Searching multiple collections in a relation widget would be a cool feature. There are a few other feature requests that could provide what you need, such as conditional fields, but no really great way to do it currently.
I’ll give you the best approach I can think of, but first the drawbacks:
- Nothing will be shown for this field in the preview pane
- Each collection that you want to search will have to be indicated in the field config (won’t automatically work for all collections - but I can’t imagine a future solution that wouldn’t require this)
- You only need one value, but this approach will use a list, so you’d only use the first value in the list
- Uses a beta feature, so the API might change
Here’s how it would work:
- Use the beta list types feature
- For each collection that you want to query, add a type whose fields consist of a relation widget
Users could then select the collection they want to query by selecting the “type” of list item they want to add, then query away. Besides the caveats mentioned above, I’m pretty sure this will work. If you try it let me know how it goes.