I am having trouble mixing c# functions with conditions in Linq-To-SQL
suppose i have a database table "things" and a local c# Function : bool isGood(thing, params)
I want to use that function to select rows from the table.
var bad = dataContext.Things.Where(t=>t.type=mytype && !isGood(t,myparams))
dataContect.Things.deleteAllOnSubmit(bad);
or
if (dataContext.Things.Any(t=>t.type=mytype && isGood(t,myparams)))
{
return false;
}
Of course this does not work, Linq has no way of translating my function into a SQL statement. So this will produce:
NotSupportedException: Method 'Boolean isGood(thing,params)' has no supported translation to SQL.
What is the best way to redesign this so that it will work?
I can split the statements and convert to list like this:
List<Things> mythings dataContext.Things.Where(t=>t.type=mytype).toList()
if (mythings.Any(t=>isGood(t,myparams)))
{
return false;
}
this works, but seems inefficient, since the whole list has to be generated in every case.
And I don't think I can do a deleteAllOnSubmit with the result
I could do a foreach over mythings instead of calling toList(), that also works. Seems inelegant though.
What other options do I have, and what would be the recommended approach here?
edit:
calling asEnumerable() seems to be another option, and seems better than toList() at least. I.e.
dataContext.Things.Where(t=>t.type=mytype).asEnumerable().Any(t=>isGood(t,myparams))
Pulling the whole list back from the database to run a local c# function on it might seem inefficient, but that what you'd have to do if your isGood() function is local.
If you can translate your isGood() function into Linq, you could apply it before the toList() call, so it would get translated into SQL and the whole list wouldn't be retrieved.
Related
I ran across some code in an older project I am working on that I've never seen before, and has me confused on it's intent.
updatables.Select(r =>
{
// some operations are done here for each element in the list
return true;
}).ToArray();
It seems like a select statement is being used to iterate the updatables collection. Also seems the ToArray call isn't doing anything.
My question is, what does calling return true in the Select statement accomplish, if anything?
This looks very much like a hack to emulate ForEach:
ToArray() call is added to ensure that updatables will be iterated to completion,
return true is added to silence the compiler that does not allow Action<T>, but allows Func<T,bool> in LINQ's Select.
I would strongly recommend against writing code like this, because it is a lot less readable than an equivalent foreach loop.
Select takes a Func<T, TResult> - which means it won't accept an Action<T>. In other words, a lambda which does not return anything will result in a compilation error when passed to Select, so the author bypassed that "limitation" by having it return a dummy value.
The intent behind this code is likely to run a foreach loop on the collection using the LINQ syntax. However, the way it's done in this code is a bad practice, as LINQ methods are expected to be pure - that is, not modify any sort of state outside of the expression.
I have some function (X) which return IQueryable with "OrderBy(x => x.Name)".
I want to use X function in new code, but in this code there is parameter which determine the order (asc/desc).
I prefer not to change (X) since it used in multiple places already.
there is option to get X(), then cancel it's "OrderBy", and apply new order?
(now it throws exception, since it is like Y.OrderBy(a => a.Name).OrderByDescending(a => a.Name))
"A column has been specified more than once in the order by list. Columns in the order by list must be unique.\r\nStatement(s) could not be prepared."
use
public void x(string order = string.empty)
{
Y.OrderBy(a => a.Name);
if (order == "desc")
{
Y = Y.Reverse();
}
}
I've tried to wiggle Expression object of IOrderedQueryable that your method is actually returning (am I wrong?) but so far no luck, and I can't see the option to manipulate System.Linq.Expression or System.Linq.EnumerableQuery easy enough to use it instead of changing X() method.
IMHO, you should choose from:
create somes kind of wrapper method using the Reverse() (#Eric Lizotte answer) or OrderByDescending() - it will be less time consuming I guess but for grouping, filtering etc. you will have to create brand new function.
remove OrderBy() from X() function body, and modify your old code - returning non-ordered result will give you more flexibility and opportunity to do what you want to do with the result via linq expressions in each scenario that you'll came across.
Can you clarify for me if you're talking about "function" you have in mind a sql function that is mapped via ORM (eg. Entity Framework) in your application or just a ordinary method which is using your custom provider to return IQueryable?
I have a question related to this previous question of mine. In an existing bit of LINQ which involves a number of joins, I'm trying to take each separate method comprising the join and convert it to a CompiledQuery.
First, the normal LINQ method:
private IQueryable<Widget> GetWidgetQuery()
{
return db.Widgets.Where(u => (!u.SomeField.HasValue || !u.SomeField.Value));
}
And here, a delegate (field) definition for a CompiledQuery along these lines:
private static readonly Func<DBDataContext, IQueryable<Widget>> GetWidgetQuery =
CompiledQuery.Compile((DBDataContext db) =>
db.Widgets.Where(u => (!u.SomeField.HasValue || !u.SomeField.Value)));
If I hover over the normal LINQ statement for the method GetWidgetQuery(), I see that it's a method as below:
(method) IQueryable<Widget> GetWidgetQuery()
However, the compiled query delegate (field) differs as follows:
(field) Func<DBDataContext, IQueryable<Widget>> GetWidgetQuery
Upon executing the latter as part of the LINQ statement, the syntax differs as follows. First, the normal LINQ's participation in the join:
var myquery =
from wxr in GetWidgetXRQuery()
join w in GetWidgetQuery() on wxr.WidgetID equals w.ID
select new DTO.WidgetList
{
...
}
And here, the invocation of the CompiledQuery in the form of the delegate:
var myquery =
from wxr in GetWidgetXRQuery()
join w in GetWidgetQuery.Invoke(myContext) on wxr.WidgetID equals w.ID
select new DTO.WidgetList
{
...
}
The former returns the expected result set; the latter, when I attempt myquery.ToList(), yields a stackoverflow exception, in part related to this limitation of .NET 3.5, I think.
Can someone please help me understand how the compiled statement existing as a field (or I guess I should say a delegate) rather than a method is killing my query? In short I know what I'm doing is wrong, but I'm not sure I understand what I misunderstand.
I tried doing roughly the same thing you're doing on EF 4, and everything seems to work fine. So it's either an EF 3.5 issue, or it has something to do with your implementation of GetWidgetXRQuery, or some combination of the two.
But the real point I'd like to make is that, as Roy Goode stated in an answer to your previous question, you lose all the advantages of a precompiled query once you extend that query in any way. By trying to perform a Join on your query, you are converting it to just a plain old query. So you might as well just use the non-compiled version which appears to work for you.
Update
Realized you were talking about LINQ to SQL. This sort of query does appear to have support in Entity Framework, but not LINQ to SQL. In .NET 4, I'm getting the following error:
An IQueryable that returns a self-referencing Constant expression is not supported.
That doesn't mean much to me, but I'm guessing that it has something to do with the way the compiled query is represented internally. I still get the same error if I evaluate the query into a variable and use that variable in the query later, so it clearly has nothing to do with the difference between a delegate and a function. I still maintain that a compiled query is not appropriate to use here. Either you need to create one big compiled query to represent the whole query you want to perform, or you need to use regular queries if you want to piece them together this way.
I just came across this same error while doing db integration testing, and to jump straight to the point without trying to explain my specific issue. Linq to Sql will create the sql query internally when using IQueryable and the moment you execute a method on that IQueryable, i.e. ToList() it executes that query on the database. So in my case I am joining to a method that returns IQueryable but is mocked to return a result, it is trying to compile that to a sql query but the IQueryable I created does not have an internal SQL query
What happens behind the curtains when I include a function into my compiled query, like I do with DataConvert.ToThema() here to convert a table object into my custom business object:
public static class Queries
{
public static Func<MyDataContext, string, Thema> GetThemaByTitle
{
get
{
var func = CompiledQuery.Compile(
(MyDataContext db, string title) =>
(from th in elan.tbl_Thema
where th.Titel == title
select DataConvert.ToThema(th)).Single()
);
return func;
}
}
}
public static class DataConvert
{
public static Thema ToThema(tbl_Thema tblThema)
{
Thema thema = new Thema();
thema.ID = tblThema.ThemaID;
thema.Titel = tblThema.Titel;
// and some other stuff
return thema;
}
}
and call it like this
Thema th = Queries.GetThemaByTitle.Invoke(db, "someTitle");
Apparently the function is not translated in to SQL or something (how could it), but it also does not hold when I set a breakpoint there in VS2010.
It works without problems, but I don't understand how or why. What exactly happens there?
Your DataConvert.ToThema() static method is simply creating an instance of a type which has a default constructor, and setting various properties, is that correct? If so, it's not terribly different from:
(from th in elan.tbl_Thema
where th.Titel == title
select new Thema{ID=th.ThemaID, Titel=th.Titel, etc...}
).Single());
When you call Queries.GetThemaByTitle, a query is being compiled. (The way you are calling this, by the way, may or may not actually be giving you any benefits from pre-compiling). That 'Query' is actually a code expression tree, only part of which is intended to generate SQL code that is sent to the database.
Other parts of it will generate IL code which is grabbing what is returned from the database and putting it into some form for your consumption. LINQ (EF or L2S) is smart enough to be able to take your static method call and generate the IL from it to do what you want - and maybe it's doing so with an internal delegate or some such. But ultimately, it doesn't need to be (much) different from what would be generated from I substituted above.
But note that this happens regardless what the type is that you get back; somewhere, IL code is being generated that puts DB values into a CLR object. That is the other part of those expression trees.
If you want a more detailed look at those expression trees and what they involved, I'd have to dig for ya, but I'm not sure from your question if that's what you are looking for.
Let me start by pointing out, that whether you compile your query or not does not matter. You would observe the very same results even if you did not pre-compile.
Technically, as Andrew has pointed out, making this work is not that complicated. When your LINQ expression is evaluated an expression tree is constructed internally. Your function appears as a node in this expression tree. No magic here. You'll be able to write this expression both in L2S and L2E and it will compile and run fine. That is until you try to actually execute the actual SQL query against the database. This is where difference begins. L2S seems to happily execute this task, whereas L2E fails with NotSupportedException, and reporting that it does not know how to convert ToThema into store query.
So what's happening inside? In L2S, as Andrew has explained, the query compiler understands that your function can be run separately from the store query has been executed. So it emits calls to your function into the object reading pipeline (where data read from SQL is transformed to the objects that are returned as the result of your call).
Once thing Andrew was not quite right, is that it matters what's inside your static method. I don't think it does.
If you put a break point in the debugger to your function, you will see that it's called once per returned row. In the stack trace you will see "Lightweight Function", which, in reality, means that the method was emitted at run time. So this is how it works for Linq to Sql.
Linq to Entity team seemed to go different route. I do not know, what was the reasoning, why they decided to ban all InvocationExpressions from L2E queries. Perhaps these were performance reason, or may be the fact that they need to support all kind of providers, not SQL Server only, so that data readers might behave differently. Or they simply thought that most people wouldn't realize that some of those are executed per returned row and preferred to keep this option closed.
Just my thoughts. If anyone has any more insight, please chime in!
I use LINQ-SQL as my DAL, I then have a project called DB which acts as my BLL. Various applications then access the BLL to read / write data from the SQL Database.
I have these methods in my BLL for one particular table:
public IEnumerable<SystemSalesTaxList> Get_SystemSalesTaxList()
{
return from s in db.SystemSalesTaxLists
select s;
}
public SystemSalesTaxList Get_SystemSalesTaxList(string strSalesTaxID)
{
return Get_SystemSalesTaxList().Where(s => s.SalesTaxID == strSalesTaxID).FirstOrDefault();
}
public SystemSalesTaxList Get_SystemSalesTaxListByZipCode(string strZipCode)
{
return Get_SystemSalesTaxList().Where(s => s.ZipCode == strZipCode).FirstOrDefault();
}
All pretty straight forward I thought.
Get_SystemSalesTaxListByZipCode is always returning a null value though, even when it has a ZIP Code that exists in that table.
If I write the method like this, it returns the row I want:
public SystemSalesTaxList Get_SystemSalesTaxListByZipCode(string strZipCode)
{
var salesTax = from s in db.SystemSalesTaxLists
where s.ZipCode == strZipCode
select s;
return salesTax.FirstOrDefault();
}
Why does the other method not return the same, as the query should be identical ?
Note that, the overloaded Get_SystemSalesTaxList(string strSalesTaxID) returns a record just fine when I give it a valid SalesTaxID.
Is there a more efficient way to write these "helper" type classes ?
Thanks!
This is probably down to the different ways LINQ handles IEnumerable<T> and IQueryable<T>.
You have declared Get_SystemSalesTaxList as returning IEnumerable<SystemSalesTaxList>. That means that when, in your first code sample, you apply the Where operator to the results of Get_SystemSalesTaxList, it gets resolved to the Enumerable.Where extension method. (Note that what matters is the declared type. Yes, at runtime Get_SystemSalesTaxList is returning an IQueryable<SystemSalesTaxList>, but its declared type -- what the compiler sees -- is IEnumerable<SystemSalesTaxList>.) Enumerable.Where runs the specified .NET predicate over the target sequence. In this case, it iterates over all the SystemSalesTaxList objects returned by Get_SystemSalesTaxList, yielding the ones where the ZipCode property equals the specified zip code string (using the .NET String == operator).
But in your last code sample, you apply the Where operator to db.SystemSalesTaxList, which is declared as being of type IQueryable<SystemSalesTaxList>. So the Where operator in that sample gets resolved to Queryable.Where, which translates the specified predicate expression to SQL and runs it on the database.
So what's different in the zip code methods is that the first one runs the C# s.ZipCode == strZipCode test in .NET, and the second translates that into a SQL query WHERE ZipCode = 'CA 12345' (parameterised SQL really but you get the idea). Why do these give different results? Hard to be sure, but the C# == predicate is case-sensitive, and depending on your collation settings the SQL may or may not be case-sensitive. So my suspicion is that strZipCode doesn't match the database zip codes in case, but in the second version SQL Server collation is smoothing this over.
The best solution is probably to change the declaration of Get_SystemSalesTaxList to return IQueryable<SystemSalesTaxList>. The major benefit of this is that it means queries built on Get_SystemSalesTaxList will be executed database side. At the moment, your methods are pulling back EVERYTHING in the database table and filtering it client side. Changing the declaration will get your queries translated to SQL and they will run much more efficiently, and hopefully will solve your zip code issue into the bargain.
The real issue here is the use of IEnumerable<T>, which breaks "composition" of queries; this has two effects:
you are reading all (or at least, more than you need) of your table each time, even if you ask for a single row
you are running LINQ-to-Objects rules, so case-sensitivity applies
Instead, you want to be using IQueryable<T> inside your data layer, allowing you to combine multiple queries with additional Where, OrderBy, Skip, Take, etc as needed and have it build the TSQL to match (and use your db's case-sensitivity rules).
Is there a more efficient way to write these "helper" type classes ?
For more efficient (less code to debug, doesn't stream the entire table, better use of the identity-map to short-circuit additional lookups (via FirstOrDefault etc)):
public IEnumerable<SystemSalesTaxList> Get_SystemSalesTaxList()
{
return db.SystemSalesTaxLists;
}
public SystemSalesTaxList Get_SystemSalesTaxList(string salesTaxID)
{
return db.SystemSalesTaxLists.FirstOrDefault(s => s.SalesTaxID==salesTaxID);
}
public SystemSalesTaxList Get_SystemSalesTaxListByZipCode(string zipCode)
{
return db.SystemSalesTaxLists.FirstOrDefault(s => s.ZipCode == zipCode);
}