What is the difference between Add and Attach in Entity Framework, and how can I fix my problem?

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:

• Attach the trainer to the context (I can check if it already exists using the key)
• Add the trainer to the context (using 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?

• If I attach the trainer/horse, then when I attach the result, I get InvalidOperationException, teling me that the trainer is already on the object context.
• If I add the trainer instead of attaching it, I get another error (can't remember it right now, but it was telling me that a Trainer was already on the database).

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.