identifies all uploads with the SHA1 hash of the file content

app/models/upload.rb

@@ -17,11 +17,18 @@ class Upload < ActiveRecord::Base
     image_info = FastImage.new(file.tempfile, raise_on_failure: true)
     # compute image aspect ratio
     width, height = ImageSizer.resize(*image_info.size)
+    # compute the sha
+    sha = Digest::SHA1.file(file.tempfile).hexdigest
+    # check if the file has already been uploaded
+    upload = Upload.where(sha: sha).first
+    # otherwise, create it
+    if upload.blank?
 
       upload = Upload.create!({
         user_id: user_id,
         original_filename: file.original_filename,
         filesize: File.size(file.tempfile),
+        sha: sha,
         width: width,
         height: height,
         url: ""
@@ -35,6 +42,8 @@ class Upload < ActiveRecord::Base
 
       upload.save
 
+    end
+
     upload
   end
 

db/migrate/20130615075557_add_sha_to_uploads.rb

@@ -0,0 +1,6 @@
+class AddShaToUploads < ActiveRecord::Migration
+  def change
+    add_column :uploads, :sha, :string, null: true
+    add_index :uploads, :sha, unique: true
+  end
+end