Count the number of times a specific user downloaded files through Active Storage

Question

Both my Post & Comment models have has_one_attached :file. What I want to do is to count the number of times a specific user downloaded either a post's file or a comment's file. Create a sort of download_counter_cache on the User model that increments whenever this user downloads a file. How can I achieve this?

Update based on Max's answer

This is what I did for the moment:

# Migration file
class CreateDownloads < ActiveRecord::Migration[6.0]
  def change
    create_table :downloads do |t|
      t.references :user, null: false, foreign_key: true
      t.references :resource, polymorphic: true

      t.timestamps
    end
  end
end

# Routes.rb
 concern :downloadable do
   resources :downloads, only: :create
 end

 resources :posts, concerns: :downloadable do
   [...] # Routes such as like/dislike
   resources :comments, concerns: :downloadable do
     [...] # Routes such as like/dislike
   end
 end

# In posts/_post.html.erb
<%= link_to([@post, :downloads], html_options = {class: 'file w-100'}, method: :post) do %>
  [...]
<% end %>

My downloads_controller is exactly the same as suggested in Max's answer, as are my Post, Comment, User & Download models.

The problem is that whenever I try to download it redirects me to downloads#index which obviously does not exist. I am not sure how I am supposed to create the resource class.

Answer 1

max has a good answer, but if you just want to have only one column on a User model to count all downloads it could be done without a polymorphic join table.

Let's add an attribute to the User module to store the counts:

  add_column :users, :download_count, :integer

Then add a controller that will handle the counts and redirect to a downloaded file:

class DownloadsController < ApplicationController
  def create
    # using find_by so it doesn't throw any errors
    resource = Post.find_by_id(params[:post_id]) || Comment.find_by_id(params[:comment_id])

    if resource
      current_user.increment!(:download_count)
      redirect_to rails_blob_path(resource.file, disposition: "attachment")
    else
      render nothing: true
    end
  end
end

Routes will look like:

  resources :downloads, only: :create

Download link in view looks like:

# for post file
<%= link_to 'download file', downloads_path(post_id: @post.id), method: :post %>

# for comment file
<%= link_to 'download file', downloads_path(comment_id: comment.id), method: :post %>

As simple as that.

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Update: changed the method from new to post to prevent double rendering.