Question 2

activity.rb

# coding: utf-8
class Activity < ActiveRecord::Base
  belongs_to :target,  polymorphic: true

  def message
    # return something
  end
end

publication.rb

# coding: utf-8
class Publication < ActiveRecord::Base
  has_many :target_activities, as: :target, class_name: 'Activity', dependent: :destroy
end

show.html.haml

- @publications.each do |pub|
  = pub.title
  - pub.target_activities.limit(7).each.with_index do |activity, i|
    - if i < 6
      = activity.message
    - else
      = "もっと見る"

skilltags_controller.rb

# coding: utf-8
class SkilltagsControlle < ApplicationController
  def show(id)
    @skilltag = Skilltag.find id
    @publications = @skilltag.publications.all
  end
end