I'd like to know how to create a method that will allow me to generically do this...
public class Repo<T> : IGenericRepo<T> where T : class
{
protected PteDotNetEntities db;
public T Get(int id)
{
//how do I dynamically get to the correct entity object and select it by
//id?
}
}
Yes you can. If you know that all your entities will have simple primary key property of type int and name Id you can do simply this:
public interface IEntity
{
int Id { get; }
}
All your entities must implement this interface. Next you can simply do:
public class Repo<T> : IGenericRepo<T> where T : class, IEntity
{
protected PteDotNetEntities db;
public T Get(int id)
{
return db.CreateObjectSet<T>().FirstOrDefault(e => e.Id == id);
}
}
This is the simplest possible solution. There are better solutions using GetObjectByKey but they are more complex. The difference between FirstOrDefault and GetObjectByKey is repeatable execution. FirstOrDefault always executes DB query whereas GetObjectByKey first checks if the entity with the same key was already loaded / attached to the context and returns it without querying the database. As reference for version using GetObjectByKey you can check similar questions:
Entity Framework Simple Generic GetByID but has differents PK Name
Generic GetById for complex PK
You can simplify those examples if you know the name of the key property upfront (forced by the interface).
In case of using code first / DbContext API you can also check this question:
Generic repository EF4 CTP5 getById
Related
I have my entity defined like this:
public class Entity : BaseModel // Has the already ID defined
{
private int? companyId;
public Company? Company { get; set; }
public int? CompanyId {
get => this.companyId == 0 ? null : this.companyId; // I tried this for debugging purposes to force this value to "null" -> made no difference
set => this.companyId = value;
}
}
public class Company : BaseModel // Has the already ID defined
{
public IEnumerable<Entity> Entities { get; set; } = new List<Entity>();
}
Anyway, if I set the CompanyId to null, my DB throws an exception with the message: "FOREIGN KEY constraint failed". If the CompanyId is set to, e.g. 123, the relationship is resolved accordingly.
I mean, it makes sense, that EF cannot find null in my DB, but how do I want to set an optional value otherwise? I am using code first annotations only, hence my OnModelCreating of my context is completely empty.
How are you loading the entities in the first place? Are you loading an Entity by ID and trying to dis-associate it from a company, or have you loaded a company with it's entities and trying to remove one association?
Normally when working with relations where you have navigation properties, you want to de-associate them (or delete them) via the navigation properties, not the FK properties. For instance if loading a company and wanting to de-associate one of the entities you should eager-load the entities then remove the desired one from the collection:
var company = _context.Companies.Include(c => c.Entitites).Single(c => c.Id == companyId);
var entityToRemove = company.Entities.SingleOrDefault(e => e.Id == entityId);
if(entityToRemove != null)
company.Entities.Remove(entityToRemove);
_context.SaveChanges();
Provided that the relationship between Company and Entity is set up properly as an optional HasMany then provided these proxies are loaded, EF should work out to set the entityToRemove's FK to null.
If you want to do it from the Entity side:
var entityToRemove = _context.Entities.Include(e => e.Company).Single(e => e.Id == entityId);
entityToRemove.Company = null;
_context.SaveChanges();
That too should de-associate the entities. If these don't work then it's possible that your mapping is set up for a required relationship, though I am pulling this from memory so I might need to fire up an example to verify. :) You also should be checking for any code that might set that CompanyId to 0 when attempting to remove one, whether that might be happening due to some mapping or deserialization. Weird behaviour like that can occur when entities are passed around in a detached state or deserialized into controller methods. (which should be avoided)
Update: Code like this can be very dangerous and lead to unexpected problems like what you are encountering:
public virtual async Task<bool> Update(TModel entity)
{
Context.Update(entity);
await Context.SaveChangesAsync();
return true;
}
Update() is typically used for detached entities, and it will automatically treat all values in the entity as Modified. If model was already an entity tracked by the Context (and the context is set up for change tracking) then it is pretty much unnecessary. However, something in the calling chain or wherever has constructed the model (i.e. Entity) has set the nullable FK to 0 instead of #null. This could have been deserialized from a Form etc. in a view and sent to a Controller as an integer value based on a default for a removed selection. Ideally entity classes should not be used for this form of data transfer from view to controller or the like, instead using a POCO view model or DTO. To correct the behaviour as your code currently is, you could try the following:
public async Task<bool> UpdateEntity(Entity entity)
{
var dbEntity = Context.Set<Entity>().Include(x => x.Customer).Single(x => x.Id == entityId);
if (!Object.ReferenceEquals(entity, dbEntity))
{ // entity is a detached representation so copy values across to dbEntity.
// TODO: copy values from entity to dbEntity
if(!entity.CustomerId.HasValue || entity.CustomerId.Value == 0)
dbEntity.Customer = null;
}
await Context.SaveChangesAsync();
return true;
}
In this case we load the entity from the DbContext. If this method was called with an entity tracked by the DbContext, the dbEntity would be the same reference as entity. In this case with change tracking the Customer/CustomerId reference should have been removed. We don't need to set entity state or call Update. SaveChanges should persist the change. If instead the entity was a detached copy deserialized, (likely the case based on that 0 value) the reference would be different. In this case, the allowed values in the modified entity should be copied across to dbEntity, then we can inspect the CustomerId in that detached entity for #null or 0, and if so, remove the Customer reference from dbEntity before saving.
The caveats here are:
This won't work as a pure Generic implementation. To update an "Entity" class we need knowledge of these relationships like Customer so this data service, repository, or what-have-you implementation needs to be concrete and non-generic. It can extend a Generic base class for common functionality but we cannot rely on a purely Generic solution. (Generic methods work where implementation is identical across supported classes.)
This also means removing that attempt at trying to handle Zero in the Entity class. It should just be:
public class Entity : BaseModel
{
public Company? Company { get; set; }
[ForeignKey("Company")]
public int? CompanyId { get; set; }
// ...
}
Marking Foreign Keys explicitly is a good practice to avoid surprises when you eventually find yourself needing to break conventions that EF accommodates in simple scenarios.
I am exploring Entity Framework 7 and I would like to know if there is a way to intercept a "SELECT" query. Every time an entity is created, updated or deleted I stamp the entity with the current date and time.
SELECT *
FROM MyTable
WHERE DeletedOn IS NOT NULL
I would like all my SELECT queries to exclude deleted data (see WHERE clause above). Is there a way to do that using Entity Framework 7?
I am not sure what your underlying infrastructure looks like and if you have any abstraction between your application and Entity Framework. Let's assume you are working with DbSet<T> you could write an extension method to exclude data that has been deleted.
public class BaseEntity
{
public DateTime? DeletedOn { get; set; }
}
public static class EfExtensions
{
public static IQueryable<T> ExcludeDeleted<T>(this IDbSet<T> dbSet)
where T : BaseEntity
{
return dbSet.Where(e => e.DeletedOn == null);
}
}
//Usage
context.Set<BaseEntity>().ExcludeDeleted().Where(...additional where clause).
I have somewhat same issue. I'm trying to intercept read queries like; select, where etc in order to look into the returned result set. In EF Core you don't have an equivalent to override SaveChanges for read queries, unfortunately.
You can however, still i Entity Framework Core, hook into commandExecuting and commandExecuted, by using
var listener = _context.GetService<DiagnosticSource>();
(listener as DiagnosticListener).SubscribeWithAdapter(new CommandListener());
and creating a class with following two methods
public class CommandListener
{
[DiagnosticName("Microsoft.EntityFrameworkCore.Database.Command.CommandExecuting")]
public void OnCommandExecuting(DbCommand command, DbCommandMethod executeMethod, Guid commandId, Guid connectionId, bool async, DateTimeOffset startTime)
{
//do stuff.
}
[DiagnosticName("Microsoft.EntityFrameworkCore.Database.Command.CommandExecuted")]
public void OnCommandExecuted(object result, bool async)
{
//do stuff.
}
}
However these are high lewel interceptors and hence you won't be able to view the returned result set (making it useless in your case).
I recommend two things, first go to and cast a vote on the implementation of "Hooks to intercept and modify queries on the fly at high and low level" at: https://data.uservoice.com/forums/72025-entity-framework-core-feature-suggestions/suggestions/1051569-hooks-to-intercept-and-modify-queries-on-the-fly-a
Second you can use PostSharp (a commercial product) by using interceptors like; LocationInterceptionAspect on properties or OnMethodBoundaryAspect for methods.
I'm using EF5 code first.
We have a method
LogHistoryTracking(DbEntityEntry entity)
to log changes when SaveChanges is called.
At SaveChanges, we get the changed entities and pass into LogHistoryTracking
var changedEntities = ChangeTracker.Entries().ToList();
But when I access
changedEntity.OriginalValues.PropertyNames
there is no properties for foreign keys object (only foreign key Id - but how can we get the data when there is only id here?).
I also tried to google for a solution, but this issue might be not so popular.
There is this article, but it does not work.
Appreciate any help. Thanks.
If you want to have your entity properties to be accessible you must 'Include' them prior to accessing them. Like in the following example which gets the orders of the first cutomer :
var orders = context.Customers
.Include("Orders")
.First().Orders;
In this example if you do not call .Include("Orders") you will not have Customer.Orders. The same goes if you have foreign key and forget to include the navigation property of the foreign key. This is because the key (the ID) is part of the object and the navigation property is not.
Let us see one real world example :
public class Employee : Entity
{
public virtual int CompanyUserId { get; set; }
public virtual CompanyUser CompanyUser { get; set; }
//... cut out for brevity
}
If you get the employees like this :
var employees = context.Employees;
You will not be able to access employees[0].CompanyUser after
context.SaveChanges() because of lazy loading. The connection is disposed after context.SaveChanges(), so no more data fetching.
But if you call :
var employees = context.Employees
.Include("CompanyUser")
.ToArray();
You will be able to access employees[0].CompanyUser.SomeProperty right away before context.SaveChanges regardless lazy loading because ToArray() will execute the query and fetch the entities with the "includes".
If you call :
var employees = context.Employees
.Include("CompanyUser");
Then you will have employee[0].CompanyUser.SomeProperty even after context.SaveChanges() with Lazy Loading because you have told EF to include "CompanyUser" property before executing the query. On execution EF will include the named property.
UPDATE
Intercepting DbContext can be done in at least two different ways.
First - override SaveChanges() or SaveChangesAsync because it is virtual:
public class MyDbContext : DbContext
{
public event Action<MyDbContext> SavingChanges = _ => { };
public override int SaveChanges()
{
this.SavingChanges(this);
return base.SaveChanges();
}
}
Second way without direct override is by hiding the DbContext inside interface like this one (this is from real project) :
public interface IUnitOfWork : IDisposable
{
void Commit();
}
Third way (somewhat different) is by intercepting the Db calls.
Fourth way exists but it depends on what IoC you use. If you use Castle Windsor you can use interceptors. I suppose that with every IoC there is its own way of intercepting this.
For a simple repository
public interface ISimpleRepository<T>
{
IApplicationState AppState { get; set; }
void Add(T instance);
void Delete(T instance);
void Delete(Guid rowGuid);
IQueryable<T> GetAll();
T Load(Guid rowGuid);
void SaveChanges();
void Update(T instance);
}
my implementation of the Load() method for specific repository for class Product might look like this:
public Product Load(Guid rowid)
{
return (from c in _ctx.Products where c.id == rowid select c).FirstOrDefault();
}
Now this is assumed when my repository implementation class looks like this:
public class EntityFrameworkProductsProvider : IRepository<Product> ...
What if I had like dozens or hundreds of this small and simple entities that would all use the same behaviour when doing CRUDs (use the same implementation of methods)? I certainly don't want to go and create a class to implement IRepository for each one of them..
I want something like this:
public class EntityFrameworkDefaultProvider<T> : IRepository<T> ...
but I don't know how to implement the LINQ Select expression then because of course I can't write from e in _ctx.T where e... or do I?
I haven't run into this scenario yet because so far I only had very specific entities with custom repository implementation.
Because you tagged your question with entity-framework and entity-framework-4 I assume you are using ObjectContext API. ObjectContext offers method CreateObjectSet<T> which is equivalent of Set<T> on DbContext.
This question is actually duplicate of either:
Generic GetById with DbContext
Generic GetById with ObjectContext
Instead of writing _ctx.Products, you can write _ctx.Set<T>. That takes care of half of the problem (you need to add a generic constraint where T: class to your repository)
Then, if rowid is the object's key, you can use _ctx.Set<T>.Find(rowid) instead of a LINQ query to retrieve by Id.
Alternatively, you can create a base interface IHaveId (or a BaseEntity class, whatever you like) which has the Id property, and then add that as an generic constraint on T, so you can use it in your queries.
If you're using EF 4.1, see the sample generic repository here:
http://www.asp.net/entity-framework/tutorials/implementing-the-repository-and-unit-of-work-patterns-in-an-asp-net-mvc-application
I know that this is possible in EF4.1 with the DbContext API, where you have a "Set" method on the context that gets you the entity set corresponding to the type T. this way, you could have your repository like this:
public class EntityFrameworkDefaultProvider<T> : IRepository<T> where T:class
{
public T Load(Guid rowId)
{
return _context.Set<T>().Find(rowId);
}
}
one more remark: I think you could use this syntax :
return _ctx.Products.FirstOrDefault(c=>c.id == rowid);
to get the entity you want instead of using the (from... in...). it's clearer (in my opinion) :)
Hope this helps
Error: The ObjectContext instance has been disposed and can no longer be used for operations that require a connection.
I am trying to create a WCF service with Entity Framework (VS 2010, .NET 4). When I run it, I get the above error.
I read something about editing the T4 template, but it appears that it already has
[DataContractAttribute(IsReference=true)]
public partial class Person : EntityObject
and
[DataMemberAttribute()]
public global::System.Int32 ID
{
get
{
return _ID;
}
I am not sure what the difference is between
[DataMemberAttribute()] and [DataMember]
or
[DataContractAttribute(IsReference=true)] and [DataContract]
either.
public Person GetPersonByID(int id)
{
using (var ctx = new MyEntities())
{
return (from p in ctx.Person
where p.ID == id
select p).FirstOrDefault();
}
}
How does WCF and EF work together, properly?
Do you have navigation properties in your Person class? Did you disable lazy loading? Otherwise it will probably try to load content for navigation properties during serialization and it fails because of closed context.
To your other questions:
[DataMemberAttribute()] and [DataMember] are same. It is just shorter name.
[DataContractAttribute(IsReference=true)] and [DataContract] are not same. IsRefrence allows tracking circular references in navigation properties. Without this parameter circular reference causes never ending recursion.