I need to call ToShortDateString in a linq query suing lambda expressions:
toRet.Notification = Repositories
.portalDb.portal_notifications.OrderByDescending(p => p.id)
.FirstOrDefault(p => p.date.ToShortDateString() == shortDateString);
but I get the error:
An exception of type 'System.NotSupportedException' occurred in
System.Data.Entity.dll but was not handled in user code
Additional information: LINQ to Entities does not recognize the method
'System.String ToShortDateString()' method, and this method cannot be
translated into a store expression.
What can I do, considering that I do need to use ToShortDateString() ?
Thanks.
Linq to Entities cannot convert ToSortDateString method into SQL code. You can't call it on server side. Either move filtering to client side (that will transfer all data from server to client), or consider to use server-side functions to take date part of date (you should pass DateTime object instead of shortDateString):
EntityFunctions.TruncateTime(p.date) == dateWithoutTime
You shouldn't be forcing a string comparison when what you're working with is Date/time data - as soon as you force string comparisons, you're suddenly having to deal with how the strings are formatted.
Instead, have something like:
var endDate = targetDate.AddDays(1);
toRet.Notification = Repositories
.portalDb.portal_notifications.OrderByDescending(p => p.id)
.FirstOrDefault(p => p.date >= targetDate && p.date < endDate);
(Assuming that targetDate is whatever DateTime variable you had that was used to produce shortDateString in your code, and is already a DateTime with no time value)
Try this,
You can also used with below code.
Activity = String.Format("{0} {1}", String.Format("{0:dd-MMM-yyyy}", s.SLIDESHEETDATE), String.Format("{0:HH:mm}", s.ENDDATETIME))
ToShortDateString() method usually used to work only with date and ignore time stamps.
You will get exactly today result-set by using the following query.
Repositories.portalDb.portal_notifications.OrderByDescending(p => p.id)
.FirstOrDefault(p => p.date.Date == DateTime.Now.Date);
By using Date property of DateTime struct you can just fetch record of that date only.
Note: Linq to Objects. Only works if you CAN (or have option) to bypass ToShortDateString() method
Related
I have a cosmos db collection. I need to query all documents and return them in order of creation date. Creation date is a defined field but for historical reason it is in string format as MM/dd/yyyy. For example: 02/09/2019. If I just order by this string, the result is chaos.
I am using linq lambda to write my query in webapi. I have tried to parse the string and try to convert the string. Both returned "method not supported".
Here is my query:
var query = Client.CreateDocumentQuery<MyModel>(CollectionLink)
.Where(f => f.ModelType == typeof(MyModel).Name.ToLower() && f.Language == getMyModelsRequestModel.Language )
.OrderByDescending(f => f.CreationDate)
.AsDocumentQuery();
Appreciate for any advice. Thanks. It will be huge effort to go back and modify the format of the field (which affects many other things). I wish to avoid it if possible.
Chen Wang.Since the order by does not support derived values or sub query(link),so you need to sort the derived values by yourself i think.
You could construct the MM/dd/yyyy to yyyymmdd by UDF in cosmos db.
udf:
function getValue(datetime){
return datetime.substring(6,10)+datetime.substring(0,2)+datetime.substring(3,5);
}
sql:
SELECT udf.getValue(c.time) as time from c
Then you could sort the array by property value of class in c# code.Please follow this case:How to sort an array containing class objects by a property value of a class instance?
I want compare datetime (i need parse it) with now in LINQ query
From i In db.Downloads
Where i.WantExpiration And DateTime.Parse(i.Expiration) < DateTime.Now
Everytime i get error: Method 'System.DateTime Parse(System.String)' has no supported translation to SQL.
How i can in LINQ to SQL parse datetime?
You could do this:
From i In db.Downloads
Where i.WantExpiration And i.Expiration < DateTime.Now
provided that Expiration is a DateTime column in your DB.
The error you are getting is expected since LINQ-SQL translates our linq queries in sql and then send them in the db, where there are executed and the results travel back to us. That being said, there isn't something similar like DateTime.Parse() in sql, so this method cannot be converted to a corresponding one in SQL. Hence your query cannot be translated in SQL.
If Expiration isn't a DateTime column in your DB, then you could make the following trick
(I see that you query is written in VB, but since I don't use VB, I will write my own in C# and subsequently, you could change it to VB.)
// Initially you will get all your data in memory.
var query = (from d in db.Downloads
select d).AsEnumerable();
// Then you will query the in memory data.
var data = (from q in query
where q.WantExpiration &&
DateTime.Parse(q.Expiration) < DateTime.Now);
I have problem with the NHibernate's second level cache. When I use query:
var items1 = Session.Query<Row>()
.Cacheable();
.Fetch(x => x.Field)
.OrderBy(x => x.Field.Value)
.ToList();
Everything is fine - the query is cached. But when I want to use Dynamic Linq (a link):
var items2 = Session.Query<Row>()
.Cacheable();
.Fetch(x => x.Field)
.OrderBy("Field.Value")
.ToList();
The query is not cached. Interesting thing is that, when I delete code line:
.Fetch(x => x.Field)
caching works again. So the problem is with using Fetch and dynamic linq OrderBy methods together.
EDIT:
When I try do debug NH code (QueryKey class), debugger tells me that these two queries do not have the same ResultTransformer (and deeper: a listTransformation private instance).
Any ideas?
Chris
Ok, I know what is the reason.
Dynamic Linq doesn't use Parameter Names in Linq Expressions. E.g. if I want to sort using lambda statemant, I write:
query.OrderBy(item => item.Name)
Above we see an item lambda parameter name.
When I use Dynamic linq:
query.OrderBy("Name")
in the result Queryable the lambda parameter in OrderBy mehod has no name (like item written above). We can illustrate the result like this:
query.OrderBy( => .Name)
And now, when NHibernate is decoding that Queryable expression and finds there an expression parameter that has no name, NH gives it a random name using GUID class. So every ordering using dynamic linq produces a Queryable Expression that has inconstant lambda parameter. This is the reason why NHibernate thinks that: query.OrderBy("Name") and query.OrderBy("Name") are not the same - they have another lamda parameters generated every time from scratch.
SOLUTION
If you want to fix it, you have to modify Dynamic Linq library.
In method ExpressionParser.ProcessParameters change line:
if (parameters.Length == 1 && String.IsNullOrEmpty(parameters[0].Name))
to:
if (parameters.Length == 1 && (parameters[0].Name == "it" || String.IsNullOrEmpty(parameters[0].Name)))
In method DynamicQueryable.OrderBy change line:
Expression.Parameter(source.ElementType, "")
to:
Expression.Parameter(source.ElementType, "it")
Now, query.OrderBy("Name") will produce query.OrderBy(it => it.Name).
Cheers!
I have two Entity Framework 4 Linq queries I wrote that make use of a custom class method, one works and one does not:
The custom method is:
public static DateTime GetLastReadToDate(string fbaUsername, Discussion discussion)
{
return (discussion.DiscussionUserReads.Where(dur => dur.User.aspnet_User.UserName == fbaUsername).FirstOrDefault() ?? new DiscussionUserRead { ReadToDate = DateTime.Now.AddYears(-99) }).ReadToDate;
}
The linq query that works calls a from after a from, the equivalent of SelectMany():
from g in oc.Users.Where(u => u.aspnet_User.UserName == fbaUsername).First().Groups
from d in g.Discussions
select new
{
UnReadPostCount = d.Posts.Where(p => p.CreatedDate > DiscussionRepository.GetLastReadToDate(fbaUsername, p.Discussion)).Count()
};
The query that does not work is more like a regular select:
from d in oc.Discussions
where d.Group.Name == "Student"
select new
{
UnReadPostCount = d.Posts.Where(p => p.CreatedDate > DiscussionRepository.GetLastReadToDate(fbaUsername, p.Discussion)).Count(),
};
The error I get is:
LINQ to Entities does not recognize the method 'System.DateTime GetLastReadToDate(System.String, Discussion)' method, and this method cannot be translated into a store expression.
My question is, why am I able to use my custom GetLastReadToDate() method in the first query and not the second? I suppose this has something to do with what gets executed on the db server and what gets executed on the client? These queries seem to use the GetLastReadToDate() method so similarly though, I'm wondering why would work for the first and not the second, and most importantly if there's a way to factor common query syntax like what's in the GetLastReadToDate() method into a separate location to be reused in several different other LINQ queries.
Please note all these queries are sharing the same object context.
I think your better of using a Model Defined Function here.
Define a scalar function in your database which returns a DateTime, pass through whatever you need, map it on your model, then use it in your LINQ query:
from g in oc.Users.Where(u => u.aspnet_User.UserName == fbaUsername).First().Groups
from d in g.Discussions
select new
{
UnReadPostCount = d.Posts.Where(p => p.CreatedDate > myFunkyModelFunction(fbaUsername, p.Discussion)).Count()
};
and most importantly if there's a way to factor common query syntax like what's in the GetLastReadToDate() method into a separate location to be reused in several different places LINQ queries.
A stored procedure would probably be one way to store that 'common query syntax"...EF, at least 4.0, works very nicely with SP's.
I'm trying to do a query like so...
query.Where(x => !string.IsNullOrEmpty(x.PropertyName));
but it fails...
so for now I have implemented the following, which works...
query.Where(x => (x.PropertyName ?? string.Empty) != string.Empty);
is there a better (more native?) way that LINQ handles this?
EDIT
apologize! didn't include the provider... This is using LINQ to SQL
http://connect.microsoft.com/VisualStudio/feedback/ViewFeedback.aspx?FeedbackID=367077
Problem Statement
It's possible to write LINQ to SQL that gets all rows that have either null or an empty string in a given field, but it's not possible to use string.IsNullOrEmpty to do it, even though many other string methods map to LINQ to SQL.
Proposed Solution
Allow string.IsNullOrEmpty in a LINQ to SQL where clause so that these two queries have the same result:
var fieldNullOrEmpty =
from item in db.SomeTable
where item.SomeField == null || item.SomeField.Equals(string.Empty)
select item;
var fieldNullOrEmpty2 =
from item in db.SomeTable
where string.IsNullOrEmpty(item.SomeField)
select item;
Other Reading:
1. DevArt
2. Dervalp.com
3. StackOverflow Post
This won't fail on Linq2Objects, but it will fail for Linq2SQL, so I am assuming that you are talking about the SQL provider or something similar.
The reason has to do with the way that the SQL provider handles your lambda expression. It doesn't take it as a function Func<P,T>, but an expression Expression<Func<P,T>>. It takes that expression tree and translates it so an actual SQL statement, which it sends off to the server.
The translator knows how to handle basic operators, but it doesn't know how to handle methods on objects. It doesn't know that IsNullOrEmpty(x) translates to return x == null || x == string.empty. That has to be done explicitly for the translation to SQL to take place.
This will work fine with Linq to Objects. However, some LINQ providers have difficulty running CLR methods as part of the query. This is expecially true of some database providers.
The problem is that the DB providers try to move and compile the LINQ query as a database query, to prevent pulling all of the objects across the wire. This is a good thing, but does occasionally restrict the flexibility in your predicates.
Unfortunately, without checking the provider documentation, it's difficult to always know exactly what will or will not be supported directly in the provider. It looks like your provider allows comparisons, but not the string check. I'd guess that, in your case, this is probably about as good of an approach as you can get. (It's really not that different from the IsNullOrEmpty check, other than creating the "string.Empty" instance for comparison, but that's minor.)
... 12 years ago :) But still, some one may found it helpful:
Often it is good to check white spaces too
query.Where(x => !string.IsNullOrWhiteSpace(x.PropertyName));
it will converted to sql as:
WHERE [x].[PropertyName] IS NOT NULL AND ((LTRIM(RTRIM([x].[PropertyName])) <> N'') OR [x].[PropertyName] IS NULL)
or other way:
query.Where(x => string.Compare(x.PropertyName," ") > 0);
will be converted to sql as:
WHERE [x].[PropertyName] > N' '
If you want to go change the type of the collection from nullable type IEnumerable<T?> to non-null type IEnumerable<T> you can use .OfType<T>().
.OfType<T>() will remove null values and return a list of the type T.
Example: If you have a list of nullable strings: List<string?> you can change the type of the list to string by using OfType<string() as in the below example:
List<string?> nullableStrings = new List<string?> { "test1", null, "test2" };
List<string> strings = nullableStrings.OfType<string>().ToList();
// strings now only contains { "test1", "test2" }
This will result in a list of strings only containing test1 and test2.