How to ensure proxies are created when using the repository pattern with entity framework? - entity-framework

Question

I have this method in my SurveyController class:
public ActionResult AddProperties(int id, int[] propertyids, int page = 1)
{
var survey = _uow.SurveyRepository.Find(id);
if (propertyids == null)
return GetPropertiesTable(survey, page);
var repo = _uow.PropertySurveyRepository;
propertyids.Select(propertyid => new PropertySurvey
{
//Setting the Property rather than the PropertyID
//prevents the error occurring later
//Property = _uow.PropertyRepository.Find(propertyid),
PropertyID = propertyid,
SurveyID = id
})
.ForEach(x => repo.InsertOrUpdate(x));
_uow.Save();
return GetPropertiesTable(survey, page);
}
The GetPropertiesTable redisplays Properties but PropertySurvey.Property is marked virtual and I have created the entity using the new operator, so a proxy to support lazy loading was never created and it is null when I access it. When we have access direct to the DbContext we can use the Create method to explicitly create the proxy. But I have a unit of work and repository pattern here. I guess I could expose the context.Create method via a repository.Create method and then I need to remember to use that instead of the new operator when I add an entity . But wouldn't it be better to encapsulate the problem in my InsertOrUpdate method? Is there some way to detect that the entity being added is not a proxy when it should be and substitute a proxy? This is my InsertOrUpdate method in my base repository class:
protected virtual void InsertOrUpdate(T e, int id)
{
if (id == default(int))
{
// New entity
context.Set<T>().Add(e);
}
else
{
// Existing entity
context.Entry(e).State = EntityState.Modified;
}
}

Answer 1

Based on the answer supplied by qujck. Here is how you can do it without having to employ automapper:
Edited to always check for proxy - not just during insert - as suggested in comments
Edited again to use a different way of checking whether a proxy was passed in to the method. The reason for changing the technique is that I ran into a problem when I introduced an entity that inherited from another. In that case an inherited entity can fail the entity.e.GetType().Equals(instance.GetType() check even if it is a proxy. I got the new technique from this answer
public virtual T InsertOrUpdate(T e)
{
DbSet<T> dbSet = Context.Set<T>();
DbEntityEntry<T> entry;
if (e.GetType().BaseType != null
&& e.GetType().Namespace == "System.Data.Entity.DynamicProxies")
{
//The entity being added is already a proxy type that supports lazy
//loading - just get the context entry
entry = Context.Entry(e);
}
else
{
//The entity being added has been created using the "new" operator.
//Generate a proxy type to support lazy loading and attach it
T instance = dbSet.Create();
instance.ID = e.ID;
entry = Context.Entry(instance);
dbSet.Attach(instance);
//and set it's values to those of the entity
entry.CurrentValues.SetValues(e);
e = instance;
}
entry.State = e.ID == default(int) ?
EntityState.Added :
EntityState.Modified;
return e;
}
public abstract class ModelBase
{
public int ID { get; set; }
}

Answer 2

I agree with you that this should be handled in one place and the best place to catch all looks to be your repository. You can compare the type of T with an instance created by the context and use something like Automapper to quickly transfer all of the values if the types do not match.
private bool mapCreated = false;
protected virtual void InsertOrUpdate(T e, int id)
{
T instance = context.Set<T>().Create();
if (e.GetType().Equals(instance.GetType()))
instance = e;
else
{
//this bit should really be managed somewhere else
if (!mapCreated)
{
Mapper.CreateMap(e.GetType(), instance.GetType());
mapCreated = true;
}
instance = Mapper.Map(e, instance);
}
if (id == default(int))
context.Set<T>().Add(instance);
else
context.Entry(instance).State = EntityState.Modified;
}