Generate a pre-signed url for S3 direct upload.

s3_pre_signed_url.rb

class SignedUrlsController < ApplicationController
  def s3
    # Create client and set bucket
    client = AWS::S3.new
    bucket = client.buckets[ENV['S3_BUCKET']]

    # Extract file extension since we're renaming the file
    extension = File.extname(params[:image_name]).downcase
    
    # Get mime-type from extension
    mime_type = Rack::Mime.mime_type(extension)
    
    # Generate custom file name
    file_name = SecureRandom.urlsafe_base64(5).downcase + extension

    # Generate form data
    # key cosists of custom directory 'u' for uploads;
    # Current time to milisecond to integer and the file name we generated earlier
    key = "u/#{(Time.now.to_f * 1000).to_i}/#{file_name}"

    form = bucket.presigned_post(key: key, content_type: mime_type)
    render json: form.fields
  end
end