On second thought, this is really two proposals, one to slice querysets where possible, and another to allow an arbitrary function to be run after materialising relationship lists.

Huge thanks for this! Really appreciate you taking the time to dig in and make a contribution.

In terms of the change itself, I think slicing a queryset and then returning just (say) the first item is something I'd be interested in including in the library. The question is, at what level do we do this? You've taken the approach of baking support deep into the guts of the library. I'm not totally sure this is necessary. I feel like this is something that it should be possible to support at the higher levels (ie pairs) without making too many deep-level changes to (say) qs. My approach to adding features to django-readers is always to start with: could this be done in application code without adding features to readers itself? If so, what features might we consider adding to make it simpler? So start at the top and work down, rather than starting at the bottom and working up.

As an example: I don't think it's necessary to include a separate slice parameter to the relationship functions in qs, because the prepare_related_queryset parameter should already be general-purpose enough to do this:

def apply_slice(slice):
    def prepare(queryset):
        return queryset[slice]

Anyway, let me have a think about this over the next few days and I'll comment with some more thoughts 🙂

Thanks again

Totally appreciate that you don't want to add a larger surface area to the library unless you need to.

I can see now that slicing querysets could be done with a slice pair, or even directly slicing the related_queryset that you would pass to the reverse_relationship pair, I think I was getting tripped up by the fact that slicing has to be done as the last step of queryset preparation, so potentially the solutions you have offered aren't as easily composable as something built in to the library.

I'm looking forward to hearing your thoughts, thanks for taking the time to consider this.