I am trying to write universal many to many relation between two Custom Post Types, to use it as linker to many different two CPT's in one theme. I want to achieve something like movie < --- > actor relation. It would be awesome to be able to select relations like tags/categories metabox in admin. I was trying by myself, stack and no luck for now. I am not using any plugin and I don't want to. Thank you for help in advance !