File upload for the classes

With one of our products in our company we were given the task of building a file uploading system which would take different type of files and upload it to our server.Instead of creating a separate class for each file and having different methods to upload the file, we instead opted for a creating a polymorphic class which can be used with different classes with the has_many relation.

First we begin by creating a migration with the polymorphic relations in the database

rails g migration CreateAssetsTable

We then fill in the migration

class CreateAssetsTable < ActiveRecord::Migration
  def change
    create_table :assets do |t|
      t.string :type
      t.references :attachable, polymorphic: true
      t.timestamps
    end
  end
end

We then run the migration

rake db:migrate

This sets up the assets table with 6 columns.Id,Type,attachable_id,attachable_type,created_at and updated_at.The important columns for the polymorphic relation are the attachable_type and attachable_id.This sets up the relation between other classes and the asset class at the database level.
Next we create the model for this class

rails g model asset

This will create the model class and set us up with an empty model class.We then will in the polymorphic relation.

class Asset < ActiveRecord::Base
  belongs_to :attachable, polymorphic: true
end

Just like in the migration the polymorphic relation is the most important feature of this class.

Now that we have the asset class ready we then create other class which will use this relation.
Example:

class Users < ActiveRecord::Base
   has_many :avatars, as: :attachable, class_name: "Asset"
end

The has many sets up the has_many relation with the user model.So now the user has many avatars which refers to the assets model for the asset with type avatar.
Now that we have dealt with the polymorphic nature of the classes,It becomes very easy to add the upload module to the asset class using the paperclip gem by thoughtbot.
We add the gem to the gem file

gem 'paperclip'
bundle install -j4

Next we generate migration for the assets table to hold your new data

class AddAttachmentToAsset < ActiveRecord::Base
  def up
    add_attachment :assets, :file
  end
  def down
    remove_attachment :assets, :file
  end
end
rake db:migrate

We the add the has_attached_file to the Asset class

class Asset < ActiveRecord::Base
  belongs_to :attachable, polymorphic: true
  has_attached_file :file
end

Since version 4 of paperclip content validation is required by default.
So we also add a simple image file validation using regex

   validates_attachment_content_type :file, content_type: /Aimage/.*z/

This check if the mime type of the file contains image/ followed by the format.
Now we have a generic file upload class.