I'm currently shifting my data access code from L2S to Entity framework db first. I have problem with queries like following
var emps = (from emp in DataContext.setupEmployees
let contacts = DataContext.setupContacts.Where(x => x.EmployeeID == emp.EmployeeID).Select(x => x.Contact)
select new
{
EmployeeName = emp.EmployeeName,
Contacts = string.Join(", ", contacts.ToArray())
}).ToList();
EF tells me it can't convert the method string.join into stored expression. One obvious work-around is to bring the objects in memory (ToList, AsEnumerable etc) and then call string.Join method like
var emps = (from emp in DataContext.setupEmployees
let contacts = DataContext.setupContacts.Where(x => x.EmployeeID == emp.EmployeeID).Select(x => x.Contact)
select new
{
EmployeeName = emp.EmployeeName,
Contacts = contacts
}).ToList().Select(x=>new{
x.EmployeeName,
Contacts = string.Join(", ",x.Contacts)
});
This solution works perfectly but it's just that it's more verbose and I'd have to duplicate select clause in lots of queries that used to work fine in L2S.
My question: is there any way I can avoid writing this ToList thing and call string.Join and similar methods in EF query?
I would recommend creating Contacts as an IEnumerable<string> datatype instead of string datatype. You can than use Contacts to produce any output you like comma seperated etc...
You can change your query to look like this:
var emps = (from emp in DataContext.setupEmployees
select new
{
EmployeeName = emp.EmployeeName,
Contacts = DataContext.setupContacts.Where(x => x.EmployeeID == emp.EmployeeID).Select(x => x.Contact)
}).ToList();
Related
I have entities like Product(Id, Name) and Keyword(Id, Description), and there is a many-to-many relationship between them.
The essence of my task is the following, I need to do a full-text search on Name and Description columns, using EF CORE 6
I already have some SQL code that works fine.
SELECT a."Id", a."Name" as name, k.txt
FROM "Products" AS a
LEFT JOIN (
SELECT x."ProductsId" as Id, string_agg(y."Description", ' ') as txt
FROM "ProductKeywords" x
JOIN "Keywords" y ON y."Id" = x."KeywordId"
GROUP BY 1
) k ON a."Id" = k.Id
WHERE to_tsvector(concat_ws(' ', a."Name", k.txt))
## to_tsquery('Some text');
And I need to write some LINQ code that will do something similar, but I have a problem with string_agg, and I don't understand how to implement it in LINQ and EF CORE will reflect it correctly
I tried to do the following
var products = _context.Products
.Select(e => new
{
Id = e.Id,
Name = e.Name,
Keywords = string.Join(" ", e.Keywords.Select(q => q.Description))
}).Where(e => EF.Functions.ToTsVector(e.Keywords).Matches("Some text")).ToList();
But I get an error, and it's most likely because of string.Join
could not be translated. Either rewrite the query in a form that can be translated, or switch to client evaluation explicitly by inserting a call to 'AsEnumerable', 'AsAsyncEnumerable', 'ToList', or 'ToListAsync'
Got the result, using linq2db
var query = _context.Products.ToLinqToDB()
.LeftJoin(_context.ProductsKeywords.ToLinqToDB().GroupBy(r => r.ProductId).Select(e => new {
Key = e.Key,
Txt = e.StringAggregate(",", t => t.Keyword.Description).ToValue()
}),
(product, productKeyword) => product.Id == productKeyword.Key,
(i, j) => new {
Id = i.Id,
Txt = j.Txt
}).Select(e => new {
Id = e.Id,
Txt = EF.Functions.ToTsVector(e.Txt)
}).Where(w => w.Txt.Matches("Some text"));
I updated my .net core 2.2 to 5
I have a error about ef that
System.InvalidOperationException: 'The LINQ expression 'x' could not
be translated. Either rewrite the query in a form that can be
translated, or switch to client evaluation explicitly by inserting a
call to 'AsEnumerable', 'AsAsyncEnumerable', 'ToList', or
'ToListAsync'. See https://go.microsoft.com/fwlink/?linkid=2101038 for
more information.'
public List<CustomerPageModel> GetCustomers(int AccountID)
{
return (from p in context.Customers
join f in context.Patients on p.ID equals f.CustomerID into ps
from t in ps.DefaultIfEmpty()
where p.AccountID == AccountID
select new CustomerPageModel
{
ID = p.ID,
Name = p.Name,
IsActive = p.IsActive,
TC = p.TC,
Surname = p.Surname,
Email = p.Email,
Address = p.Address,
Phone = p.Phone,
Note = p.Note,
AccountID = p.AccountID,
Pats = string.Join(",", ps.Select(x => x.Name)),
PatCount = ps.Count()
})
.GroupBy(p => p.ID)
.Select(g => g.First())
.ToList();
}
How can I convert that code?
Your problem line is:
Pats = string.Join(",", ps.Select(x => x.Name)),
Specifically, the string.Join method doesn't translate to SQL, so in previous versions of EF, it had to retrieve the data from the database then in-memory preform the string.Join function. Now EF explicitly tells you it can't run that on the database server - this is a breaking change but a design decision that tells you (the developer) that it may not have been running as efficiently as you thought it was...
To "fix" this, and based on your particular code example I'd recommend the following:
Add a pet names array property to your CustomerPageModel:
public string[] PetNames {get;set;}
And turn the Pets property into a readonly calculated string:
public string Pets { get => string.Join(",", PetNames); }
And change the problem line in your LINQ expression to:
PetNames = ps.Select(x => x.Name).ToArray()
I changed (lamb to subquery)
string.Join(",", ps.Select(x => x.Name))
to
string.Join(",", (from y in PatientList where y.CustomerID == p.ID select y.Name).ToArray()),
I made the group later (after tolist)
var test = mylist.GroupBy(p => p.ID)
.Select(g => g.First())
.ToList();
problem solved
I have a table called "UserAnswers".below screenshot contains table data
I want to get data by surveyId and group by CreatedBy column.
for an example
There is a user called "amara#gmail.com".this user contains 4 records for a SurveyId.
I want to get this like below
Answers : [
{"2"},
{"1","0","1","1"},
{"1","2","4","3"},
{"Blue"}]
But my code returns this array for every rows.I meant duplicate records returning.
Here is my code
var qstns = await (from uans in _context.UserAnswers
where uans.SurveyId == id
select new UserAnswersReturnDto
{
UserEmail = uans.CreatedBy,
Qustns = (from ans in _context.UserAnswers
where ans.CreatedBy == uans.CreatedBy
select new UserAnswersSet
{
QNo = ans.QNo,
Ansrs = JsonConvert.DeserializeObject<JArray>(string.IsNullOrEmpty(ans.Answers) ? "[]" : ans.Answers)
}).ToArray()
}).ToListAsync();
So how to solve this issue.I opened many questions for this problem,but no one answered.Please help me.Thanks in advanced
You need to actually group your data before returning:
I used LINQ Lambda notation, but it should be quite easy to translate back to query if you're so inclined:
var qstns = _context.UserAnswers.Where(uans => uans.SurveyId == id)
.GroupBy(uans => uans.CreatedBy)
.Select(grans => new UserAnswersReturnDto {
UserEmail = grans.Key,
Qustions = grans.Select(ans => new UserAnswersSet() {
QNo = ans.QNo,
Ansrs = ans.Answers
}).ToList()
} ).ToList();
I didn't have time to double-check this, but I hope it serves as a guide to help you solve your issue!
There is no group by statement in your linq query.
I have the following EF code
Func<CxForumArticle, bool> whereClause = a => a.CreatedBy == authorId;
IEnumerable<CxForumArticle> articlesCol = ctx.Articles
.Where(whereClause)
.Where(a => a.PublishingStatus == EnPublishStatus.PUBLISHED)
.OrderByDescending(a => a.ModifiedOn).Skip(offset).Take(pageSize);
It produces the following SQL
SELECT
[Extent1].[ArticleId] AS [ArticleId],
[Extent1].[Alias] AS [Alias],
[Extent1].[MigratedId] AS [MigratedId],
[Extent1].[Title] AS [Title],
[Extent1].[Teaser] AS [Teaser],
[Extent1].[ClobId] AS [ClobId],
[Extent1].[UnifiedContentId] AS [UnifiedContentId],
[Extent1].[EditorComments] AS [EditorComments],
[Extent1].[CreatedOn] AS [CreatedOn],
[Extent1].[CreatedBy] AS [CreatedBy],
[Extent1].[ModifiedOn] AS [ModifiedOn],
[Extent1].[ModifiedBy] AS [ModifiedBy],
[Extent1].[PublishingStatus] AS [PublishingStatus]
FROM [dbo].[ForumArticle] AS [Extent1]
As you see, there is no ordering and paging in this SQL. So EF orders and pages data in memory.
This doesn't seem to be a good thing to do.
I read an article, claiming that I have to use expression in OrderBy clause. I did that
Func<CxForumArticle, bool> whereClause = a => a.CreatedBy == authorId;
Expression<Func<CxForumArticle, DateTime>> orderByFunc = a => a.ModifiedOn;
IEnumerable<CxForumArticle> articlesCol = ctx.Articles
.Where(whereClause)
.Where(a => a.PublishingStatus == EnPublishStatus.PUBLISHED)
.OrderByDescending(orderByFunc.Compile()).Skip(offset).Take(pageSize)
;
But I got the same result. Any ideas how can I force EF to sort and page the data in DB?
I know this is pretty old, but you actually need to have the whereClause use an expression as well. Entity Framework only knows how to convert expressions to SQL. If you use a delegate instead of an expression, it is going to query the entire table and then then filter using LINQ to Objects.
I have an extension method defined like so:
public static TSource MaxBy<TSource, TResult>(this IEnumerable<TSource> collection, Func<TSource, TResult> func) where TSource : class
{
var comparer = Comparer<TSource>.Default;
TSource maxItem = null;
foreach (var item in collection)
{
if (comparer.Compare(item, maxItem) > 0)
maxItem = item;
}
return maxItem;
}
which I then use in the following LINQ-to-Entities query:
var balancesQuery = from t in db.Transactions
where t.UserId == userId
group t by t.CurrencyCode into tg
let tMaxDate = tg.MaxBy(i => i.TsCreate)
join c in db.Currencies on tg.Key equals c.CurrencyCode
select new { Currency = c, Balance = tMaxDate.Balance }
So what I'm doing is - get the newest transaction (MaxBy TsCreate) in each currency (group by CurrencyCode) and then select the balance against each of those transactions.
My problem is - this does not work with Entity Framework (LINQ-to-Entities; I get:
LINQ to Entities does not recognize the method 'Transaction MaxBy[Transaction,DateTime](System.Collections.Generic.IEnumerable'[Transaction], System.Func'2[Transaction,System.DateTime])' method, and this method cannot be translated into a store expression.
The same query works with LINQ to SQL.
My questions are:
Is there a way to make it work with Entity Framework?
Or maybe there is a better way of querying for the same information, which would work with Entity Framework?
Thanks in advance for any help!
Isn't the query same as:
var balancesQuery = from t in db.Transactions
where t.UserId == userId
group t by t.CurrencyCode into tg
join c in db.Currencies on tg.Key equals c.CurrencyCode
select new {
Currency = c,
Balance = tg.OrderByDescending(i => i.TsCreate).Take(1).Balance
};
You can't use extensions with LINQ queries that actually pull data from the database as it's impossible for this to be turned into SQL. However, you can do this by returning the results into memory using ToArray() or ToList() to trigger the database query executing and then call your extension functions on resultant real data in memory.