identifies all uploads with the SHA1 hash of the file content

app/models/upload.rb

@@ -17,23 +17,32 @@ 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),
-      width: width,
-      height: height,
-      url: ""
-    })
+      upload = Upload.create!({
+        user_id: user_id,
+        original_filename: file.original_filename,
+        filesize: File.size(file.tempfile),
+        sha: sha,
+        width: width,
+        height: height,
+        url: ""
+      })
 
-    # make sure we're at the beginning of the file (FastImage is moving the pointer)
-    file.rewind
+      # make sure we're at the beginning of the file (FastImage is moving the pointer)
+      file.rewind
 
-    # store the file and update its url
-    upload.url = Upload.store_file(file, image_info, upload.id)
+      # store the file and update its url
+      upload.url = Upload.store_file(file, image_info, upload.id)
 
-    upload.save
+      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