that match at least one of the selected Topics. Each Activity can have 0-to-many Topics.
I can write a straight SQL query that gives me the results I want like so:
SELECT *
FROM dbo.ACTIVITY_VERSION av
WHERE ACTIVITY_VERSION_ID IN (
SELECT ACTIVITY_VERSION_ID
FROM dbo.ACTIVITY_TOPIC at
WHERE at.TOPIC_ID IN (3,4)
)
What I can't figure out is how to write a LINQ query (we are using Linq to Sql) that returns the same results.
I've tried:
activities.Where(x => criteria.TopicIds.Intersect(x.TopicIds).Any());
this works if activities is a list of in memory objects (i.e. a Linq to Objects query), but I get an error if I try to use the same code in a query that hits the database. The error I receive is:
Local sequence cannot be used in LINQ to SQL implementations of query operators except the Contains operator.
I believe that this means that Linq to Sql doesn't know how to translate either Intersect or Any (or possibly both). If that is the case, I understand why it isn't working, but I still don't know how to make it do what I want it to and my Google-fu has not provided me with anything that works.
Haven't tested it. But this is how you ll go about it.
List<int> IDs = new List<int>();
IDs.Add(3);
IDs.Add(4);
var ACTIVITY_VERSION_IDs = ACTIVITY_TOPIC
.Where(AT => IDs.Contains(AT.TOPIC_ID))
.Select(AT=>AT.ACTIVITY_VERSION_ID)
var results = ACTIVITY_VERSION
.Where(AV => ACTIVITY_VERSION_IDs.Contains(AV.ACTIVITY_VERSION_ID))
I'm fairly new to nHibernate having come from an EF background and I'm struggling with the following query :
_patientSearchResultModel = (from patient in _patientRepository.Query(patientSearch.BuildPatientSpecification())
join admission in _admissionRepository.Query(patientSearch.BuildAdmissionSpecification())
on patient.Id equals admission.Patient.Id
orderby admission.AdmissionDate
select new PatientSearchResultModel(patient.Id,
admission.Id,
false,
_phaseTypeMapper.GetPhaseTypeModel(admission.PhaseType),
patient.Last, patient.First,
admission.InPatientLocation,
admission.AdmissionDate,
admission.DischargeDate,
admission.RRI,
null,
admission.CompletionStatus,
admission.FollowupStatus)).ToList();
The intent of this query is to allow users to filter the two queries on parameters built up using the two Build???Specification functions and return the resultset. There could be many admission records and I would only like one PatientSearchResultModel per patient object, with the admission object being the newest one by Admission Date.
These objects are coming from nHibernate and it keeps return a Not Supported exception. There is also an association between Patient and Admissions thus : Patient.Admissions but i couldn't figure out how to then add the query filters return from the function Build???Specifications.
I'd be really grateful if someone could point me in the right direction; am I up against the Linq provider implementation here in nHibernate and need to move to Criteria or is it my Linq query ?
If anyone has any links or suggestions for good books or other learning materials in this area that would also be really helpful too.
I see several potential problems:
If you're using NHibernate 2.x + Linq2NHibernate explicit joins like that are not supported; in other versions they're just considered a smell.
I dont think NHibernate supports calling parameterized constructors in select clauses
I'm very sure NHibernate does not support calling instance methods in the select lambda
I'd suggest using the lambda syntax and SelectMany to alleviate potential join issues. Points #2 & #3 can be solved by projecting into an anonymous type, calling AsEnumerable then projecting into your model type.
Overall I'd suggest restructuring your code like:
var patientSpec = patientSearch.BuildPatientSpecification();
var admissionSpec = patientSearch.BuildAdmissionSpecification();
_patientSearchResultModel = _patientRepository.Where(patientSpec)
.SelectMany(p=>p.Admissions).Where(admissionSpec)
.Select(a=> new {
PatientId = a.Patient.Id,
AdminssionId = a.Id,
a.PhaseType,
a.Patient.Last,
a.Patient.First,
a.InPatientLocation,
a.AdmissionDate,
a.DischargeDate,
a.RRI,
a.CompletionStatus,
a.FollowupStatus
}).AsEnumerable()
.Select(x=> new PatientSearchResultModel(x.PatientId, x.AdmissionId ...))
.ToList();
Divide your query into parts and check which part runs and which doesn't.
My take on this is that select new ... is not supported in Linq to nHibernate.
I would recomend using something else, because it is simply too imature and feature-less to use seriously.
As with most popular LINQ-to-Database query providers, NHibernate will try to translate the whole query into a SQL statement to run against the database. This requires that all elements of your query are possible to express in the SQL flavour you're using.
In your query, the select new statement cannot be expressed in SQL, because you're making a call to the constructor of your PatientSearchResultModel class and are making a call to a GetPhaseTypeModel method.
You should restructure your query to express what you want to execute on the SQL database, then call AsEnumerable() to force the remainder of the query to be evaluated in-memory. After that call, you can call the constructor of your class and any .NET methods, and they will be executed as native code.
This query is too complex to describe it using Linq. It would give wrong result finally (if Patient has more than one admission records, result would have duplicate entries).
I see two steps for solution:
1) At development stage, use in-memory query. So, take Patients using ToList() first (query db at this moment). Some predicates (Patient filter like MRN, First, Last) could be used at this stage.
And then do search in-memory. Not performance, but working solution. Mark it for refactor to optimize later.
2) Finally, use NHibernate IQuery (ISQLQuery) and build sql query manually to make sure it would work as expected and work fast enough on SQL Server side. This is just read-only query and do not require Nhibernate query engine (Linq to Nhibernate) at all.
I have an IQueryable that has a list of pages.
I want to do: Pages.OrderByDescending(o => CalculateSort(o.page));
the method calculate sort is similar to that here is a plain english version:
public int calculatesort(page p)
{
int rating = (from r in db.rating select r). sum();
int comments = //query database for comments;
float timedecayfactor = math.exp(-page.totalhoursago);
return sortscore = (rating +comments)* timedecayfactor;
}
when I run a code similar to the one above an error is thrown that the mothode calculatesort cannot be converted to sql.
How can I do a conver the function above to be understood by sql so that I can use it to sort the pages?
Is this not a good approach for large data? Is there another method used to sort sets of results other than dynamically at the database?
I havent slept for days trying to fix this one :(
your code is nowhere near compiling so I'm guessing a lot here but I hope this gives an idea none the less.
As several have posted you need to give Linq-2-Sql an expression tree. Using query syntax that's what happens (by compiler magic)
from p in pages
let rating = (from r in db.rating
where r.PageId == p.PageId
select r.Value).Sum()
let comments = (from c in db.Comments
where c.PageId == p.PageId
select 1).Count()
let timedecayfactor = Math.Exp(-(p.totalhoursago))
orderby (rating + comments)*timedecayfactor descending
select p;
I haven't actually tried this against a database, there's simply too many unknown based on your code, so there might still be stuff that can't be translated.
The error occurs because LINQ cannot convert custom code/methods into SQL. It can convert only Expression<Func<>> objects into SQL.
In your case, you have a complex logic to do while sorting, so it might make sense to do it using a Stored Procedure, if you want to do it in the DB Layer.
Or load all the objects into main memory, and run the calculate sort method on the objects in memory
EDIT :
I don't have the code, so Describing in english is the best I can do :
Have table with structure capable of temporarily storing all the current users data.
Have a calculated field in the Pages table that holds the value calculated from all the non-user specific fields
Write a stored procedure that uses values from these two sources (temp table and calc field) to actually do the sort.
Delete the temp table as the last part in the stored proc
You can read about stored procs here and here
var comments = db.comments.Where(...);
Pages.OrderByDescending(p=>(db.rating.Sum(r=>r.rate) + comments.Count()) * Math.Exp(-p.totalhoursago))
Linq is expecting Calculatesort to return a "queryable" expression in order to generate its own SQL.
In can embed your 'calculatesort' method in this lambda expression. (I replaced your variables with constants in order to compile in my environment)
public static void ComplexSort(IQueryable<string> Pages)
{
Pages.OrderByDescending(p =>
{
int rating = 99;//(from r in db.rating select r). sum();
int comments = 33;//query database for comments;
double timedecayfactor = Math.Exp(88);
return (rating + comments) * timedecayfactor;
});
}
Also, you can even try to run that in parallel (since .net 4.0) replacing the first line with
Pages.AsParallel().OrderByDescending(p =>
Yes, counting previous answers: the LINQ to SQL doesn't know how to translate CalculateSort method. You should convert LINQ to SQL to ordinary LINQ to Object before using custom method.
Try to use this in the way you call the CalculateSort by adding AsEnumerable:
Pages.AsEnumerable().OrderByDescending(o => CalculateSort(o.page));
Then you're fine to use the OrderByDescending extension method.
UPDATE:
LINQ to SQL will always translate the query in the code into Expression tree. It's quite almost the same concept as AST of any programming language. These expression trees are further translated into SQL expression specific to SQL Server's SQL, because currently LINQ to SQL only supports SQL Server 2005 and 2008.
I dont't know how to describe the title better (so feel free to change) but essntial I want to produce the equivilent of the following statement in LINQ to SQL:
select * from products where category in ('shoes','boots')
The fields will be coming from the query string essentially (more secure than this but for ease of explanation) i.e
string[] myfields = request.querystring["categories"].split(',');
Thanks
Yes, you use Contains:
db.Products.Where(product => myFields.Contains(product.Category))
aka
from product in db.Products
where myFields.Contains(product.Category)
select product
As other have mentioned, yes it does using the .Contains method. To benefit the other random people that may arrive here via Bing (or any of the other search engines): Linq-To-Entities does not support the .Contains method in the current version. However, with a simple extension method, you can do so:
http://george.tsiokos.com/posts/2007/11/30/linq-where-x-in/
In theory you would use contains:
var productList = from product in context.Products
where myfields.contains(product.category)
select product
However you'll need to test this - I seem to recall there being a bug in the Linq2Sql optimizer when trying to deal with List<> and array values being used like this (it may have only occured if you tried to cast them as IQueryable)