class Project < ApplicationRecord
  has_many :sales, -> { order(:date) }, inverse_of: :project, dependent: :destroy
end

class Sale < ApplicationRecord
  belongs_to :project, inverse_of: :sales, touch: true
  validates :hours, numericality: { only_integer: true }
end

class ProjectsController < ApplicationController
  def index
    @q = Project.ransack(params[:q])
    @projects = @q.result
  end
end

<th><%= sort_link(@q, <SUM_OF_SOLD_HOURS>, 'Hours') %></th>