Getting related objects

I have 2 entites: ActivityParameter and Activity and some logic:

Activity entity has relationship:

    $this->hasMany('id', 'ActivityParameter', 'activity_id', [

        'alias' => 'parameters',
        'reusable' => true,
        'foreignKey' => [
            'action' => Relation::ACTION_CASCADE
        ]
    ]);

Logic:

    $parameter = new ActivityParameter();
    $parameter->setValue(10.101);
    $parameter->setUnitId(1);

    $activity = new Activity();
    $activity->parameters = $parameter;

My problem is that:

count($activity->parameters);

gives 1, but:

count($activity->getParameters());

gives 0.

How to resolve this problem? I need to use getParameters() beacause later I use:

$activity->getParameters("value = '10.101'");

I found it is needed to make $activity->save(); to use $activity->getParameters(). I don't want to save these objectd because it's unit test. How to fix it?