I have the following LINQ query; I want to remove dot character if available at the end of string
var hatoList = (from L in db.vHatoList select L.HatoFullName.Trim('.')).ToList();
You're currently trying to perform the query in the database. We don't know what database technology you're using, so it's hard to know whether there's some way to make it actually happen in the database, but it's probably simplest to do in memory instead (given that it won't change how many records will be returned). The AsEnumerable() method is used to just change the compile-time type to IEnumerable<T> (instead of IQueryable<T>) forcing the rest of the query to be evaluated in .NET code instead of in the database:
var hatoList = db.vHatoList
.AsEnumerable()
.Select(entry => entry.HatoFullName.TrimEnd('.'))
.ToList();
(I've also changed the call from Trim to TrimEnd to avoid it removing periods at the start of the string.)
You are using linq to sql and database does not understand TrimEnd or Trim.
So first you have need to load data in memory and then use TrimEnd method.
var hatoList = (from L in db.vHatoList select L.HatoFullName).ToList().
Select(t=>t.TrimEnd('.'));
Related
I'm writing a timesheet application (Silverlight) and I'm completely stuck on getting linq queries working. I'm netw to linq and I just read, and did many examples from, a Linq book, including Linq to Objects, linq to SQl and linq to Entities.(I assume, but am not 100% sure that the latter is what Lightswitch uses). I plan to study a LOT more Linq, but just need to get this one query working.
So I have an entity called Items which lists every item in a job and it's serial no
So: Job.ID int, ID int, SerialNo long
I also have a Timesheets entity that contains shift dates, job no and start and end serial no produced
So Job.ID int, ShiftDate date, Shift int, StartNo long, EndNo long
When the user select a job from an autocomplete box, I want to look up the MAX(SerialNo) for that job in the timesheets entity. If that is null (i.e. none have been produced), I want to lookup the MIN(SerialNo) from the Items entity for that job (i.e. what's the first serial no they should produce)
I realize I need a first or default and need to specify the MIN(SerialNo) from Items as a default.
My Timesheet screen uses TimesheetProperty as it's datasource
I tried the following just to get the MAX(SerialNo) from Timesheets entity:
var maxSerialNo =
(from ts in this.DataWorkspace.SQLData.Timesheets
where ts.Job.ID == this.TimesheetProperty.Job.ID
select ts.StartNo).Min();
but I get the following errors:
Instance argument: cannot convert from 'Microsoft.LightSwitch.IDataServiceQueryable' to 'System.Collections.Generic.IEnumerable
'Microsoft.LightSwitch.IDataServiceQueryable' does not contain a definition for 'Min' and the best extension method overload 'System.Linq.Enumerable.Min(System.Collections.Generic.IEnumerable)' has some invalid arguments
I also don't get why I can't use this:
var maxSerialNo = this.DataWorkspace.SQLData.Timesheets.Min(ts => ts.StartNo);
Can anyone point me in the right direction?
Thanks
Mark
IDataServiceQueryable doesn't support full set of LINQ operator like IEnumerable has.
IDataServiceQueryable – This is a LightSwitch-specific type that allows a restricted set of “LINQ-like” operators that are remote-able to the middle-tier and ultimately issued to the database server. This interface is the core of the LightSwitch query programming model. IDataServiceQueryable has a member to execute the query, which returns results that are IEnumerable. [Reference]
Possible solution is, execute your query first to get collection of type IEnumerable by calling .ToList(), then you can call .Min() against the first query result. But that isn't good idea if you have large amount of data, because .ToList() will retrieve all data match the query and do further processing in client side, which is inefficient.
Another way is, change your query using only operators supported by IDataServiceQueryable to avoid retrieving unnecessary data to client. For example, to get minimum StartNo you can try to use orderby descending then get the first data instead of using .Min() operator :
var minStartNo =
(
from ts in this.DataWorkspace.SQLData.Timesheets
where ts.Job.ID == this.TimesheetProperty.Job.ID
orderby ts.StartNo descending select ts
).FirstOrDefault();
Is there a way to do
"UPDATE Item SET start_date = CURRENT_TIMESTAMP" ?
in Nhibernate without using hql/sql.
I am trying to avoid hql/sql because the rest of my code is in criteria. I want to do something like :
var item = session.get<Item>(id)
item.start_date = current_timestamp
There are two ways and sql is correct one.
Either you will
load all entities, change, update and commit, or
write sql query and let dbms handle most of the work
I am trying to avoid hql/sql because the rest of my code is in criteria
That is not a valid argument. Criteria is an API intended for relational search, and it does not support mass updates.
Different tasks, different APIs.
In this case, you can use either HQL or SQL, as the syntax is the same. I recommend the former, because you'll be using your entity/property names instead of table/column ones.
suppose my LINQ query is like
var qry = from c in nwEntitiesContext.CategorySet.AsEnumerable()
let products = this.GetProducts().WithCategoryID(c.CategoryID)
select new Model.Category
{
ID = c.CategoryID,
Name = c.CategoryName,
Products = new Model.LazyList<Core.Model.Product>(products)
};
return qry.AsQueryable();
i just want to know what query it will generate at runtime....how to see what query it is generating from VS2010 IDE when we run the code in debug mode....guide me step by step.
There is not much to see here - it will just select all fields from the Category table since you call AsEnumerable thus fetching all the data from the Category table into memory. After that you are in object space. Well, depending on what this.GetProducts() does - and my guess it makes another EF query fetching the results into memory. If that's the case, I would strongly recommend you to post another question with this code and the code of your GetProducts method so that we can take a look and rewrite this in a more optimal way. (Apart from this, you are projecting onto a mapped entity Model.Category which again won't (and should not) work with Linq-to-Entities.)
Before reading into your query I was going to recommend doing something like this:
string sqlQueryString = ((ObjectQuery)qry).ToTraceString();
But that won't work since you are mixing Linq-to-Entities with Linq-to-objects and you will actually have several queries executed in case GetProducts queries EF. You can separate the part with your EF query and see the SQL like this though:
string sqlString = nwEntitiesContext.CategorySet.ToTraceString();
but as I mentioned earlier - that would just select everything from the Categories table.
In your case (unless you rewrite your code in a drastic way), you actually want to see what queries are run against the DB when you execute the code and enumerate the results of the queries. See this question:
exact sql query executed by Entity Framework
Your choices are SQL Server Profiler and Entity Framework Profiler. You can also try out LinqPad, but in general I still recommend you to describe what your queries are doing in more detail (and most probably rewrite them in a more optimal way before proceeding).
Try Linqpad
This will produce SELECT * FROM Categories. Nothing more. Once you call AsEnumerable you are in Linq-to-objects and there is no way to get back to Linq-to-entities (AsQueryable doesn't do that).
If you want to see what query is generated use SQL Profiler or any method described in this article.
I'm tasked with adding an option to our search, which will return results where a given field doesn't begin with a letter of the alphabet. (The .StartsWith(letter) part wasn't so hard).
But I'm rather unsure about how to get the results that don't fall within the A-Z set, and equally hoping it generates some moderately efficient SQL underneath.
Any help appreciated - thanks.
In C# use the following construct, assuming db as a data context:
var query = from row in db.SomeTable
where !System.Data.Linq.SqlClient.SqlMethods.Like(row.SomeField, "[A-Z]%")
select row;
This is only supported in LINQ to SQL queries. All rules of the T-SQL LIKE operator apply.
You could also use less effective solution:
var query = from row in db.SomeTable
where row.SomeField[0] < 'A' || row.SomeField[0] > 'Z'
select row;
This gets translated into SUBSTRING, CAST, and UNICODE constructs.
Finally, you could use VB, where there appears to be a native support for the Like method.
Though SQL provides the ability to check a range of characters in a LIKE statement using bracket notation ([a-f]% for example), I haven't seen a linq to sql construct that supports this directly.
A couple thoughts:
First, if the result set is relatively small, you could do a .ToList() and filter in memory after the fact.
Alternatively, if you have the ability to change the data model, you could set up additional fields or tables to help index the data and improve the search.
--EDIT--
Made changes per Ruslan's comment below.
Well, I have no idea if this will work because I have never tried it and don't have a compiler nearby to try it, but the first thing I would try is
var query = from x in db.SomeTable
where x.SomeField != null &&
x.SomeField.Length >= 1 &&
x.SomeField.Substring(0, 1).All(c => !Char.IsLetter(c))
select x;
The possiblility exists that LINQ to SQL fails to convert this to SQL.
I have a LINQ query mapped with the Entity Framework that looks something like this:
image = this.Context.ImageSet
.Where(n => n.ImageId == imageId)
.Where(n => n.Albums.IsPublic == true)
.Single();
This returns a single image object and works as intended.
However, this query returns all the properties of my Image table in the DB.
Under normal circumstances, this would be fine but these images contain a lot of binary data that takes a very long time to return.
Basically, in it current state my linq query is doing:
Select ImageId, Name, Data
From Images
...
But I need a query that does this instread:
Select ImageId, Name
From Images
...
Notice i want to load everything except the Data. (I can get this data on a second async pass)
Unfortunately, if using LINQ to SQL, there is no optimal solution.
You have 3 options:
You return the Entity, with Context tracking and all, in this case Image, with all fields
You choose your fields and return an anonymous type
You choose your fields and return a strongly typed custom class, but you lose tracking, if thats what you want.
I love LINQ to SQL, but thats the way it is.
My only solution for you would be to restructure your DataBase, and move all the large Data into a separate table, and link to it from the Image table.
This way when returning Image you'd only return a key in the new DataID field, and then you could access that heavier Data when and if you needed it.
cheers
This will create a new image with only those fields set. When you go back to get the Data for the images you select, I'd suggest going ahead and getting the full dataset instead of trying to merge it with the existing id/name data. The id/name fields are presumably small relative to the data and the code will be much simpler than trying to do the merge. Also, it may not be necessary to actually construct an Image object, using an anonymous type might suit your purposes just as well.
image = this.Context.ImageSet
.Where(n => n.ImageId == imageId)
.Where(n => n.Albums.IsPublic == true)
.Select( n => new Image { ImageId = n.ImageId, Name = n.Name }
.Single();
[If using Linq 2 SQL] Within the DBML designer, there is an option to make individual table columns delay-loaded. Set this to true for your large binary field. Then, that data is not loaded until it is actually used.
[Question for you all: Does anyone know if the entity frameworks support delayed loaded varbinary/varchar's in MSVS 2010? ]
Solution #2 (for entity framework or linq 2 sql):
Create a view of the table that includes only the primary key and the varchar(max)/varbinary(max). Map that into EF.
Within your Entity Framework designer, delete the varbinary(max)/varchar(max) property from the table definition (leaving it defined only in the view). This should exclude the field from read/write operations to that table, though you might verify that with the logger.
Generally you'll access the data through the table that excludes the data blob. When you need the blob, you load a row from the view. I'm not sure if you'll be able to write to the view, I'm not sure how you would do writes. You may be able to write to the view, or you may need to write a stored procedure, or you can bust out a DBML file for the one table.
You cannot do it with LINQ at least for now...
The best approach I know is create View for the table you need without large fields and use LINQ with that View.
Alternatively you could use the select new in the query expression...
var image =
(
from i in db.ImageSet
where i.ImageId == imageId && i.Albums.IsPublic
select new
{
ImageId = i.ImageId,
Name = i.Name
}
).Single()
The LINQ query expressions actually get converted to the Lambda expression at compile time, but I prefair using the query expression generally because i find it more readable and understandable.
Thanks :)