Ruby on Rails4で複数キーワードのOR検索をしたいときはArelを使う。 ref: http://qiita.com/budougumi0617/items/e6102f0d424d4031f7e2

add_more_keyword

    for i in 1...keyword_arrays.length
      seminars_sel = seminars_sel.or(seminars.matches("\%#{keyword_arrays[i]}\%"))
    end

index.html.erb

    <div id="condition-cards" class="clearfix">
      <%= form_tag seminars_path, :method => 'get' do %>
      <div id="keyword">
        <dl class="clearfix">
          <dt><%= text_field_tag :search, params[:search] %></dt>
          <dd><%= submit_tag "検索", :name => nil %></dd>
        </dl>
      </div>
      <% end %>
    </div>

schema.rb

ActiveRecord::Schema.define(version: 20140713000000) do

  # These are extensions that must be enabled in order to support this database
  enable_extension "plpgsql"

  create_table "seminars", force: true do |t|
    t.string   "summary"
  end

end

seminar.rb

# encoding: utf-8

class Seminar < ActiveRecord::Base

  def self.search(params)
    params[:search] ||= ""
    keyword_arrays = params[:search].gsub(/　/," ").split()
    seminars = Seminar.arel_table[:summary]
    seminars_sel = seminars.matches("\%#{keyword_arrays[0]}\%")
    for i in 1...keyword_arrays.length
      seminars_sel = seminars_sel.or(seminars.matches("\%#{keyword_arrays[i]}\%"))
    end
    logger.debug("SQL: #{Seminar.where(seminars_sel).to_sql}")

    Seminar.where(seminars_sel)
  end
end