post__in not working with pre_get_posts, but post__not_in does work

Question

I have an action setup on pre_get_posts which I would like to use post__in on.

I used the following code as a test:

$query->set('post__not_in', array(74485,1528,71460));

This works, the total number of posts is reduced by those 3. However, what if I want to use post__in to just return those 3 posts? I tried:

$query->set('post__in', array(74485,1528,71460));

But that didn't work. I thought maybe it was because post__not_in is already being used (as the codex says post__in and post__not_in can't be used together. So I tried:

$query->set('post__not_in', '');
$query->set('post__in', array(74485,1528,71460));

But this still doesn't work.

Any help please? Thanks

Answer 1

I just ran into this with WooCommerce. I ended up passing the IDs I needed to a variable for safe keeping, and then hooking a function with extremely low priority. That worked fine. I figure woocommerce or what of my cleint's 8000000 plugins was overwriting post__in.

add_filter('pre_get_posts', 'assign_ids', 1000);