getting this error on where clause.
trying to fetch records from sql server with linQ query in entity framework.
var Stud = contextSchool.Student.Where(x => x.STDNT_ID == lstStudent[i].StudentID).FirstOrDefault();
if i store value of list in a variable and then use in where clause, it works but not with list.
complete error:
linQ to entities does not recognize the method Models.Repository.Student get_Item(Int32)' method, and this method cannot be translated into a store expression.
i also get this error on some other projects, the problem is that u cant use methods like get_Item or f.e. AddDays() in ur linq command.
So u kinda have to play around it.
U have to create an object (list, or whatever) before ur linq command and fill it with the data u need (per methods) and use the variable in ur linq command.
I know some differences of LINQ to Entities and LINQ to Objects which the first implements IQueryable and the second implements IEnumerable and my question scope is within EF 5.
My question is what's the technical difference(s) of those 3 methods? I see that in many situations all of them work. I also see using combinations of them like .ToList().AsQueryable().
What do those methods mean, exactly?
Is there any performance issue or something that would lead to the use of one over the other?
Why would one use, for example, .ToList().AsQueryable() instead of .AsQueryable()?
There is a lot to say about this. Let me focus on AsEnumerable and AsQueryable and mention ToList() along the way.
What do these methods do?
AsEnumerable and AsQueryable cast or convert to IEnumerable or IQueryable, respectively. I say cast or convert with a reason:
When the source object already implements the target interface, the source object itself is returned but cast to the target interface. In other words: the type is not changed, but the compile-time type is.
When the source object does not implement the target interface, the source object is converted into an object that implements the target interface. So both the type and the compile-time type are changed.
Let me show this with some examples. I've got this little method that reports the compile-time type and the actual type of an object (courtesy Jon Skeet):
void ReportTypeProperties<T>(T obj)
{
Console.WriteLine("Compile-time type: {0}", typeof(T).Name);
Console.WriteLine("Actual type: {0}", obj.GetType().Name);
}
Let's try an arbitrary linq-to-sql Table<T>, which implements IQueryable:
ReportTypeProperties(context.Observations);
ReportTypeProperties(context.Observations.AsEnumerable());
ReportTypeProperties(context.Observations.AsQueryable());
The result:
Compile-time type: Table`1
Actual type: Table`1
Compile-time type: IEnumerable`1
Actual type: Table`1
Compile-time type: IQueryable`1
Actual type: Table`1
You see that the table class itself is always returned, but its representation changes.
Now an object that implements IEnumerable, not IQueryable:
var ints = new[] { 1, 2 };
ReportTypeProperties(ints);
ReportTypeProperties(ints.AsEnumerable());
ReportTypeProperties(ints.AsQueryable());
The results:
Compile-time type: Int32[]
Actual type: Int32[]
Compile-time type: IEnumerable`1
Actual type: Int32[]
Compile-time type: IQueryable`1
Actual type: EnumerableQuery`1
There it is. AsQueryable() has converted the array into an EnumerableQuery, which "represents an IEnumerable<T> collection as an IQueryable<T> data source." (MSDN).
What's the use?
AsEnumerable is frequently used to switch from any IQueryable implementation to LINQ to objects (L2O), mostly because the former does not support functions that L2O has. For more details see What is the effect of AsEnumerable() on a LINQ Entity?.
For example, in an Entity Framework query we can only use a restricted number of methods. So if, for example, we need to use one of our own methods in a query we would typically write something like
var query = context.Observations.Select(o => o.Id)
.AsEnumerable().Select(x => MySuperSmartMethod(x))
ToList – which converts an IEnumerable<T> to a List<T> – is often used for this purpose as well. The advantage of using AsEnumerable vs. ToList is that AsEnumerable does not execute the query. AsEnumerable preserves deferred execution and does not build an often useless intermediate list.
On the other hand, when forced execution of a LINQ query is desired, ToList can be a way to do that.
AsQueryable can be used to make an enumerable collection accept expressions in LINQ statements. See here for more details: Do i really need use AsQueryable() on collection?.
Note on substance abuse!
AsEnumerable works like a drug. It's a quick fix, but at a cost and it doesn't address the underlying problem.
In many Stack Overflow answers, I see people applying AsEnumerable to fix just about any problem with unsupported methods in LINQ expressions. But the price isn't always clear. For instance, if you do this:
context.MyLongWideTable // A table with many records and columns
.Where(x => x.Type == "type")
.Select(x => new { x.Name, x.CreateDate })
...everything is neatly translated into a SQL statement that filters (Where) and projects (Select). That is, both the length and the width, respectively, of the SQL result set are reduced.
Now suppose users only want to see the date part of CreateDate. In Entity Framework you'll quickly discover that...
.Select(x => new { x.Name, x.CreateDate.Date })
...is not supported (at the time of writing). Ah, fortunately there's the AsEnumerable fix:
context.MyLongWideTable.AsEnumerable()
.Where(x => x.Type == "type")
.Select(x => new { x.Name, x.CreateDate.Date })
Sure, it runs, probably. But it pulls the entire table into memory and then applies the filter and the projections. Well, most people are smart enough to do the Where first:
context.MyLongWideTable
.Where(x => x.Type == "type").AsEnumerable()
.Select(x => new { x.Name, x.CreateDate.Date })
But still all columns are fetched first and the projection is done in memory.
The real fix is:
context.MyLongWideTable
.Where(x => x.Type == "type")
.Select(x => new { x.Name, DbFunctions.TruncateTime(x.CreateDate) })
(But that requires just a little bit more knowledge...)
What do these methods NOT do?
Restore IQueryable capabilities
Now an important caveat. When you do
context.Observations.AsEnumerable()
.AsQueryable()
you will end up with the source object represented as IQueryable. (Because both methods only cast and don't convert).
But when you do
context.Observations.AsEnumerable().Select(x => x)
.AsQueryable()
what will the result be?
The Select produces a WhereSelectEnumerableIterator. This is an internal .Net class that implements IEnumerable, not IQueryable. So a conversion to another type has taken place and the subsequent AsQueryable can never return the original source anymore.
The implication of this is that using AsQueryable is not a way to magically inject a query provider with its specific features into an enumerable. Suppose you do
var query = context.Observations.Select(o => o.Id)
.AsEnumerable().Select(x => x.ToString())
.AsQueryable()
.Where(...)
The where condition will never be translated into SQL. AsEnumerable() followed by LINQ statements definitively cuts the connection with entity framework query provider.
I deliberately show this example because I've seen questions here where people for instance try to 'inject' Include capabilities into a collection by calling AsQueryable. It compiles and runs, but it does nothing because the underlying object does not have an Include implementation anymore.
Execute
Both AsQueryable and AsEnumerable don't execute (or enumerate) the source object. They only change their type or representation. Both involved interfaces, IQueryable and IEnumerable, are nothing but "an enumeration waiting to happen". They are not executed before they're forced to do so, for example, as mentioned above, by calling ToList().
That means that executing an IEnumerable obtained by calling AsEnumerable on an IQueryable object, will execute the underlying IQueryable. A subsequent execution of the IEnumerable will again execute the IQueryable. Which may be very expensive.
Specific Implementations
So far, this was only about the Queryable.AsQueryable and Enumerable.AsEnumerable extension methods. But of course anybody can write instance methods or extension methods with the same names (and functions).
In fact, a common example of a specific AsEnumerable extension method is DataTableExtensions.AsEnumerable. DataTable does not implement IQueryable or IEnumerable, so the regular extension methods don't apply.
ToList()
Execute the query immediately
AsEnumerable()
lazy (execute the query later)
Parameter: Func<TSource, bool>
Load EVERY record into application memory, and then handle/filter them. (e.g. Where/Take/Skip, it will select * from table1, into the memory, then select the first X elements) (In this case, what it did: Linq-to-SQL + Linq-to-Object)
AsQueryable()
lazy (execute the query later)
Parameter: Expression<Func<TSource, bool>>
Convert Expression into T-SQL (with the specific provider), query remotely and load result to your application memory.
That’s why DbSet (in Entity Framework) also inherits IQueryable to get the efficient query.
Do not load every record, e.g. if Take(5), it will generate select top 5 * SQL in the background. This means this type is more friendly to SQL Database, and that is why this type usually has higher performance and is recommended when dealing with a database.
So AsQueryable() usually works much faster than AsEnumerable() as it generate T-SQL at first, which includes all your where conditions in your Linq.
ToList() will being everything in memory and then you will be working on it.
so, ToList().where ( apply some filter ) is executed locally.
AsQueryable() will execute everything remotely i.e. a filter on it is sent to the database for applying.
Queryable doesn't do anything til you execute it. ToList, however executes immediately.
Also, look at this answer Why use AsQueryable() instead of List()?.
EDIT :
Also, in your case once you do ToList() then every subsequent operation is local including AsQueryable(). You can't switch to remote once you start executing locally.
Hope this makes it a little bit more clearer.
Encountered a bad performance on below code.
void DoSomething<T>(IEnumerable<T> objects){
var single = objects.First(); //load everything into memory before .First()
...
}
Fixed with
void DoSomething<T>(IEnumerable<T> objects){
T single;
if (objects is IQueryable<T>)
single = objects.AsQueryable().First(); // SELECT TOP (1) ... is used
else
single = objects.First();
}
For an IQueryable, stay in IQueryable when possible, try not be used like IEnumerable.
Update. It can be further simplified in one expression, thanks Gert Arnold.
T single = objects is IQueryable<T> q?
q.First():
objects.First();
How can I achieve following query method in Entity Framework,
below is a snippet from NHibernate documentation http://docs.jboss.org/hibernate/orm/3.3/reference/en/html/querycriteria.html
Example example = Example.create(cat)
.excludeZeroes() //exclude zero valued properties
.excludeProperty("color") //exclude the property named "color"
.ignoreCase() //perform case insensitive string comparisons
.enableLike(); //use like for string comparisons
List results = session.createCriteria(Cat.class)
.add(example)
.list();
Entity framework is LINQ based. Linq is said to be a declarative language, which means so much as telling what to do in stead of how to do it (imperative). A statement like
context.Orders.Select(o => o.OrderDate).Distinct();
is a declarative shortcut, if you like, for a 'ceremonial' foreach statement in which OrderDates are added to a list if they were not added to it before.
I'm not an expert in NHibernate or its criteria API, but the criteria API seems to be even more declarative than linq. That makes it hard to compare them. A few differences:
The main one: query by example is not possible in EF.
There is no way in linq to set behaviours for a whole query. For instance, if you want to exclude zero valued properties, you'll have to specify each one of them in a where predicate (which is closer to telling how to filter).
Case sensitivity is downright underdeveloped in EF. For example, a statement like
People.Where(c => string.Compare( c.Name, "z", false) > 0)
will generate the same SQL as
People.Where(c => string.Compare( c.Name, "z", true) > 0)
The database collation determines the case sensitiveness of string comparisons.
You can do LIKE queries, but, again, specified for each individual predicate:
People.Where (c => c.Name.Contains("a"))
(again: no differentiation in case)
So I can't really give a linq translation of your criteria query. I'd have to know the class properties to be able to specify all individual predicates.
I made a SqlDependency service in my application. It works perfectly when I type the queries by hand but I cannot include wildcards (I don't really know why).
For example:
//Using this SqlCommand will work
new SqlCommand("SELECT [employees].[name] FROM [dbo].[employees]", sqlNotificationConn)
//But this one won't
new SqlCommand("SELECT [employees].* FROM [dbo].[employees]", sqlNotificationConn)
//And this one won't either
new SqlCommand("SELECT * FROM [dbo].[employees]", sqlNotificationConn)
So basically, I want to get my DbContext to generate a full SELECT command with every fields it deals with.
In Linq 2 SQL, I used this service using dbContext.GetCommand(.....);
In EF 4.0 (or was it 4.1?), I used dbContext.employee.ToTraceString();
But in EF 4.4, I can't find anything to generate that SELECT query string....
With DbContext (DbQuery) it is as simple as:
query.ToString()
With ObjectContext (ObjectQuery):
((ObjectQuery)query).ToTraceString()
By the way, query can be any expression based on a DbSet (or ObjectSet, respectively). So something like dbContext.employee.Where(e => e.Name == "Gates").ToString() will also show the generated SQL query.
A LINQ statement that forces execution, like ToList(), Single(), FirstOrDefault(), etc, creates a new object and ToString() will return the object's type name.
ToTraceString() is still in EF, same place it always was. However, it's on ObjectQuery, not DbQuery. Having said that, I've never seen the SQL Server provider for EF use *; it always uses discrete fields in SQL.
I am trying to run following query in entity framework 3.5
var test = from e in customers where IsValid(e) select e;
Here IsValid function takes current customer and validate against some conditions and returns false or true. But when I am trying to run the query it is giving error "LINQ Method cannot be translated into a store expression." Can any body tell me any other approach?
One approach I can think of is to write all validation conditions here, but that will make the code difficult to read.
Thanks
Ashwani
As the error text implies, the EF Linq provider can not translate the IsValid method into SQL.
If you want the query to execcute in the DB, then you would have to write your validation conditions in the linq statement.
Or if you simply want to fetch all customers and then check if they are valid you could do:
var test = from e in customers.ToList() //execute the query directly w/o conditions
where IsValid(e)
select(e);