2014XXX_add_attachment_columns_to_photos.rb

class AddAttachmentColumnsToPhotos < ActiveRecord::Migration
  def self.up
    change_table :photos do |t|
      t.has_attached_file :attachment
    end
  end

  def self.down
    drop_attached_file :photos, :attachment
  end
end

2014XXX_create_photos.rb

class CreatePhotos < ActiveRecord::Migration
  def change
    create_table :photos do |t|
      t.integer :post_id

      t.timestamps
    end
  end
end 

_form.html.erb

<%= f.simple_fields_for :photos do |photo| %>
  <%= f.file_field :attachment, :id => "photo_attachment" %>
<% end %>

_post.html.erb

<div class="photos">
  <% for photo in post.photos %>
    <%= image_tag photo.attachment.url(:medium) %>
  <% end %>
</div>

photo.rb

class Photo < ActiveRecord::Base
  belongs_to :post, :class_name => "Forem::Post"

  has_attached_file :attachment,
    path: ":rails_root/public/system/attachments/:id/:style/:filename",
    url: "/system/attachments/:id/:style/:filename",
    styles: {
      display: "480x480>",
      thumbnail: "70x70#"
    },
    convert_options: { all: "-quality 100" },
    processors: [:thumbnail, :compression]

  validates_attachment_presence :attachment
  validates_attachment :attachment, content_type: {
    content_type: ["image/jpg", "image/jpeg", "image/png", "image/gif"]
  }
end

post_decorator.rb

Forem::Post.class_eval do
  has_many :photos

  accepts_nested_attributes_for :photos
end

posts_controller_decorator.rb

Forem::PostsController.class_eval do
protected
  def post_params
    params.require(:post).permit(:text, :password, :email, photos_attributes: [])
  end
end

topics_controller_decorator.rb

Forem::TopicsController.class_eval do
  def new
    authorize! :create_topic, @forum
    @topic = @forum.topics.build
    post = @topic.posts.build
    post.photos.build
  end
end