Implicit binding of nested route parameter

[shaedrich]

I'm currently writing a laravel application with a nested route parameter that is dependent on the previous route parameter. While reading the docs, I stumbled across the following paragraph:

When using a custom keyed implicit binding as a nested route parameter, Laravel will automatically scope the query to retrieve the nested model by its parent using conventions to guess the relationship name on the parent.

It's nice—no doubt about that—but nonetheless, odd. Why does this only happen with custom keys? That seems a bit arbitrary. I currently use "id" as a custom key to make use of that behavior. Is it possible to make use of it without that non-custom custom key AND without explicit binding?