You can’t do a query in WP that queries posts based on fields in other posts.
You would need to query the field in the other post and then query the posts you want based on the results of that query. Not a quick solution at all.
So basically:
- Query the the possible related posts
- Dynamically build a new query
- Query the posts
If you are interested I created a new plugin recently that will pull the data from the related post into the post you want to query so that you can query the those values directly and I’m looking for input on it. I posted the information here: http://support.advancedcustomfields.com/forums/topic/field-converter/
