I'm trying to build a custom module that contains an EAV entity in Magento 2 for learning purposes.
I have troubles performing a proper save from an admin controller.
I'm trying to avoid code like:

$data = get data from post here; 
$myObj->addData($data)->save();

Because the save method is deprecated and because I'm trying to work with interfaces instead of their implementations.

For a flat entity I could do this:

$entity = either get it from a repository or via factory;
$data = get data from post
$this->dataObjectHelper->populateWithArray($entity, $data, EntityInterface::class);

Where dataObjectHelper is an instance of \Magento\Framework\Api\DataObjectHelper that reads the setters of the interface passed as the 3rd parameter and maps the corresponding to the methods from the $data array to my entity.

So if $data looks like this:

$data = [
    'title'=>'Some title',
    'description' => 'Some description'
]

will just call $entity->setTitle('Some title')->setDescription('Some description').

But I cannot do that for the EAV entities because my entity interface will contain only the system attributes and not the custom ones.

I need to get the data from POST and pass it to the $entity instance and then call $entityRepository->save($entity) but I don't want to lose any custom attributes.

How can I proceed in this case?
I looked in the core and for products and categories the controllers still call $entity->save() which is not OK.

I'm trying to build a custom module that contains an EAV entity in Magento 2 for learning purposes.
I have troubles performing a proper save from an admin controller.
I'm trying to avoid code like:

$data = get data from post here; 
$myObj->addData($data)->save();

Because the save method is deprecated and because I'm trying to work with interfaces instead of their implementations.

For a flat entity I could do this:

$entity = either get it from a repository or via factory;
$data = get data from post
$this->dataObjectHelper->populateWithArray($entity, $data, EntityInterface::class);

Where dataObjectHelper is an instance of \Magento\Framework\Api\DataObjectHelper that reads the setters of the interface passed as the 3rd parameter and maps the corresponding to the methods from the $data array to my entity.

So if $data looks like this:

$data = [
    'title'=>'Some title',
    'description' => 'Some description'
]

will just call $entity->setTitle('Some title')->setDescription('Some description').

But I cannot do that for the EAV entities because my entity interface will contain only the system attributes and not the custom ones.

I need to get the data from POST and pass it to the $entity instance and then call $entityRepository->save($entity) but I don't want to lose any custom attributes.

How can I proceed in this case?
I looked in the core and for products and categories the controllers still call $entity->save() which is not OK.