I recently started using Entity Framework, and it has been kind of a pain to check if I really need to add new records to the database or not.
If the Entity I need to add to the database is already on it, I will know, because I do a query before inserting it, and if it exists, then I keep that instance because I need to use it in some relationships.
Let's suppose my entity name is Book
.
The problem comes when an entity isn't in the database, and I do:
Book b = //...
modelContainer.AddToBooks(b);
I could easily do:
modelContainer.SaveChanges()
everytime I add a new entity (no matter what entity it is), and this will work fine, because as I'm inserting one kind of entry at a time, and checking if it already is in the database, I won't have duplication problems.
But what if I want to avoid calling SaveChanges()
so often?
In this question: Is is possible to check if an object is already attached to a data context in Entity Framework?, the author of the question provides a method that kind of helps me in my case, but it does not work if I Add
the object to the context instead of Attaching
it.
My question (maybe two, but very related) is: What is the difference between Add and Attach and how can I solve my problem?
Edit:
Here is an example of the problem I'm having.
I have an entity Result
that has a relationship with two more entities: Trainer
and Horse
.
I get the data from an external source, so I have to create manually all the entities.
Everytime I need to insert a new Trainer
, I do:
var trainer = Trainer.CreateTrainer(Id)
Then I query the database to see if a trainer with that Id
is already on the database. If it is, then I replace the trainer
variable with the one that is on the database.
If it isn't, I can do two things here:
AddToTrainers(...)
)The same process for Horse
.
Now, when I need to create a new Result
(that contains a Trainer
and a Horse
), I assign the previous trainer & horse to that result instance.
What should I do here to be able to add to the context that new Result
?
InvalidOperationException
, teling me that the trainer is already on the object context.Important:
The first error is given when attaching the result, and the second one when doing SaveChanges()
.
What I want to avoid here is calling SaveChanges()
everytime I add a new result.
Attach is used to repopulate a context with an entity that is known to already exist in the database. SaveChanges will therefore not attempt to insert an attached entity into the database because it is assumed to already be there.
Use the DbSet. Add method to add a new entity to a context (instance of DbContext ), which will insert a new record in the database when you call the SaveChanges() method.
AsNoTracking(IQueryable)Returns a new query where the entities returned will not be cached in the DbContext or ObjectContext. This method works by calling the AsNoTracking method of the underlying query object.
No-Tracking query using AsNoTracking() extention method The AsNoTracking() extension method returns a new query and returned entities do not track by the context. It means that EF does not perform any additional task to store the retrieve entities for tracking.
ObjectContext
internally tracks all entities which was either loaded by context, attached or added. Only these entities can be modified in database when SaveChanges
is invoked. Each such entity has a ObjectStateEntry
in the ObjectStateManager
. One of the main properties of the ObjectStateEntry
is a State
. The state is of enum type EntityState
which offers these values:
Each entity loaded from the database is in Unchanged
state. Detached is special state. You will not find ObjectStateEntry
with Detached state in the ObjectStateManager
. But if you ask ObjectStateManager
for the ObjectStateEntry
for entity not tracked by the context it will create a new ObjectStateEntry
with Detached
state.
Now the difference between Attach
and AddObject
:
Attach
- if you call this method ObjectContext
will start tracking whole object graph (main entity and all related entities). All entities which were not tracked yet will be set to Unchanged
state. AddObject
- if you call this method ObjectContext
will also start tracking whole object graph (main entity and all related entities). The difference is that all entities which were not tracked yet will be set to Added
state (= new objects which must be set to database).I know that I am a little late to this post, but was looking for similar solution... and then I found this Microsoft article which may succinctly answers most of the OP's questions, and may help future SO viewers?:
Add/Attach and Entity States == ( http://msdn.microsoft.com/en-us/data/jj592676.aspx )
From the article:
This topic will cover how to add and attach entities to a context and how Entity Framework processes these during SaveChanges.
And specifically see the last section on that page:
Insert or update pattern: A common pattern for some applications is to either Add an entity as new (resulting in a database insert) or Attach an entity as existing and mark it as modified (resulting in a database update) depending on the value of the primary key.
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