Uniq cannot be used directly on an ActiveRecord model. 'DISTINCT fiel…

…d' is the only pluck query that translates into the aforementioned SQL

activerecord/lib/active_record/relation/calculations.rb

@@ -135,7 +135,7 @@ def calculate(operation, column_name, options = {})
     #   # SELECT people.id, people.name FROM people
     #   # => [[1, 'David'], [2, 'Jeremy'], [3, 'Jose']]
     #
-    #   Person.uniq.pluck(:role)
+    #   Person.pluck('DISTINCT role')
     #   # SELECT DISTINCT role FROM people
     #   # => ['admin', 'member', 'guest']
     #