It is not a ruby keyword, but it appears to be a method defined on scopes.
Try this:
@issue_books.method(:taken).owner
#=> ActiveRecord::Delegation
@issue_books.method(:taken).source_location
#=> (...)/gems/activerecord-3.2.6/lib/active_record/relation/delegation.rb
So the scope taken
you have defined is probably overshadowed by a definition in ActiveRecord::Delegate
.
Update:
I did some digging, and taken
seems to be defined as an alias for limit
in Arel::SelectorManager
, a dependency of ActiveRecord
.