Ransackで A and (B or C) ref: http://qiita.com/acro5piano/items/9f792d58f0647892d612

file2.sql

SELECT  "users".* FROM "users"
WHERE ("users"."name" ILIKE '%山田%'
       AND ("users"."email" ILIKE '%gmail.com%'
            OR "users"."address" ILIKE '%東京都%'))

index.html.haml

= search_form_for @q do |f|
  = f.search_field :name_cont
  = f.grouping_fields do |g|
    = g.hidden_field :m, value: 'or'
    = g.search_field :email_cont
    = g.search_field :address_cont

users_controller.rb

def index
  @q = User.search(params[:q]))
  @q.build_grouping unless @q.groupings.any?
  @users = @q.result
end