Use Ohm to illustrate how to manage an index within Redis. See http://stackoverflow.com/a/19289987/1688185.

colour.rb

require "ohm"

class Doc < Ohm::Model
  attribute :colour
  index :colour
end

Doc.create(:colour => "red")
Doc.create(:colour => "green")
Doc.create(:colour => "red")

# redis> SMEMBERS "Doc:indices:colour:red"
# 1) "1"
# 2) "3"

Doc.create(:colour => "red")

# redis> SMEMBERS "Doc:indices:colour:red"
# 1) "1"
# 2) "3"
# 3) "4"

Doc.find(:colour => "red").each {|d| puts d.id}
# 1
# 3
# 4