I am writing a web application which is readonly, I mean data is read from a database and presented in the UI, so my BLL service methods consist mainly of linq queries like:
return context.SomeLinqQuery
If I need some calculation I do it within Select() method and I wonder is it correct?
The other question is - is it a better way to write below example query? I would like to write the code in line with good practices and I am not sure if putting everything in Select() is one of them.
I wonder too, if calling _context within Select() is wrong or not. If it is wrong, how can I resolve it? Should I load SomeProperty in additional loop?
_context.SomeEntity
.Where(some condition)
.Select(x => new MyModel()
{
Id = x.Id,
Name = x.Name,
SomeProperty = x.Status == 1
? _context.DbViewModel.Where(condition).Select...
: _context.AnotherDbViewModel.Where(condition).Select...
});
If I need some calculation I do it within Select() method and I wonder is it correct?
Yes! Correct! as long as EF Core is able to convert the expression generated by your LINQ query into T-SQL.
The other question is - is it a better way to write below example query? I would like to write the code in line with good practices and I am not sure if putting everything in Select() is one of them.
It will depend on the relationship between your entities. Better to build the query using navigation properties.
I wonder too, if calling _context within Select() is wrong or not. If it is wrong, how can I resolve it? Should I load SomeProperty in additional loop?
Nothing wrong if EF Core is able to evaluate the query in database level.
The way to improve EF Query is to let DB do all Query and return expected view directly .
I cannot contribute more code, but you code should simliar like this:
_context.SomeEntity
.Join(DbViewModel,condition)...
.Join(AnotherDbViewModel,condition)...
.Where(some condition)
.Select({
Id = x.Id,
Name = x.Name,
SomeProperty = x.SomePropety.
})
Another solution is to use DBQuery, Here is a good example. and also, use ExecuteSqlCommand or FromSQL
if your LINQ is very complex, you can also use CompiledQuery to improve more.
Related
(as advised re-posting this question here... originally posted in msdn forum)
I am striving to write a "generic" routine for some simple CRUD operations using EF/Linq to Entities. I'm working in ASP.NET (C# or VB).
I have looked at:
Getting a reference to a dynamically selected table with "GetObjectByKey" (But I don't want anything from cache. I want data from database. Seems like not what this function is intended for).
CRM Dynamic Entities (here you can pass a tablename string to query) looked like the approach I am looking for but I don't get the idea that this CRM effort is necessarily staying current (?) and/or has much assurance for the future??
I looked at various ways of drilling thru Namespaces/Objects to get to where I could pass a TableName parameter into the oft used query syntax var query = (from c in context.C_Contacts select c); (for example) where somehow I could swap out the "C_Contacts" TEntity depending on which table I want to work with. But not finding a way to do this ??
Slightly over-simplyfing, I just want to be able to pass a tablename parameter and in some cases some associated fieldnames and values (perhaps in a generic object?) to my routine and then let that routine dynamically plug into LINQ to Entity data context/model and do some standard "select all" operations for parameter table or do a delete to parameter table based on a generic record id. I'm trying to avoid calling the various different automatically generated L2E methods based on tablename etc...instead just trying to drill into the data context and ultimately the L2E query syntax for dynamically passed table/field names.
Has anyone found any successful/efficient approaches for doing this? Any ideas, links, examples?
The DbContext object has a generic Set() method. This will give you
from c in context.Set<Contact>() select c
Here's method when starting from a string:
public void Test()
{
dynamic entity = null;
Type type = Type.GetType("Contract");
entity = Activator.CreateInstance(type);
ProcessType(entity);
}
public void ProcessType<TEntity>(TEntity instance)
where TEntity : class
{
var result =
from item in this.Set<TEntity>()
select item;
//do stuff with the result
//passing back to the caller can get more complicated
//but passing it on will be fine ...
}
I have DbContext (called "MyContext") with about 100 DbSets within it.
Among the domain classes, I have a Document class with 10 direct subclasses (like PurchaseOrder, RequestForQuotation etc).
The heirarchy is mapped with a TPT strategy.
That is, in my database, there is a Document table, with other tables like PurchaseOrder, RequestForQuotation for the subclasses.
When I do a query like:
Document document = myContext.Documents.First();
the query took 5 seconds, no matter whether it's the first time I run it or subsequently.
A query like:
Document document = myContext.Documents.Where(o => o.ID == 2);
also took as long.
Is this an issue with EF4.1 (if so, will EF4.2 help) or is this an issue with the query codes?
Did you try using SQL Profile to see what is actually sent to the DB? It could be that you have too many joins on your Document that are not set to lazy load, and so the query has to do all the joins in one go, bringing back too many columns. Try to send a simple query with just one return column.
As you can read here, there are some performance issues regarding TPT in EF.
The EF Team annouced several fixes in the June 2011 CTP, including TPT queries optimization, but they are not included in EF 4.2, as you can read in the comments to this answer.
In the worst case, these fixes will only be released with .NET 4.5. I'm hoping it will be sooner...
I'm not certain that the DbSet exposed by code-first actually using ObjectQuery but you can try to invoke the .ToTraceString() method on them to see what SQL is generated, like so:
var query = myContext.Documents.Where(o => o.ID == 2);
Debug.WriteLine(query.ToTraceString());
Once you get the SQL you can determine whether it's the query or EF which is causing the delay. Depending on the complexity of your base class the query might include a lot of additional columns, which could be avoided using projection. With using projections, you can perform a query like this:
var query = from d in myContext.Documents
where d.ID == 2
select new
{
o.Id
};
This should basically perform a SELECT ID FROM Documents WHERE ID = 2 query and you can measure how long this takes to gain further information. Of course the projected query might not fit your needs but it might get you on the right track. If this still takes up to 5 seconds you should look into performance problems with the database itself rather than EF.
Update
Apparently with code-first you can use .ToString() instead of .ToTraceString(), thanks Slauma for noticing.
I've just had a 5 sec delay in ExecuteFunction, on a stored procedure that runs instantaneously when called from SQL Management Studio. I fixed it by re-writing the procedure.
It appears that EF (and SSRS BTW) tries to do something like a "prepare" on the stored proc and for some (usually complex) procs that can take a very long time.
A quick and dirty solution is to duplicate and then replace your SP parameters with internal variables:
create proc ListOrders(#CountryID int = 3, #MaxOrderCount int = 20)
as
declare #CountryID1 int, #MaxOrderCount1 int
set #CountryID1 = #CountryID
set #MaxOrderCount1 = #MaxOrderCount
select top (#MaxOrderCount1) *
from Orders
where CountryID = #CountryID1
Sorry if this is a really stupid question, or if it's been answered anywhere else but I'm having trouble finding an answer.
I want to be able to filter a list of entities by a foreign key of a related entity without having to eager load that entity using .Include("ParentEntity"). This doesn't seem like an edge case to me, selecting a complete object when all I want is the FK, which is hidden - it's even in the returned data when that the generated SQL is executed.
This is what a I want to do:
from s in EfContext.Child
where s.Parent.Id == 1
select s;
This is what I have to do:
from s in EfContext.Child
.Include("Parent")
where s.Parent.Id == 1
select s;
Now you may think this isn't a big deal, but when you're dealing with the rubbish that is the Entity Framework inheritance implementation, that Include statement generates several hundred lines of SQL if the entity you're including is the base class to a bunch of other entities :( - So I'm trying to find a workaround.
I found this tip which talks about faking the FK, but it suggests you can't use the faked property in a LINQ statement.
I can't use EF4 either as I'm stuck with .net 3.5 sp1
Thanks all.
UPDATE:
So I've developed a work around for my problem, what I'm after is someone to smack me upside the head and tell me that this is a totally ridiculous way of doing things :)
Parent parent = new Parent{Id = 1};
_ctx.AttachTo("Parent",parent);
parent.Children.Load();
//This requirement wasn't in the original question, but I've included for reference
foreach (Child child in Parent.Children)
{
child.GrandChildReference.Load();
}
return from p in parent.Children
select p;
You say:
This is what a I want to do:
... and show this code:
from s in EfContext.Child
where s.Parent.Id == 1
select s;
Have you tried it? This works fine in EF 1. No Include() needed.
Greetings,
Considering the Northwind sample tables Customers, Orders, and OrderDetails I would like to eager load the related entities corresponding to the tables mentioned above and yet I need ot order the child entities on the database before fetching entities.
Basic case:
var someQueryable = from customer in northwindContext.Customers.Include("Orders.OrderDetails")
select customer;
but I also need to sort Orders and OrderDetails on the database side (before fetching those entities into memory) with respect to some random column on those tables. Is it possible without some projection, like it is in T-SQL? It doesn't matter whether the solution uses e-SQL or LINQ to Entities. I searched the web but I wasn't satisfied with the answers I found since they mainly involve projecting data to some anonymous type and then re-query that anonymous type to get the child entities in the order you like. Also using CreateSourceQuery() doesn't seem to be an option for me since I need to get the data as it is on the database side, with eager loading but just by ordering child entities. That is I want to do the "ORDER BY" before executing any query and then fetch the entities in the order I'd like. Thanks in advance for any guidance. As a personal note, please excuse the direct language since I am kinda pissed at Microsoft for releasing the EF in such an immature shape even compared to Linq to SQL (which they seem to be getting away slowly). I hope this EF thingie will get much better and without significant bugs in the release version of .NET FX 4.0.
Actually I have Tip that addresses exactly this issue.
Sorting of related entities is not 'supported', but using the projection approach Craig shows AND relying on something called 'Relationship Fixup' you can get something very similar working:
If you do this:
var projection = from c in ctx.Customers
select new {
Customer = c,
Orders = c.Orders.OrderByDescending(
o => o.OrderDate
)
};
foreach(var anon in projection )
{
anon.Orders //is sorted (because of the projection)
anon.Customer.Orders // is sorted too! because of relationship fixup
}
Which means if you do this:
var customers = projection.AsEnumerable().Select(x => x.Customer);
you will have customers that have sorted orders!
See the tip for more info.
Hope this helps
Alex
You are confusing two different problems. The first is how to materialize entities in the database, the second is how to retrieve an ordered list. The EntityCollection type is not an ordered list. In your example, customer.Orders is an EntityCollection.
On the other hand, if you want to get a list in a particular order, you can certainly do that; it just can't be in a property of type EntityCollection. For example:
from c in northwindContext.Customers
orderby c.SomeField
select new {
Name = c.Name,
Orders = from o in c.Orders
orderby c.SomeField
select new {
SomeField = c.SomeField
}
}
Note that there is no call to Include. Because I am projecting, it is unnecessary.
The Entity Framework may not work in the way you expect, coming from a LINQ to SQL background, but it does work. Be careful about condemning it before you understand it; deciding that it doesn't work will prevent you from learning how it does work.
Thank you both. I understand that I can use projection to achieve what I wanted but I thought there might be an easy way to do it since in T-SQL world it's perfectly possible with a few nested queries (or joins) and order bys. On the other hand seperation of concerns sounds reasonable and we are in the entity domain now so I will use the way you two both recommended though I have to admit this is easier and cleaner to achieve in LINQ to SQL by using AssociateWith.
Kind regards.
In a repository, I do this:
public AgenciesDonor FindPrimary(Guid donorId) {
return db.AgenciesDonorSet.Include("DonorPanels").Include("PriceAdjustments").Include("Donors").First(x => x.Donors.DonorId == donorId && x.IsPrimary);
}
then down in another method in the same repository, this:
AgenciesDonor oldPrimary = this.FindPrimary(donorId);
In the debugger, the resultsview shows all records in that table, but:
oldPrimary.Count();
is 1 (which it should be).
Why am I seeing all table entries retrieved, and not just 1? I thought row filtering was done in the DB.
If db.EntitySet really does fetch everything to the client, what's the right way to keep the client data-lite using EF? Fetching all rows won't scale for what I'm doing.
You will see everything if you hover over the AgenciesDonorSet because LINQ to Entities (or SQL) uses delayed execution. When the query is actually executed, it is just retrieving the count.
If you want to view the SQL being generated for any query, you can add this bit of code:
var query = queryObj as ObjectQuery; //assign your query to queryObj rather than returning it immediately
if (query != null)
{
System.Diagnostics.Trace.WriteLine(context);
System.Diagnostics.Trace.WriteLine(query.ToTraceString());
}
Entity Set does not implement IQueryable, so the extension methods that you're using are IEnumerable extension methods. See here:
http://social.msdn.microsoft.com/forums/en-US/linqprojectgeneral/thread/121ec4e8-ce40-49e0-b715-75a5bd0063dc/
I agree that this is stupid, and I'm surprised that more people haven't complained about it. The official reason:
The design reason for not making
EntitySet IQueryable is because
there's not a clean way to reconcile
Add\Remove on EntitySet with
IQueryable's filtering and
transformation ability.