1👍
I do not think the select_related method will accomplish the goal you have set out to achieve with this ModelChoiceField. You are quite correct that the two queries below return the same resulting queryset:
Catalog.objects.all().select_related('article_products'))
Catalog.objects.all()
The select_related method of Django querysets serves a different function, specifically as a performance booster to reduce the number of database accesses required to obtain the data you want to retrieve from a model instance. The Django reference about this method contains very good documentation, with examples explaining why you would use the select_related method for performance purposes.
With that being said, your original purpose remains: The form field would display all of the articles related to a given catalog.
In order to achieve this goal, it seems best to filter the queryset of the Article objects being given to the form field. First of all, if we want to display Article objects within the ModelChoiceField, we should certainly give the ModelChoiceField a queryset containing Article objects rather than Catalog objects, like so:
article = forms.ModelChoiceField(queryset=Article.objects.all())
But this queryset argument is not quite right, either. We are still passing the queryset of all Article objects that exist in the database. Instead, we want to pass only the articles that are associated with a given Catalog object. To achieve this goal, we can filter the Article queryset to obtain only the Article objects that are related to a certain Catalog object, like so:
# cat is some catalog object
article = forms.ModelChoiceField(queryset=Article.objects.filter(catalog=cat))
In this example, the queryset filter returns only Article objects which contain a reference to the given Catalog object. This queryset will be used to populate the ModelChoiceField.
For more information about filtering by field lookup, see the Django documentation here.