I have a `state` (state machine value) property on an entity and am setting datetime fields based on when the entity transitions to a given state.

Task states: (simplified)

* new
* in\_progress
* complete

A task can transition back into `in_progress` after it is set to `complete`, but I want to use timestampable to set a `$dateStarted` property only on first time it transitions to `in_progress`.

This code will always set `$dateStarted` to the last time `$state` transitions to `in_progress`.

Is there a way to configure it so `$dateStarted` is only set the first time the task transitions to `in_progress`?

```
/**
 * @var \DateTime
 *
 * @ORM\Column(type="datetime", nullable=true)
 * @Gedmo\Timestampable(on="change", field="state", value="in_progress")
 */
private $dateStarted;

```