In my DB I have tables who have an attribute int DeleteState. I want a generic method to query those tables. In other words a method who does this: Context.Table.Where(x => x.DeleteState == 0).
I thought I could do this:
public static class Extensions
{
public static IQueryable<T> Exists<T>(this IQueryable<T> qry) where T : IDeletable
{
return qry.Where(x => x.DeleteState == 0);
}
}
Where IDeletable is this:
public interface IDeletable
{
int DeleteState { get; set; }
}
Now I only have to add the IDeletable in the EF model:
public partial class Table : EntityObject, IDeletable { ... }
I did this with the templating mechanism.
Unfortunately, it doesn't work :( It compiles fine, but throws at runtime:
Unable to cast the type 'Table' to type 'IDeletable'. LINQ to Entities only supports casting Entity Data Model primitive types
if I call it like that:
Context.Table.Exists();
How can I solve this problem? Could you think of a fix or a different method to achieve similar results? Thx
The problem you have is that the Entity Framework can only work with an Expression Tree. Your function executes a query directly instead of building an Expression Tree.
A simpler solution would be to add a Model Defined Function.
A model defined function can be called directly on an instance of your context.
Maybe:
public static IQueryable<T> Exists<T>(this IQueryable<T> qry)
{
return qry.Where(x => (!typeof(IDeletable).IsAssignableFrom(x.GetType()) || typeof(IDeletable).IsAssignableFrom(x.GetType()) && ((IDeletable)x).DeleteState == 0));
}
Tsss, this is the answer: Linq Entity Framework generic filter method
I forgot about the class here:
... where T : class, IDeletable
Have you tried converting your objects to IDeletable before you actually query? e.g.
public static IQueryable<T> Exists<T>(this IQueryable<T> qry)
{
return qry.Select<T, IDeletable>(x => x).Where(x => x.DeleteState == 0).Cast<T>();
}
I haven't tested this code, however, the error rings a bell and I remember I had to do something similar.
Related
I have been reading posts about Expression<Func<TModel,TResult>> for about an hour and I really do not understand this. My apologizes, but I just don't.
I have an issue that I have a abstract class that has a call to EF6 where I need to order by some property that I would like to define in the child class. That said I will add an example below.
public abstract MyController<TModel>:ApiController
{
protected IRepository<TModel> Repository {get;}
protected MyController(IRepository<TModel> repo)
{
Repository = repo;
}
protected Expression<Func<TModel,TResult>> OrderBy {get; set}
public IHttpActionResult GetItems()
{
return Ok(Repository.Get().OrderBy(x=>OrderBy(x)).ToList()); //with lots of other cool stuff.
}
}
public PersonController:MyController<Person>
{
public PersonControler(IRepository<Person> repo):base(repo)
{
OrderBy = //I need help here
}
}
Okay, so after all of this I have been reading about Expressions and I do not understand why OrderBy = (person)=> person.LastName will not work here. Can someone please explain how the Expression works and how to make this work?
Declare your property as
protected Expression<Func<TModel, object>> OrderBy { get; set; }
Assign it in PersonController
OrderBy = p => p.LastName
And use it like
Repository.Get().OrderBy(OrderBy).ToList()
Under the hood (of EF/LINQ query translation engine) Expression (actually all methods call chain) is translated to SQL query. There is expression visitor (see pattern 'visitor'), which does the job.
To get things work:
method Get of IRepository should return type DbSet<TModel> (for EF) or Table<TModel> (for LINQ to SQL)
type of ordering key should be defined either in abstract class (public abstract MyController<TModel, TKey> : ApiController) or in expression directly (protected Expression<Func<TModel, int>> OrderBy {get; set})
I would like to pass an IQueryable and an array of ids to a method which filters the IQueryable based on those ids.
As the ids can be either long's or int's it should be solved generically.
I came up with the following:
public static IEnumerable<T> GetModified<TId, T>(IQueryable<T> objects, TId[] ids) where T : class
{
return objects.Where(j => ids.Contains((TId)j.GetType().GetProperty("Id").GetValue(j)));
}
Unfortunately I'm getting the exception:
LINQ to Entities does not recognize the method 'System.Object GetValue(System.Object)' method, and this method cannot be translated into a store expression.
The exception is normal, as getting properties through reflection is something that clearly cannot be translated to SQL.
One thing I would try is to create a generic interface that exposes an Id property of a given type:
public interface HasId<T> {
T Id { get; set; }
}
Now you could declare your entity as implementing HasId<int>, for example, if the Id was of type int.
The next step is to modify your method like so:
public static IEnumerable<T> GetModified<TId, T>
(IQueryable<T> objects, TId[] ids) where T : class, HasId<TId>
{
return objects.Where(j => ids.Contains(j.Id));
}
Note the added generic restriction: where T : class, HasId<TId>. This enables you to write the simplified j.Id, which returns a TId value, instead of resorting to reflection.
Please note that I haven't run or tested this code; it's just an idea that I got when I saw your problem and I hope it helps.
Update:
Here's another possible solution that doesn't require that you declare interfaces or change your classes in any way:
public static IEnumerable<T> GetModified<TId, T>
(IQueryable<T> objects, TId[] ids, Expression<Func<T, TId>> idSelector)
where T : class
{
return objects.Where(j => ids.Contains(idSelector(j)));
}
What I've done here is add the Expression<Func<T, TId>> idSelector parameter, an expression that can return the Id of a given instance of T.
You would call the method like that:
var modified = GetModified(dbObjects, yourIdArray, entity => entity.Id);
(only the third parameter being new; keep the others as you have them now).
Again, I haven't tested if this works or even compiles, as I don't have a computer with VS here :(.
Entity Framework doesn't support some of the .NET methods such as GetValue() since it does not translate to SQL (which is the code actually executed to the IQueryable. Try calling ToList to get the CLR object before doing reflection:
public static IEnumerable<T> GetModified<TId, T>(IQueryable<T> objects, TId[] ids) where T : class
{
return objects.ToList().Where(j => ids.Contains((TId)j.GetType().GetProperty("Id").GetValue(j)));
}
I'm trying to filter a LINQ-to-entities query in a generic way, but I keep getting an error. Here is a piece of code:
private IQueryable<T> FilterDeletedEntities<T>(IQueryable<T> entities)
{
if (typeof(IDeletable).IsAssignableFrom(typeof(T)))
{
var deletableEntities = (IQueryable<IDeletable>)entities;
deletableEntities = deletableEntities.Where(entity => !entity.Deleted);
entities = (IQueryable<T>)deletableEntities;
}
return entities;
}
Basically I'm trying to filter out deleted entities (i.e. 'Deleted' field is 'true'), if and only if the entity is IDeletable (i.e. it has the 'Deleted' field). The problem is that I can't cast IQueryable< IDeletable > back to IQueryable< T >.
Any ideas on how to fix this? And before you ask: yes, this method has to be generic.
Thanks in advance!
But you can use Cast<T>() to convert it.
entities = deletableEntities.Cast<T>();
You could also use it to case to IDeletable as well, for example,
private IEnumerable<T> FilterDeletedEntities<T>(IQueryable<T> entities)
{
if (typeof(IDeletable).IsAssignableFrom(typeof(T)))
{
return entities.ToList()
.Cast<IDeletable>()
.Where( e => !e.Deleted )
.Cast<T>();
}
return entities.ToList();
}
I was able to solve my problem by doing this:
private IQueryable<T> FilterDeletedEntities<T>(IQueryable<T> entities)
{
if (typeof(IDeletable).IsAssignableFrom(typeof(T)))
{
var deletableEntities = (IQueryable<IDeletable>)entities;
return deletableEntities.Where(entity => !entity.Deleted).Cast<T>();
}
return entities;
}
Thanks to tvanfosson for the inspiration.
If you can assume that no one will need to call this method with T that does not implement IDeletable, you can restrict T:
private IQueryable<T> FilterDeletedEntities<T>(IQueryable<T> entities) where T : IDeletable
As a bonus, you won't need to cast anything or use reflection to test for IDeletable.
In my Custom ObjectContext class I have my entity collections exposed as IObjectSet so they can be unit-tested. I have run into a problem when I use this ObjectContext in a compiled query and call the "Include" extension method (From Julie Lerman's blog http://thedatafarm.com/blog/data-access/agile-entity-framework-4-repository-part-5-iobjectset/) and in her book Programming Entity Framework 2nd edition on pages 722-723.
Here is the code:
Query:
public class CommunityPostsBySlugQuery : QueryBase<IEnumerable<CommunityPost>>
{
private static readonly Expression<Func<Database, string, IEnumerable<CommunityPost>>> expression = (database, slug) => database.CommunityPosts.Include("Comments").Where(x => x.Site.Slug == slug).OrderByDescending(x => x.DatePosted);
private static readonly Func<Database, string, IEnumerable<CommunityPost>> plainQuery = expression.Compile();
private static readonly Func<Database, string, IEnumerable<CommunityPost>> compiledQuery = CompiledQuery.Compile(expression);
private readonly string _slug;
public CommunityPostsBySlugQuery(bool useCompiled, string slug): base(useCompiled)
{
_slug = slug;
}
public override IEnumerable<CommunityPost> Execute(Database database)
{
return base.UseCompiled ? compiledQuery(database, _slug) : plainQuery(database, _slug);
}
}
Extension
public static class ObjectQueryExtension
{
public static IQueryable<T> Include<T>(this IQueryable<T> source, string path)
{
var objectQuery = source as ObjectQuery<T>;
return objectQuery == null ? source : objectQuery.Include(path);
}
}
LINQ to Entities does not recognize the method 'System.Linq.IQueryable1[MyPocoObject] Include[MyIncludedPocoObject](System.Linq.IQueryable1[MyPocoObject], System.String)' method, and this method cannot be translated into a store expression.
If I use this same query on ObjectSet collections rather than IObjectSet it works fine. If I simply run this query without precompiling it works fine. What am I missing here?
I really don't know but have asked if someone on the EF team can answer it.
Response by EF Team:
This is a known issue with CTP4, Include is an instance method on ObjectSet but when your set is typed as IObjectSet you are actually using an extension method on IQueryable that is included in CTP4. This extension method doesn't work with compiled queries but we will try and support this in the next release.
I'm trying to create a generic repository for my models. Currently i've 3 different models which have no relationship between them. (Contacts, Notes, Reminders).
class Repository<T> where T:class
{
public IQueryable<T> SearchExact(string keyword)
{
//Is there a way i can make the below line generic
//return db.ContactModels.Where(i => i.Name == keyword)
//I also tried db.GetTable<T>().Where(i => i.Name == keyword)
//But the variable i doesn't have the Name property since it would know it only in the runtime
//db also has a method ITable GetTable(Type modelType) but don't think if that would help me
}
}
In MainViewModel, I call the Search method like this:
Repository<ContactModel> _contactRepository = new Repository<ContactModel>();
public void Search(string keyword)
{
var filteredList = _contactRepository.SearchExact(keyword).ToList();
}
Solution:
Finally went with Ray's Dynamic Expression solution:
public IQueryable<TModel> SearchExact(string searchKeyword, string columnName)
{
ParameterExpression param = Expression.Parameter(typeof(TModel), "i");
Expression left = Expression.Property(param, typeof(TModel).GetProperty(columnName));
Expression right = Expression.Constant(searchKeyword);
Expression expr = Expression.Equal(left, right);
}
query = db.GetTable<TModel>().Where(Expression.Lambda<Func<TModel, bool>>(expr, param));
Interface solution
If you can add an interface to your object you can use that. For example you could define:
public interface IName
{
string Name { get; }
}
Then your repository could be declared as:
class Repository<T> where T:class, IName
{
public IQueryable<T> SearchExact(string keyword)
{
return db.GetTable<T>().Where(i => i.Name == keyword);
}
}
Alternate interface solution
Alternatively you could put the "where" on your SearchExact method by using a second generic parameter:
class Repository<T> where T:class
{
public IQueryable<T> SearchExact<U>(string keyword) where U: T,IName
{
return db.GetTable<U>().Where(i => i.Name == keyword);
}
}
This allows the Repository class to be used with objects that don't implement IName, whereas the SearchExact method can only be used with objects that implement IName.
Reflection solution
If you can't add an IName-like interface to your objects, you can use reflection instead:
class Repository<T> where T:class
{
static PropertyInfo _nameProperty = typeof(T).GetProperty("Name");
public IQueryable<T> SearchExact(string keyword)
{
return db.GetTable<T>().Where(i => (string)_nameProperty.GetValue(i) == keyword);
}
}
This is slower than using an interface, but sometimes it is the only way.
More notes on interface solution and why you might use it
In your comment you mention that you can't use an interface but don't explain why. You say "Nothing in common is present in the three models. So i think making an interface out of them is not possible." From your question I understood that all three models have a "Name" property. In that case, it is possible to implement an interface on all three. Just implement the interface as shown and ", IName" to each of your three class definitions. This will give you the best performance for both local queries and SQL generation.
Even if the properties in question are not all called "Name", you can still use the nterface solution by adding a "Name" property to each and having its getter and setter access the other property.
Expression solution
If the IName solution won't work and you need the SQL conversion to work, you can do this by building your LINQ query using Expressions. This more work and is significantly less efficient for local use but will convert to SQL well. The code would be something like this:
class Repository<T> where T:Class
{
public IQueryable<T> SearchExact(string keyword,
Expression<Func<T,string>> getNameExpression)
{
var param = Expression.Parameter(typeof(T), "i");
return db.GetTable<T>().Where(
Expression.Lambda<Func<T,bool>>(
Expression.Equal(
Expression.Invoke(
Expression.Constant(getNameExpression),
param),
Expression.Constant(keyword),
param));
}
}
and it would be called thusly:
repository.SearchExact("Text To Find", i => i.Name)
Ray's method is quite good, and if you have the ability to add an interface definitely the superior however if for some reason you are unable to add an interface to these classes (Part of a class library you can't edit or something) then you could also consider passing a Func in which could tell it how to get the name.
EG:
class Repository<T>
{
public IQueryable<T> SearchExact(string keyword, Func<T, string> getSearchField)
{
return db.GetTable<T>().Where(i => getSearchField(i) == keyword);
}
}
You'd then have to call it as:
var filteredList = _contactRepository.SearchExact(keyword, cr => cr.Name).ToList();
Other than these two options you could always look into using reflection to access the Name property without any interface, but this has the downside that there's no compile-time check that makes sure the classes you're passing actually DO have a Name property and also has the side-effect that the LINQ will not be translated to SQL and the filtering will happen in .NET (Meaning the SQL server could get hit more than is needed).
You could also use a Dynamic LINQ query to achieve this SQL-side effect, but it has the same non type-safe issues listed above.