When does the entity get detached in Entity Framework? - entity-framework

Question

I have seen in many posts that if the tracking is not enabled, the entity is detached.
What I would like to know is: how can there be few objects which are not tracked and a few which are tracked?
Can someone share the code snippet which shows that this entity is not tracked by a context.
According to MSDN:
Detached: the entity is not being tracked by the context
https://msdn.microsoft.com/en-us/library/jj592676(v=vs.113).aspx
According to following post which I read:
http://blog.maskalik.com/entity-framework/2013/12/23/entity-framework-updating-database-from-detached-objects/
var entry = _context.Entry<T>(entity);
if (entry.State == EntityState.Detached)
{
_context.Set<T>().Attach(entity);
entry.State = EntityState.Modified;
}
Detached objects, or objects that are created outside of Entity Framework (EF), don’t have automatic tracking enabled.
And creating a POCO class in code-first approach is one such example of a detached entity.
Is this the only scenario?

Answer 1

There are more scenarios for the Detached Object.
1.You dont want to track an entity.
var entity= context.MyEntities.AsNoTracking().Where(...).FirsOrDefault();
In this query entities retrieved are not tracked hence any changes on the entities will not be recorded to database.
Consider this.
entity.Name = "1";
context.SaveChanges();
As this entities are not tracked the changes will not be saved
unless you attach this.
var entry = _context.Entry<T>(entity);
if (entry.State == EntityState.Detached)
{
_context.Set<T>().Attach(entity);
entry.State = EntityState.Modified;
}
2.Consider your working on disconnected architecture (API,Web). Consider an employee API which have PUT endpoint.
This would attach the employee to the context and update the entity as context is not aware of this entity.
Advantage : No need to fetch the employee entity from the database.
Disadvantage : Someother user changes the entity between the transaction might be losed (you can still update property that are only changed)
public void UpdateEmployee(Employee entity)
{
var entry = _context.Entry<Employee>(entity);
if (entry.State == EntityState.Detached)
{
_context.Attach(entity);
entry.State = EntityState.Modified;
}
Context.SaveChanges()
}
Second Version
public void UpdateEmployee(Employee entity)
{
var dbItem = context.EmployeeEnities.FirstOrDefault(g=>g.Id==entity.Id);
//Context is already have track of this entity, you can just update properties you have changed.
dbItem.Name = entity.Name;
Context.SaveChanges()
}