so here's the code with irrelevant bits left out:
public IEnumerable<T> GetByQuery(Expression<Func<T, bool>> filter
{
try
{
return Session.Linq<T>().Where(filter);
}
catch(Exception ex)
{
// custom exception handling here
}
finally
{
CloseSession();
}
return null;
}
and an example of it being called looks like this:
IEnumerabl<ClientReport> clientReports =
clientReportRepository.GetByQuery(item => item.ClientId = id);
So as you can see, nothing fancy and being called in this way, we're hitting one table in the database with no relationships to any other tables. But when I have show_sql = true in the configuration, It's displaying 2 of the same query.
Any ideas?
Thanks
clientReports will probably execute the query every time you enumerate it (or get the Count(), for example).
To avoid that, use .ToList() in the assignment.
Related
I am using rxjava2 for the first time on an Android project, and am doing SQL queries on a background thread.
However I am having trouble figuring out the best way to do a simple SQL query, and being able to handle the case where the record may or may not exist. Here is the code I am using:
public Observable<Record> createRecordObservable(int id) {
Callable<Record> callback = new Callable<Record>() {
#Override
public Record call() throws Exception {
// do the actual sql stuff, e.g.
// select * from Record where id = ?
return record;
}
};
return Observable.fromCallable(callback).subscribeOn(Schedulers.computation());
}
This works well when there is a record present. But in the case of a non-existent record matching the id, it treats it like an error. Apparently this is because rxjava2 doesn't allow the Callable to return a null.
Obviously I don't really want this. An error should be only if the database failed or something, whereas a empty result is perfectly valid. I read somewhere that one possible solution is wrapping Record in a Java 8 Optional, but my project is not Java 8, and anyway that solution seems a bit ugly.
This is surely such a common, everyday task that I'm sure there must be a simple and easy solution, but I couldn't find one so far. What is the recommended pattern to use here?
Your use case seems appropriate for the RxJava2 new Observable type Maybe, which emit 1 or 0 items.
Maybe.fromCallable will treat returned null as no items emitted.
You can see this discussion regarding nulls with RxJava2, I guess that there is no many choices but using Optional alike in other cases where you need nulls/empty values.
Thanks to #yosriz, I have it working with Maybe. Since I can't put code in comments, I'll post a complete answer here:
Instead of Observable, use Maybe like this:
public Maybe<Record> lookupRecord(int id) {
Callable<Record> callback = new Callable<Record>() {
#Override
public Record call() throws Exception {
// do the actual sql stuff, e.g.
// select * from Record where id = ?
return record;
}
};
return Maybe.fromCallable(callback).subscribeOn(Schedulers.computation());
}
The good thing is the returned record is allowed to be null. To detect which situation occurred in the subscriber, the code is like this:
lookupRecord(id)
.observeOn(AndroidSchedulers.mainThread())
.subscribe(new Consumer<Record>() {
#Override
public void accept(Record r) {
// record was loaded OK
}
}, new Consumer<Throwable>() {
#Override
public void accept(Throwable throwable) {
// there was an error
}
}, new Action() {
#Override
public void run() {
// there was an empty result
}
});
I'm trying to get some data from a database whose results can be more than one row.
I've the following code for that:
public System.Linq.IQueryable<Users> getUser2(string idUser)
{
try
{
using (Entities c = new Entities())
{
c.ContextOptions.LazyLoadingEnabled = false;
c.ContextOptions.ProxyCreationEnabled = false;
return c.Users.Include("Empresas").Where(x => x.Login == idUser && x.Empresas.Activa == true);
}
}
catch (Exception ex)
{
throw ex;
}
}
But it doesn't seem to get any result, it shows something like a badly formed Iqueryable, I mean if I expand its results view I can see a message that says "ObjectContext instance has been eliminated and cannot be used for operations that need a connection" If I try to access any Users element with the function ElementAt(index) I get an IndexOutOfBounds error as it looks like it has no data if watched on debug mode.
I've deduced that it's Where fault because this code Works fine in returning the first user it finds that fulfills the condition:
public Users getUser(string idUser)
{
try
{
using (Entities c = new Entities())
{
c.ContextOptions.LazyLoadingEnabled = false;
c.ContextOptions.ProxyCreationEnabled = false;
return c.Users.Include("Empresas").FirstOrDefault(x => x.Login == idUser && x.Empresas.Activa == true);
}
}
catch (Exception ex)
{
throw ex;
}
}
Does that Where work differently than what I think I should? If then, how could I get several data that fulfills the conditions I'm passing the same as in getUser but for several rows?
Thanks for your attention.
You need to enumerate the result, so after the "where" statement add. ToList() which will enumerate and execute the query against your database. FirstOrDefault is executing the query thats why you get a result.
You need to check the deferred methods and understand how they work.
EDIT
The following are some links to show you the deference between the Deferred method vs Immediate methods in LINQ
1- http://www.dotnetcurry.com/showarticle.aspx?ID=750
2- http://www.codeproject.com/Articles/627081/LINQ-Deferred-Execution-Lazy-Evaluation
3- http://visualcsharptutorials.com/linq/deferred-execution
Hope that helps.
Lets say I have a method like this:
IQueryable<MyFlatObject> GetMyFlatObjects()
{
using (var context = new MyEntities())
{
return context.MyEntities.Select(x => new MyFlatObject()
{
Property1 = x.PropertyA,
Property2 = x.PropertyB,
Property3 = x.PropertyC,
});
}
}
Now if I call:
MyService.GetMyFlatObjects().Where(x => x.Property1 == "test");
Sanity check. This filter will not propagate to my database store (like if I had just queried my entities), but instead I will get all results back and be using LINQ-to-objects to filter. Right?
I think, it's not right. First, it doesn't query anything because you are only extending an IQueryable<T> to a new IQueryable<T>. If you call ToList() or anything else that causes the query to execute you'll get an exception because the context has already been disposed at the end of the using block. If you don't dispose the context the Where filter will be translated to SQL and executed in the database. I believe it will behave the same way as if you would apply the Where to PropertyA before the Select.
Suppose following codes:
IEnumerable<MyClass> MakeQuery()
{
var query = from m in session.Linq<MyClass>()
select m;
return query;
}
List<MyClass> m1()
{
return MakeQuery()
.Skip(10)
.Take(20)
.ToList<MyClass>();
}
List<MyClass> m2()
{
var query = from m in session.Linq<MyClass>()
select m;
return query
.Skip(10)
.Take(20)
.ToList<MyClass>();
}
Supposing all queries are same, it seems that in m1(), Skip and Take does not work. Indeed its like they do not exist all.
Why this happens and how can be fixed?
I'm using linq-to-nhibernate and this methods are used for paging. Thanks.
Why not use IQueryable for the MakeQuery() method?
IQueryable<MyClass> MakeQuery()
{
return session.Linq<MyClass>();
}
Not that the actual query makes a lot of sense. But I'll leave that to you.
But this is also the only difference between m1() and m2()
I have a piece of code which combines an in-memory list with some data held in a database. This works just fine in my unit tests (using a mocked Linq2SqlRepository which uses List).
public IRepository<OrderItem> orderItems { get; set; }
private List<OrderHeld> _releasedOrders = null;
private List<OrderHeld> releasedOrders
{
get
{
if (_releasedOrders == null)
{
_releasedOrders = new List<nOrderHeld>();
}
return _releasedOrders;
}
}
.....
public int GetReleasedCount(OrderItem orderItem)
{
int? total =
(
from item in orderItems.All
join releasedOrder in releasedOrders
on item.OrderID equals releasedOrder.OrderID
where item.ProductID == orderItem.ProductID
select new
{
item.Quantity,
}
).Sum(x => (int?)x.Quantity);
return total.HasValue ? total.Value : 0;
}
I am getting an error I don't really understand when I run it against a database.
Exception information:
Exception type: System.NotSupportedException
Exception message: Local sequence cannot be used in LINQ to SQL
implementation of query operators
except the Contains() operator.
What am I doing wrong?
I'm guessing it's to do with the fact that orderItems is on the database and releasedItems is in memory.
EDIT
I have changed my code based on the answers given (thanks all)
public int GetReleasedCount(OrderItem orderItem)
{
var releasedOrderIDs = releasedOrders.Select(x => x.OrderID);
int? total =
(
from item in orderItems.All
where releasedOrderIDs.Contains(item.OrderID)
&& item.ProductID == orderItem.ProductID
select new
{
item.Quantity,
}
).Sum(x => (int?)x.Quantity);
return total.HasValue ? total.Value : 0;
}
I'm guessing it's to do with the fact
that orderItems is on the database
and releasedItems is in memory.
You are correct, you can't join a table to a List using LINQ.
Take a look at this link:
http://flatlinerdoa.spaces.live.com/Blog/cns!17124D03A9A052B0!455.entry
He suggests using the Contains() method but you'll have to play around with it to see if it will work for your needs.
It looks like you need to formulate the db query first, because it can't create the correct SQL representation of the expression tree for objects that are in memory. It might be down to the join, so is it possible to get a value from the in-memory query that can be used as a simple primitive? For example using Contains() as the error suggests.
You unit tests work because your comparing a memory list to a memory list.
For memory list to database, you will either need to use the memoryVariable.Contains(...) or make the db call first and return a list(), so you can compare memory list to memory list as before. The 2nd option would return too much data, so your forced down the Contains() route.
public int GetReleasedCount(OrderItem orderItem)
{
int? total =
(
from item in orderItems.All
where item.ProductID == orderItem.ProductID
&& releasedOrders.Contains(item.OrderID)
select new
{
item.Quantity,
}
).Sum(x => (int?)x.Quantity);
return total.HasValue ? total.Value : 0;
}