Question

The Ruby and C tags exist in the db.

Tag.where(:name => ["Ruby", "C", "foo", "bar"]).map(&:name) 

Returns: ["Ruby", "C"].

How do I return the tags that aren’t in the db. So the opposite. i.e. ["foo", "bar"]

Looking for an elegant Ruby one liner solution. If possible.

Answer 1

search_tags = ["Ruby", "C", "foo", "bar"]
not_found = search_tags - Tag.where(:name => search_tags).map(&:name)

Just a quick answer with mostly copy/pasting what you already had, adjust as needed.