Skip calculating fingerprint when it is not used

The MD5 sum of an attachment is only necessary if the model has a corresponding
_fingerprint column. If this column is absent, there is no need to calculate the
MD5, which can be an expensive operation for large files.

Accomplish this by deferring the fingerprint calculation using a block. If the
_fingerprint column is absent, the block is never called, and the calculation
is avoided.

lib/paperclip/attachment.rb

@@ -427,7 +427,7 @@ module Paperclip
     def assign_attributes
       @queued_for_write[:original] = @file
       assign_file_information
-      assign_fingerprint(@file.fingerprint)
+      assign_fingerprint { @file.fingerprint }
       assign_timestamps
     end
 
@@ -437,9 +437,9 @@ module Paperclip
       instance_write(:file_size, @file.size)
     end
 
-    def assign_fingerprint(fingerprint)
+    def assign_fingerprint
       if instance_respond_to?(:fingerprint)
-        instance_write(:fingerprint, fingerprint)
+        instance_write(:fingerprint, yield)
       end
     end
 
@@ -465,7 +465,7 @@ module Paperclip
 
     def reset_file_if_original_reprocessed
       instance_write(:file_size, @queued_for_write[:original].size)
-      assign_fingerprint(@queued_for_write[:original].fingerprint)
+      assign_fingerprint { @queued_for_write[:original].fingerprint }
       reset_updater
     end
 

spec/paperclip/attachment_spec.rb

@@ -1374,6 +1374,12 @@ describe Paperclip::Attachment do
     end
 
     it "does not calculate fingerprint" do
+      Digest::MD5.stubs(:file)
+      @dummy.avatar = @file
+      expect(Digest::MD5).to have_received(:file).never
+    end
+
+    it "does not assign fingerprint" do
       @dummy.avatar = @file
       assert_nil @dummy.avatar.fingerprint
     end