Uploading a file in Rails

I'm new to rails, and I'm writing a RESTful website using the CRUD technique. So far I have created three pages, all of which allow the user to create, edit, and delete a row from the database. However, my fourth page will need to include an upload file form, but a) I don't know how the filesystem works with Rails thus I don't know where files should be stored. The file would be around 100kb and couldn't be stored in temporary storage because it will be constantly downloaded. And b) I don't know how to write to a file.

It would be great if you could tell me how to do what I mentioned above - create an upload input on an input form, and to then write the file to a filepath in a separate directory.

Update 2018

While everything written below still holds true, Rails 5.2 now includes active_storage, which allows stuff like uploading directly to S3 (or other cloud storage services), image transformations, etc. You should check out the rails guide and decide for yourself what fits your needs.

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While there are plenty of gems that solve file uploading pretty nicely (see https://www.ruby-toolbox.com/categories/rails_file_uploads for a list), rails has built-in helpers which make it easy to roll your own solution.

Use the file_field-form helper in your form, and rails handles the uploading for you:

<%= form_for @person do |f| %>
  <%= f.file_field :picture %>
<% end %>

You will have access in the controller to the uploaded file as follows:

uploaded_io = params[:person][:picture]
File.open(Rails.root.join('public', 'uploads', uploaded_io.original_filename), 'wb') do |file|
  file.write(uploaded_io.read)
end

It depends on the complexity of what you want to achieve, but this is totally sufficient for easy file uploading/downloading tasks. This example is taken from the rails guides, you can go there for further information: http://guides.rubyonrails.org/form_helpers.html#uploading-files

Sept 2018

For anyone checking this question recently, Rails 5.2+ now has ActiveStorage by default & I highly recommend checking it out.

Since it is part of the core Rails 5.2+ now, it is very well integrated & has excellent capabilities out of the box (still all other well-known gems like Carrierwave, Shrine, paperclip,... are great but this one offers very good features that we can consider for any new Rails project)

Paperclip team deprecated the gem in favor of the Rails ActiveStorage.

Here is the github page for the ActiveStorage & plenty of resources are available everywhere

Also I found this video to be very helpful to understand the features of Activestorage

There is a nice gem especially for uploading files : carrierwave. If the wiki does not help , there is a nice RailsCast about the best way to use it . Summarizing , there is a field type file in Rails forms , which invokes the file upload dialog. You can use it , but the 'magic' is done by carrierwave gem .

I don't know what do you mean with "how to write to a file" , but I hope this is a nice start.

Okay. If you do not want to store the file in database and store in the application, like assets (custom folder), you can define non-db instance variable defined by attr_accessor: document and use form_for - f.file_field to get the file,

In controller,

 @person = Person.new(person_params)

Here person_params return whitelisted params[:person] (define yourself)

Save file as,

dir = "#{Rails.root}/app/assets/custom_path"
FileUtils.mkdir(dir) unless File.directory? dir
document = @person.document.document_file_name # check document uploaded params
File.copy_stream(@font.document, "#{dir}/#{document}")

Note, Add this path in .gitignore & if you want to use this file again add this path asset_pathan of application by application.rb

Whenever form read file field, it get store in tmp folder, later you can store at your place, I gave example to store at assets

note: Storing files like this will increase the size of the application, better to store in the database using paperclip.

In your intiallizer/carrierwave.rb

if Rails.env.development? || Rails.env.test?
    config.storage = :file
    config.root = "#{Rails.root}/public"
    if Rails.env.test?
      CarrierWave.configure do |config|
        config.storage = :file
        config.enable_processing = false
      end
    end
 end

use this to store in a file while running on local