Django models filter by foreignkey

Question

I'm having some trouble in filtering objects from a set of models. Here is the problem:

I have 3 classes:

class Autor(models.Model):    
    nome = models.CharField(max_length=50)
    slug = models.SlugField(max_length=50, blank=True, unique=True)
    foto = models.ImageField(upload_to='autores/', null=True, blank=True)
    ...

class CategoriaRecolha(models.Model):
    categoria = models.CharField(max_length=30)
    descricao = models.TextField()
    slug = models.SlugField(max_length=30, blank=True, unique=True)
    ...

class Recolha(models.Model):    
    titulo = models.CharField(max_length=100)
    slug = models.SlugField(max_length=100, blank=True, unique=True)
    descricao = models.TextField()
    ficha_tec = models.TextField()
    categoria = models.ForeignKey(CategoriaRecolha)
    autor = models.ForeignKey(Autor)
    ....

What I'm trying to retrieve is the fields of the class Autor, in which the field categoria of the class Recolha is equal to a specific value.

In a more simple way, I need to get the all the autor that have participated in a specific categoria.

Thanks

Answer 1

A more direct alternative:

autors = Autor.objects.filter(recolha__categoria=MyCategoria)

where MyCategoria is the relevant CategoriaRecolha instance. Or, if you want to match agains the specific category name, you can extend the query another level:

autors = Autor.objects.filter(recolha__categoria__categoria='my_category_name')

Answer 2

in django 2 is ForeignKey.limit_choices_to docs

staff_member = models.ForeignKey(
    User,
    on_delete=models.CASCADE,
    limit_choices_to={'is_staff': True},
)

Answer 3

cat = CategoriaRecolha.objects.get(field=value)
rows = Recolha.filter(categoria=cat)
autors = [row.autor for row in rows]

The Django Docs explain this pretty well.