I am new to Entity Framework, and ORM's for that mather.
In the project that I'm involed in we have a legacy database,
with all its keys as strings, case-insensitive.
We are converting to MSSQL and want to use EF as ORM,
but have run in to a problem.
Here is an example that illustrates our problem:
TableA has a primary string key,
TableB has a reference to this primary key.
In LINQ we write something like:
var result = from t in context.TableB select t.TableA;
foreach( var r in result )
Console.WriteLine( r.someFieldInTableA );
if TableA contains a primary key that reads "A", and TableB contains two rows that references TableA but with different cases in the referenceing field, "a" and "A".
In our project we want both of the rows to endup in the result, but only the one
with the matching case will end up there.
Using the SQL Profiler, I have noticed that both of the rows are selected.
Is there a way to tell Entity Framework that the keys are case insensitive?
Edit:We have now tested this with NHibernate and come to the conclution that NHibernate works with case-insensitive keys. So NHibernate might be a better choice for us.I am however still interested in finding out if there is any way to change the behaviour of Entity Framework.
Thanks for your answer!
Problem is that if we add that constraint to the database now,
the legacy application might stop working because of how it is built.
Best for us would be, if possible, to change the behavior of EF.
I'm guessing it is not possible, but I'm giving it a shot.
Regards,
Fredrik
edit: The reason why I added an answer to my own question was that I added this question before I was a registerd user, and when I had registred my account I couldn't add comments or edit my post. Now the accounts are merged.
I think you need to make the change to the schema in SQL Server, not in EF. This post's answer, on how to make a column case-sensitive, looks like it will do the trick: T-SQL: How do I create a unique key that is case sensitive?
I know this isn't a perfect solution, but in LINQ why not do the join yourself. EF doesn't work because the .Designer.cs file returns objA.Equals(objB) when doing the join. .Equals is case sensitive.
var result = from t1 in context.TableB
join t2 in context.TableA on t1.someFieldInTableB.ToUpper() equals t2.someFieldInTableA.ToUpper();
Hackish I know, but LINQ to Entities is still in its infancy and the object classes that are designed are designed for specific reasons that do not handle exceptional cases in a design such as this.
Another alternative is that you can create your own code generator using T4 templates. Since everything is a public partial class you can create a navigation property that actually does the case insensitive comparisson that you are looking for.
To answer your question truthfully though, there is no "out of the box" way to get EF to do a navigation using case insensitive searching.
I came up with a workaround that "stitches up" the string based association in memory after the context has retrieved the rows from the database (hint: making using of the context.[EntityTypeCollection].Local property. You can see my answer at https://stackoverflow.com/a/12557796/62278
I know this isn't a perfect solution, but in LINQ why not do the join yourself. EF
doesn't work because the .Designer.cs file returns objA.Equals(objB) when doing the >> join. .Equals is case sensitive.
Well, not if you override the Equals method
The generated domain classes in EF are partial no? So it's fairly easy to replace the default Equals implementation of these classes by your own implementations (which of course would render it case insensitive )
BTW : a technique dat dates back from .NET 1.0
With all this .NET 3.5/4.0, Linq and Lambda violence, people tend to forget about the basics
As an alternative to the Entity Framework, you can use LINQ to SQL, which works well with relations involving case sensitive collations. Although this ORM does not offer all the flexibility of EF or NHibernate, it can be sufficient in many cases.
I've recently posted a thread on the official Microsoft Entity Framework forum:
http://social.msdn.microsoft.com/Forums/en-US/adodotnetentityframework/thread/d4aa6880-31b3-4ff2-b7f5-e2694d76772e
Related
I have some entities in my EF data model where the entity's key is a composite of two columns.
Made-up (but reasonably close) example... Three entities
Patients
PatientID (PK)
PatientName, etc
Identifiers
PatientID (PK)
IdentifierTypeID (PK)
Code
StartDate, etc
IdentifierTypes
IdentifierTypeID (PK)
Description
The Identifiers table has a composite key. Basically a many-to-many between Patients and IdentifierTypes but with data on the join table. It ought not to matter, but in case it does, the IDs are GUIDs (uniqueidentifier in SQL Server).
I can bend my database a bit to make a new column in Identifiers such as "IdentifierID" that could be the primary key, but we have a legacy code base I'd rather not modify if I can avoid it.
I can see several solutions that avoid modifying the tables although I'm not sure if all are actually possible in WebAPI
a)
Create a view in the database and have this as the basis of my entity. The view is of the form
select
*,
IdentifierID = cast( PatientID as varchar(50) ) + '_' + cast( IdentifierTypeID as varchar(50) )
from Identifiers
This has the problem that seeks on IdentifierID from Entity Framework will be slow. I can rectify this somewhat with an indexed view I suppose.
b)
Same as (a), but in my controller's [Queryable] Get() method I instead use ODataQueryOptions and some expression parsing magic (not very experienced with it at all) to see if we're filtering on IdentifierID and, if so, split that up into filters on PatientID and IdentifierTypeID
c)
I keep things as they are and discover some unknown support for Tuple as entity key in the WebAPI OData stack. That would be awesome :)
I'm inheriting from EntitySetController<>, so it would look like I'm inheriting from EntitySetController>
d)
Some blend of (c) that's not quite as clever but still achieves the same outcome
It seems possible to have composite keys in the OData spec as this other question on StackOverflow says how to address such an entity in OData (using WCF Data Services). How to address entity that uses composite identity key in OData Url?
I don't mind using nightly builds or even using the ASP.Net source - I've spent the weekend stepping through OData requests already to solve another issue (my fault) formatting GUIDs in query strings :P I'm hoping someone's already invested the hours understanding this and I can avoid another day hitting breakpoints in the framework source.
Thanks for your help,
Ian
It is possible to do composite keys with web API OData. You don't even need to use the nightly build to do that. RTM bits should be good. Refer to this great sample by Hongye for details.
I have a dumb question. It would be great if this could be done, but I am not holding my breath.
I need a single column from a table linked to my JPA entity to be a collection in said JPA entity. Is there any way, that I can just get back that column alone that is related to that entity, instead of having to get back an entire table (which could be very costly?)
Can I perform a query inside that JPA entity that will be performed and loaded eagerly into a collection?
I am trying to avoid having to make several calls to the database by just executing a couple of queries.
What are your thoughts on this?
#ElementCollection(fetch=FetchType.EAGER)
#CollectionTable(name="QUICK_LAUNCH_DISTLIST",joinColumns=#JoinColumn(name="QUICK_LAUNCH_ID"))
#Column(name="LIST_ID")
private List<Long> distListIDs;
The ElementCollection attribute is what I was looking for. It seems to work pretty well in addition to that.
Thanks for the help and inspiration guys.
Suppose a Category has many products:
select product.name from Category c inner join c.products product where ...
If that's not what you want, please show an example in your question.
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.
This is a follow-up to an earlier question I posted on EF4 entity keys with SQL Compact. SQL Compact doesn't allow server-generated identity keys, so I am left with creating my own keys as objects are added to the ObjectContext. My first choice would be an integer key, and the previous answer linked to a blog post that shows an extension method that uses the Max operator with a selector expression to find the next available key:
public static TResult NextId<TSource, TResult>(this ObjectSet<TSource> table, Expression<Func<TSource, TResult>> selector)
where TSource : class
{
TResult lastId = table.Any() ? table.Max(selector) : default(TResult);
if (lastId is int)
{
lastId = (TResult)(object)(((int)(object)lastId) + 1);
}
return lastId;
}
Here's my take on the extension method: It will work fine if the ObjectContext that I am working with has an unfiltered entity set. In that case, the ObjectContext will contain all rows from the data table, and I will get an accurate result. But if the entity set is the result of a query filter, the method will return the last entity key in the filtered entity set, which will not necessarily be the last key in the data table. So I think the extension method won't really work.
At this point, the obvious solution seems to be to simply use a GUID as the entity key. That way, I only need to call Guid.NewGuid() method to set the ID property before I add a new entity to my ObjectContext.
Here is my question: Is there a simple way of getting the last primary key in the data store from EF4 (without having to create a second ObjectContext for that purpose)? Any other reason not to take the easy way out and simply use a GUID? Thanks for your help.
I ended up going with a GUID.
The size/performance issues aren't
critical (or even noticeable) with SQL Compact, since
it is a local, single-user system.
It's not like the app will be
managing an airline reservation
system.
And at least at this point, there
seems to be no way around the "no
server-generated keys" limitation of
the SQL Compact/EF4 stack. If someone has a clever hack, I'm still open to it.
That doesn't mean I would take the same approach in SQL Server or SQL Express. I still have a definite preference for integer keys, and SQL Compact's bigger siblings allow them in conjunction with EF4.
Use a Guid. AutoIncrement is not supported on Compact Framework with Entity Framework.
Also, if you ever want to create a application which uses multiple data sources, int PK's are going to fall apart on you very, very quickly.
With Guid's, you can juse call Guid.NewGuid() to get a new key.
With int's, you have to hit the database to get a valid key.
If you store data in multiple databases, int PK's will cause conflicts.
What I've done for SQL CE before, and I assume we have a single application accessing the database, is to calculate the MAX value on startup and put it in a static variable. You can now hand out sequential values easily and you can make the code to generate them thread safe very easily.
One reason to avoid Guids would be size = memory and storage space consumption.
You could also query SQL Compact metadata like so:
SELECT AUTOINC_NEXT FROM INFORMATION_SCHEMA.COLUMNS
WHERE TABLE_NAME = 'Categories' AND AUTOINC_NEXT IS NOT NULL
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.