search.rb

module Search
  def run(query, params)
    filters.inject(query) do |memo, filter|
      filter.call(memo, params)
    end
  end

  def filter(arg, options = {})
    case arg
    when Symbol
      def_simple_filter(arg, options)
    when Hash
      def_complex_filter(arg, options)
    end
  end

  private

  def filters
    @filters ||= []
  end

  def def_simple_filter(attr, options = {})
    filters.push(-> (query, params) do
      params.key?(attr) ?
        query.where(attr => params[attr]) :
        query
    end)
  end

  def def_complex_filter(hash, options = {})
    raise NotImplementedError
  end
end

usage.rb

UserSearch.run(User.all, name: 'Pepe', email: 'Lapone@gmail.com')

user_search.rb

module User::Search
  extend ::Search

  filter :name
  filter :email, operation: :matches
end