Update only the fields that were changed in Laravel 5

I'd like to update only the fields that were changed(but I'm passing all fields), like the method fill(). But, When I do this in polymorphic relations, it doesn't work.

Fill():

$this->empresa->fill($input)->save();

ex: updating only updated_atfield: Log:

> `update `empresas` set `updated_at` = ? where `id` = ? [["2017-08-24
> 11:26:04",1],0.31]`

but in child, even if I change only 1 field, the model updates all fields.

$empresa->contatos()->update($input);

Log:

> update `contatos` set `endereco` = ?, `numero` = ?, `bairro` = ?,
> `cep` = ?, `cidade` = ?, `complemento` = ?, `tipo_end` = ?,
> `estado_id` = ?, `updated_at` = ? where `contatos`.`contatoable_id` =
> ? and `contatos`.`contatoable_id` is not null and
> `contatos`.`contatoable_type` = ? [["Rua Um","2145","Vila
> Toledo","27590000","city","Portão Branco","2","19","2017-08-24
> 11:26:04",1,"empresas"],0.39]

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