Develop Reference

Ignore null values in context update

I have a large model which has been partially updated via deserialization. Since it has only been partially updated I would like to ignore any null values when I pass this to my entity framework update. Ultimately the EntityState.Modified is set but the issue I am having is that all fields are updated. This means anything that was null is now blanked in the database.
Is it possible to change this default behavior through a setting or override a method to check for null? It seems that since the context is expecting the full model I cannot simply set only a few values.
I've verified this by mapping only what I need to modify and the same behavior occurs.

You could implement something like this.
In this case I'm using a generic repository with reflection, to iterate through the properties and exclude null values in the update method.
public virtual TEntity Update(TEntity entity)
{
dbSet.Attach(entity);
dbContext.Entry(entity).State = EntityState.Modified;
var entry = dbContext.Entry(entity);
Type type = typeof(TEntity);
PropertyInfo[] properties = type.GetProperties();
foreach (PropertyInfo property in properties)
{
if (property.GetValue(entity, null) == null)
{
entry.Property(property.Name).IsModified = false;
}
}
dbContext.SaveChanges();
return entity;
}

public IHttpActionResult PutProduct(int id, Product product)
{
NorthwindEntities db = new NorthwindEntities();
if (!ModelState.IsValid)
{
return BadRequest(ModelState);
}
db.Products.Attach(product);
// Only the fields you want to update, will change.
db.Entry(product).Property(p => p.ProductName).IsModified = true;
db.Entry(product).Property(p => p.UnitPrice).IsModified = true;
db.Entry(product).Property(p => p.UnitsInStock).IsModified = true;
// only if if the value is not null, the field will change.
db.Entry(product).Property(p => p.UnitsOnOrder).IsModified =
product.UnitsOnOrder != null;
db.SaveChanges();
return Ok(product);
}

This is a somewhat tedious problem I've had to solve.
For lack of a more straightforward solution, eventually I decided I'd rather not want to try solving it as much downstream as when EF's SaveChanges-and-the-likes' get called (i.e., no need to "hook into" EF so late), but instead as much higher upstream / earlier on as possible --
that is, to do so right after I obtain a satisfying deserialization, which is meaningful to mutate the model, on a per-entity instance basis (in my use case, none of the updatable properties would represent relationships, but only independent attributes, in E/R parlance -- YMMV)
So, I opted for a "Populate" helper, along the lines of:
static void Populate(object from, object to)
{
var sourceType = from.GetType();
foreach (PropertyInfo target in to.GetType().GetProperties())
{
// Is the property at the target object writable and *not* marked
// as `[NotMapped]'?
var isUpdatable =
target.CanWrite &&
(target.GetCustomAttribute<NotMappedAttribute>(true) == null);
if (isUpdatable)
{
// If so, just find the corresp. property with the same name at the source object
// (caller is assumed responsible to guarantee that there is one, and of the same type, here)
var source = sourceType.GetProperty(target.Name);
var #default = sourceType.IsValueType ? Activator.CreateInstance(sourceType) : null;
var equality = (IEqualityComparer)typeof(EqualityComparer<>).MakeGenericType(sourceType).GetProperty("Default", BindingFlags.Public | BindingFlags.Static).GetValue(null);
var value = source.GetValue(from);
// Test for <property value> != default(<property type>)
// (as we don't want to lose information on the target because of a "null" (or "default(...)") coming from the source)
if (!equality.Equals(value, #default))
{
target.SetValue(to, value, null);
}
}
}
}
where "from" is the fresh entity instance that just got partially populated by whatever deserialization code, and where "to" is the actual target entity that lives in the DbContext (be it an EF proxy or not);
and where NotMappedAttribute is the EF's usual.
You'd typically have Populate to be called some time after the deserialization (&/or DTO-mapping) onto your "from" instance is done, but anyway before SaveChanges() gets call on your DbContext for all the "to" entities -- obviously, we assume there is a feasible 1-to-1 mapping "from" ... "to", that the caller of Populate knows about / could figure out.
Note I still don't know if there is a more elegant (more straightforward) way to do that, without recourse to reflection -- so, there, FWIW.
Remarks
1) above code can (or should) be made more defensive in various ways, depending on the caller assumptions;
2) one may want to cache those IEqualityComparer's (&/or the PropertyInfo's) for whatever (good) reason may arise -- in my case, I didn't have to;
3) finally, my understanding is that third-party librairies such as AutoMapper are also specially designed for that sort of task, if you can afford the additional dependency
'HTH,

Audit Entity Framework deleted entries preserving previous value

I've been struggling for a while with a problem that consists on auditing generically database entities when they're saved. I have a project that uses EF 6 and it was required to me to create a "non-invasive" method to audit entities when they're added, modified or deleted. I have to store a JSON of the inserted entity, modified entity or deleted entity without interfering with the normal flow. The project has a Database First implementation.
My solution was simple, add a partial class of any entity that the rest of the programmers want to audit implementing IAudit which is basically an empty interface to get all changes from entities that implement it.
public interface IAudit {}
I have a Currencies entity that just implement it without any other code (I could do something else in the future but I don't need it)
public partial class Currencies : IAudit
I override the SaveChanges method to look for entities to audit
public override int SaveChanges()
{
ChangeTracker.DetectChanges();
// This linq looks for new entities that were marked for audit
CreateAuditLog(System.Data.Entity.EntityState.Added);
CreateAuditLog(System.Data.Entity.EntityState.Modified);
CreateAuditLog(System.Data.Entity.EntityState.Deleted);
return base.SaveChanges();
}
The solution calls 3 times the CreateAuditLog because in the near future I need to implement a configuration to audit whatever the user decides, might be from a database configuration that is activated/deactivated by users.
Everything worked perfectly, I was able to get saved entities in the specified state:
private void CreateAuditLog(System.Data.Entity.EntityState state)
{
var auditedEntities = ChangeTracker.Entries<IAudit>()
.Where(p => p.State == state)
.Select(p => p.Entity);
... some code that do something else
foreach (var auditedEntity in auditedEntities)
{
... some information I required to add
strJSON = JsonConvert.SerializeObject(auditedEntity, new EFNavigationPropertyConverter());
... some code to save audit information
}
}
The problem is I lose every value in the Deleted state, I only get the ID, there's no information in the properties except the ID and there is no any possibility of extract it in any way. I looked for every single solution in StackOverflow and other websites and there is nothing to recover the original information.
How can I get the previous deleted values to store them in the same way I'm storing Added and Modified entities?

It took me a couple of days to figure it out. Might be the solution is a bit complex but I tried several less complex options with not a good result.
First, as I'm just auditing Delete in a different way I separated Deleted state from Added and Modified that work well with no change. Deleted state is a particular case and I treat it like that.
First, I needed to obtain the original values from the database. In the Deleted state they're gone, there's not any possibility of recovering them from the entity. It's possible to obtain them with the following code:
var databaseValues = this.Entry(auditedEntity).GetDatabaseValues();
The result is just a collection of DB property values (DbPropertyValues). If I can get the original values I set the original values from the deleted entity:
dbEntityEntry.OriginalValues.SetValues(databaseValues);
This line just fills the entity original values, it doesn't modify the current value at all. It's useful to do it that way because it takes some code to check every property and set it ourselves, it's an interesting shortcut.
Now, the problem is I don't have the entity to serialize, so I need a new one which in my case I create by reflection because I don't know the type (I receive entities that implement IAudit)
Type type = auditedEntity.GetType();
var auditDeletedEntity = Activator.CreateInstance(type);
This is the entity I will serialize to store the audit later.
Now, the complex part, I need to get the entity properties and fill them by reflection from the original values set in the entity:
foreach (var propertyInfo in type.GetProperties())
{
if (!propertyInfo.PropertyType.IsArray && !propertyInfo.PropertyType.IsGenericType)
{
var propertyValue = originalValues.GetValue<object>(propertyInfo.Name);
auditDeletedEntity.GetType().InvokeMember(propertyInfo.Name,
BindingFlags.Instance | BindingFlags.Public | BindingFlags.SetProperty,
Type.DefaultBinder, auditDeletedEntity, new[] { propertyValue });
}
}
I had to check generic and array types to avoid following EF relations that are not going to work with this method and I also don't need (I need the object not the whole tree)
After that I simply need to serialize the audited deleted entity:
strJSON = JsonConvert.SerializeObject(auditDeletedEntity, new EFNavigationPropertyConverter());
The code looks like this:
string strJSON = string.Empty;
if (state == System.Data.Entity.EntityState.Deleted)
{
var databaseValues = this.Entry(auditedEntity).GetDatabaseValues();
// Get original values from the database (the only option, in the delete method they're lost)
DbEntityEntry dbEntityEntry = this.Entry(auditedEntity);
if (databaseValues != null)
{
dbEntityEntry.OriginalValues.SetValues(databaseValues);
var originalValues = this.Entry(auditedEntity).OriginalValues;
Type type = auditedEntity.GetType();
var auditDeletedEntity = Activator.CreateInstance(type);
// Get properties by reflection
foreach (var propertyInfo in type.GetProperties())
{
if (!propertyInfo.PropertyType.IsArray && !propertyInfo.PropertyType.IsGenericType)
{
var propertyValue = originalValues.GetValue<object>(propertyInfo.Name);
auditDeletedEntity.GetType().InvokeMember(propertyInfo.Name,
BindingFlags.Instance | BindingFlags.Public | BindingFlags.SetProperty,
Type.DefaultBinder, auditDeletedEntity, new[] { propertyValue });
}
}
strJSON = JsonConvert.SerializeObject(auditDeletedEntity, new EFNavigationPropertyConverter());
}
}
else
{
strJSON = JsonConvert.SerializeObject(auditedEntity, new EFNavigationPropertyConverter());
}
Might be there's a better way but I seriously spent a good amount of time looking for options and I couldn't find anything better.
Any suggestion or optimization is appreciated.

Failure to attach a detached entity (entity with the same key is already in the context)

I'm using Entity Framework 6, Code First approach. I'll try to present my problem with a simple piece of code:
public void ViewEntity(MyEntity Entity) // Want to read properties of my entity
{
using (var Db = new MyDbContext())
{
var DummyList = Db.MyEntities.ToList(); // Iteration on this DbSet
Db.MyEntities.Attach(Entity); // Exception
}
}
The exception message is: Attaching an entity of type 'MyProgram.MyEntity' failed because another entity of the same type already has the same primary key value.
From what I've read on MSDN it's an expected behaviour. But what I want on that last line is to first check if there is an entity with the same key already attached to a context; if it is, use it instead, and only otherwise attach my entity to context.
But I've failed to find a way to do so. There are many utility methods on ObjectContext instance (for example GetObjectByKey). I can't test them all 'cause they all ultimately need a qualifiedEntitySetName, and I don't have any in my real imlpementation, because this method should be on an abstract class and it should work for all entity types. Calling Db.Entity(this) is no use, there is no EntityKey which would have EntitySetName.
So all of this became complex really fast. And in my terms I just want to check if the object is already in "cache" (context), use it, otherwise use my object and attach it to this context.
To be clear, I have a detached object from a TreeNode.Tag in the first place, and I just want to use it again, or if it's impossible; if there already is one in the context), use that one instead. Maybe I'm missing some crucial concepts of EF6, I'm just starting out with EF.

I've found a solution for me. As I guessed correctly ObjectContext.GetObjectByKey method does what I need, but first I needed to construct qualifiedEntitySetName, and I found a way to do so. A tad bit cumbersome (using reflection, iterating properties of MyDbContext), but does not compare to a headache of a problem I made out of all this. Just in case, here's the patch of code that is a solution for me:
public SdsAbstractObject GetAttachedToContext()
{
var ObjContext = (SdsDbContext.Current as IObjectContextAdapter).ObjectContext;
var ExistingItem = ObjContext.GetObjectByKey(GetEntityKey()) as SdsAbstractObject;
if (ExistingItem != null)
return ExistingItem;
else
{
DbSet.Attach(this);
return this;
}
}
public EntityKey GetEntityKey()
{
string DbSetName = "";
foreach (var Prop in typeof(SdsDbContext).GetProperties())
{
if (Prop.PropertyType.IsGenericType
&& Prop.PropertyType.GenericTypeArguments[0] == ObjectContext.GetObjectType(GetType()))
DbSetName = Prop.Name;
}
if (String.IsNullOrWhiteSpace(DbSetName))
return null;
else
return new EntityKey("SdsDbContext." + DbSetName, "Id", Id);
}

An Entity can be in one of five stages : Added, Unchanged, Modified, Deleted, Detached.
public void ViewEntity(MyEntity entity) // Want to read properties of my entity
{
using (var Db = new MyDbContext())
{
var DummyList = Db.MyEntities.ToList(); // Iteration on this DbSet
// Set the Modified state of entity or you can write defensive code
// to check it before set the state.
if (Db.Entry(entity).State == EntityState.Modified) {
Db.Entry(entity).State = EntityState.Modified
}
// Attached it
Db.MyEntities.Attach(Entity);
Db.SaveChanges();
}
}
Since EF doesn't know which properties are different from those in the database, it will update them all.

A generic way to save an entity in Entity Framework

I am trying to write a GenericEFRepository which will be used by other Repositories. I have a Save method as below.
public virtual void Save(T entity) // where T : class, IEntity, new() And IEntity enforces long Id { get; set; }
{
var entry = _dbContext.Entry(entity);
if (entry.State != EntityState.Detached)
return; // context already knows about entity, don't do anything
if (entity.Id < 1)
{
_dbSet.Add(entity);
return;
}
var attachedEntity = _dbSet.Local.SingleOrDefault(e => e.Id == entity.Id);
if (attachedEntity != null)
_dbContext.Entry(attachedEntity).State = EntityState.Detached;
entry.State = EntityState.Modified;
}
You can find the problem in comments of below code
using (var uow = ObjectFactory.GetInstance<IUnitOfWork>()) // uow is implemented like EFUnitOfWork which gives the DbContext instance to repositories in GetRepository
{
var userRepo = uow.GetRepository<IUserRepository>();
var user = userRepo.Get(1);
user.Name += " Updated";
userRepo.Save(user);
uow.Save(); // OK only the Name of User is Updated
}
using (var uow = ObjectFactory.GetInstance<IUnitOfWork>())
{
var userRepo = uow.GetRepository<IUserRepository>();
var user = new User
{
Id = 1,
Name = "Brand New Name"
};
userRepo.Save(user);
uow.Save();
// NOT OK
// All fields (Name, Surname, BirthDate etc.) in User are updated
// which causes unassigned fields to be cleared on db
}
The only solution I can think of is creating Entities via repository like userRepo.CreateEntity(id: 1) and repository will return an Entity which is attached to DbContext. But this seems error prone, still any developer may create an entity using new keyword.
What are your solution suggestions about this particular problem?
Note: I already know about cons and pros of using a GenericRepository and an IEntity interface. So, "Don't use a GenericRepository, don't use an IEntity, don't put a long Id in every Entity, don't do what you are trying to do" comments will not help.

Yes it is error prone but simply that is the problem with EF and repositories. You must either create entity and attach it before you set any data you want to update (Name in your case) or you must set modified state for each property you want to persist instead of whole entity (as you can imagine again developer can forget to do that).
The first solution leads to special method on your repository doing just this:
public T Create(long id) {
T entity = _dbContext.Set<T>().Create();
entity.Id = id;
_dbContext.Set<T>().Attach(entity);
return entity;
}
The second solution needs something like
public void Save(T entity, params Expression<Func<T, TProperty>>[] properties) {
...
_dbContext.Set<T>().Attach(entity);
if (properties.Length > 0) {
foreach (var propertyAccessor in properties) {
_dbContext.Entry(entity).Property(propertyAccessor).IsModified = true;
}
} else {
_dbContext.Entry(entity).State = EntityState.Modified;
}
}
and you will call it like:
userRepository(user, u => u.Name);

This is kind of a fundamental problem of this approach because you expect the repository to magically know which fields you changed and which ones you didn't. Using null as a signal for "unchanged" does not work in case null is a valid value.
You'd need to tell the repository which fields you want to have written, for example sending a string[] with the field names. Or one bool for each field. I do not think this is a good solution.
Maybe you can invert the control flow like this:
var entity = repo.Get(1);
entity.Name += "x";
repo.SaveChanges();
That would allow change tracking to work. It is closer to how EF wants to be used.
Alternative:
var entity = repo.Get(1);
entity.Name += "x";
repo.Save(entity);

While the other two answers provide good insight into how perhaps you can avoid this issue I think its worth pointing out a couple of things.
What you are trying to do (ie a proxy entity update) is extremely EF-centeric and IMO actually doesn't make sense outside of the EF context and hence it doesnt make sense that a generic repository would be expected to behave in this way.
You actually haven't even gotten the flow quite right for EF, if you attach an object with a few fields already set EF will conciser what you told it to be the current DB state unless you modify a value or set a modified flag. To do what you are attempting without a select you would normally attach an object without the name and then set the name after attaching the ID object
Your approach is normally used for performance reasons, I would suggest that by abstracting over the top of an existing framework you are almost always going to suffer some logical performance degradation. If this is a big deal maybe you shouldn't be using a repository? The more you add to your repository to cater to performance concerns the more complex and restrictive it becomes and the harder it gets to provide more than one implementation.
All that being said I do think you can handle this particular case in a generic situation.
This is one possible way you could do it
public void UpdateProperty(Expression<Func<T,bool>> selector, FunctionToSetAProperty setter/*not quite sure of the correct syntax off the top of my head*/)
{
// look in local graph for T and see if you have an already attached version
// if not attach it with your selector value set
// set the property of the setter
}
Hope this makes some sense, I'm not by my dev box atm so I cant really do a working sample.
I think this is a better approach for a generic repository as it allows you to implement this same behavior in multiple different ways, the abovc may work for EF but there will be different methods if you have an in memory repository (for example). This approach allows you to implement different implementations that fulfill the intent rather than restrict your repository to only act like EF.

Avoid locking an Object that's already in the session (UniqueObjectException)

I call this function on my objects when they need to be initialized again:
public virtual void Initialize()
{
if (!HibernateSessionManager.Instance.GetSession().Contains(this)) {
try
{
HibernateSessionManager.Instance.GetSession()
.Lock(this, NHibernate.LockMode.None);
}
catch (NonUniqueObjectException e) { }
}
}
I thought I can prevent initializing something twice with checking Contains(this), but it happens sometimes that Lock(this, NHibernate.LockMode.None) throws a NonUniqueObjectException. So far I ignore it because it works, but I'd like to know the reason and a better way to Lock my objects.
Best regards, Expecto

This most likely means you violate the Identity map somewhere. This means you have two instances of an object hanging around with the same database ID, but different referential identity.
Session.Contains will check reference equality, but Lock will throw an exception if there's anything with the same type & id already in the session, which is a much less strict comparison.
Consider the following test on the AdventureWorks database, with a (very naive and unrecommended) simple implementation of Equals & GetHashCode
using (ISession session = SessionFactory.Factory.OpenSession())
{
int someId = 329;
Person p = session.Get<Person>(someId);
Person test = new Person() { BusinessEntityID = someId };
Assert.IsTrue(p.Equals(test)); //your code might think the objects are equal, so you'd probably expect the next line to return true
Assert.IsFalse(session.Contains(test)); //But they're not the same object
Assert.Throws<NonUniqueObjectException>(() =>
{
session.Lock(test, LockMode.None); //So when you ask nhibernate to track changes on both objects, it gets very confused
});
}
NHibernate (and I'd guess any ORM) works by tracking changes to objects. So in Get'ing Person 329, you ask NHibernate to pay attention to whatever happens to that particular instance of a Person. Let's say we change his first-name to Jaime.
Next, we get another instance of person with the same Id (in this case we just new'ed it up, but there are many insidious ways to get such an object). Imagine NHibernate would let us attach this to the session as well. We could even set the first-name of this second object to something like Robb.
When we flush the session NHibernate has no way of knowing whether the database row needs to be synched to either Robb or Jaime. So it throws the non-unique your way before that could happen.
Ideally these situations shouldn't crop up, but if you're very sure what's happening, you might want to check out session.Merge, which lets you force the tracked state to whatever happens to be merged in last (Robb in the example).

the problem was a completely different - contains checks for equality by reference if I don't override Equals(). Now it works with the code from my question!
public override bool Equals(object obj)
{
if (this == obj) {
return true;
}
if (GetType() != obj.GetType()) {
return false;
}
if (Id != ((BaseObject)obj).Id)
{
return false;
}
return true;
}