How to add entity with existing attached to it - entity-framework

Question

I want to add entity payment object, containing EXISTING Currency object to EF database:
public Payment()
{
int Id {get;set;}
public int Value {get;set;}
public Currency SelectedCurrency{get;set;}
}
public Currency()
{
int Id {get;set;}
string Name;
}
Suppose that I have existing Currency attached to new entity Payment(). When I add such entity Payment(), the error appears
Violation of PRIMARY KEY constraint 'PK_dbo.Currency'. Cannot insert duplicate key in object 'dbo.MwbeCurrency'. The duplicate key value is (GBP).\r\nThe statement has been terminated."}
How to add higher-level entity with attached existing lower-level entity?
My code for adding entity is:
public virtual TEntity Add(TEntity entity)
{
return DbSet.Add(entity);
}
public void SaveChanges()
{
Context.SaveChanges();
}

Answer 1

I suspect you retrieved Currency with a different instance than the one that retrieved Payment and did something like this :
payment.Currency = retrievedCurrency;
Therefore, the Payment context things that Currency is a new object and tries to persist it. Since it already exists, you are getting a PRIMARY KEY violation.
If you want to persist Payment correctly, add the following lines:
if (payment.Currency != null && payment.Currency.Id != 0)
{
context.Entry(payment.Currency).State = EntityState.Unchanged;
}
although it would probably be cleaner if you retrieved Payment and Currency with the same context, so you can persist them appropriately.

Answer 2

Calling DbSet.Add(entity) adds the entire graph for persistence, which means it will go through all the navigation properties of entity and set each one's state to EntityState.Added.
While the other answer might work, a better approach is to change the way you are adding the objects, and be explicit about what entities you are adding / updating / etc.
To do this, change:
public virtual void Add(TEntity entity)
{
DbSet.Add(entity);
}
To:
public virtual void Add(TEntity entity)
{
context.Entry(entity).State = EntityState.Added;
}
This will add only the supplied entity. If one of your navigation properties objects is also new, you call .Add(entity) on it as well.
If you do need to add the entire graph in other situations, you can add an additional method that works the way your original one does, but has a better name to indicate it's function:
public virtual void AddGraph(TEntity entity)
{
DbSet.Add(entity);
}
Good Luck
Update
Additionally, since it looks like you are using a repository, I prefer to disable auto detect changes by setting context.Configuration.AutoDetectChangesEnabled = false; If you modify a property on an entity that you want persisted, you would need to set the state of the entity to modified like so:
public virtual void Update(TEntity entity)
{
context.Entry(entity).State = EntityState.Modified;
}