Using relatedTo, and/or and :empty/:notempty in queries

Question

I got users that has 3 Entries fields in their profile: breadForMonday, breadForWednesday and breadForFriday.

I want to differentiate between users with 1, 2 or 3 selected entries. How would I use relatedTo together with and/or and :empty/:notempty?

Something like this:

{% set bread = craft.entries({ section: 'bread' }).limit(null) %}
{# Find users that have selected entries for two out of three fields #}
{% set users = craft.users({
    group: 'customers',
    relatedTo: [
        'and',
        { targetElement: bread, field: 'breadForMonday' },
        { targetElement: bread, field: 'breadForWednesday' }
    ]
}) %}

Answer 1

Please don't laugh, but I thought to myself, let's give this a go in Twig and see where it takes me. It very soon began to become clear, that this will end up in lots of criteria models, db queries and a huge pile of Twig code.

So my better answer should be, don't do it in Twig. This is something you should better do in PHP with custom database queries (→ Yii query builder). But now that I've done it, here's a gist of that Twig code.