# Simple model

We briefly introduced [MementoEntity and MementoEntityCollection](/release-1/memento/change-tracking-service/memento-entities.md) and seen a code snipped that allows us to attach the memento tracking system to an entity:

```csharp
var memento = new ChangeTrackingService();

var person = new Person();
memento.Attach( person );

person.FirstName = "first name value";
person.LastName = "last name value";

var isChanged = memento.IsChanged; //true
var canUndo = memento.CanUndo; //true
```

What is happening is that each change performed on each tracked entity will be recorded by che `ChangeTrackingService`, on each tracked entity means that the following will work as expected:

```csharp
var memento = new ChangeTrackingService();

var person = new Person();
var customer = new Customer();
memento.Attach( person );
memento.Attach( customer );

person.FirstName = "first name value";
person.LastName = "last name value";
customer.CompanyName = "sample company";

var isChanged = memento.IsChanged; //true
var canUndo = memento.CanUndo; //true
```

Tracking at the same time both the `Person` instance and the `Customer` instance. The following sample highlights how changes are tracked:

```csharp
var memento = new ChangeTrackingService();

var person = new Person();
memento.Attach( person );

person.FirstName = "a name";
person.LastName = "last name";

var isChanged = memento.IsChanged;
var state = memento.GetEntityState( person );

memento.Undo();
var _lastNameAfterUndo = person.LastName; //null

memento.Redo();
var _lastNameAfterRedo = person.LastName; //"last name"
```

The memento keeps tracks of a stack of changes in the exact same order they happened to the tracked models, each time `Undo()` is called the last change in the stack will be reverted and moved into the forward changes stack allowing the caller to call `Redo()` in order to apply it once again.

Calling `AcceptChanges()` on a memento instance will flush all the recorded changes considering the models as unchanged. Calling `RejectChanges()` will revert all the tracked models to their original state, or to the state we called `AcceptChanges()` last time.

The same applies also to collections: [Handling Change Tracking in collections](/release-1/memento/change-tracking-service/handling-change-tracking/collections.md)


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