Uploading a file in Rails

Question

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.

Answer 1

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