Added support for keeping a specified amount of documents in GridFS instead of removing all or none

lib/mongo/gridfs/grid_file_system.rb

@@ -71,6 +71,9 @@ def initialize(db, fs_name=Grid::DEFAULT_FS_NAME)
     #  GridFileSystem#delete.
     # @option opts [Boolean] :safe (false) When safe mode is enabled, the chunks sent to the server
     #   will be validated using an md5 hash. If validation fails, an exception will be raised.
+    # @option opts [Integer] :versions (false) deletes all versions which exceed the number specified to 
+    #   retain ordered by uploadDate. This option only works in 'w' mode. Certain precautions must be taken when 
+    #   deleting GridFS files. See the notes under GridFileSystem#delete.
     #
     # @example
     #
@@ -97,17 +100,22 @@ def initialize(db, fs_name=Grid::DEFAULT_FS_NAME)
     def open(filename, mode, opts={})
       opts = opts.dup
       opts.merge!(default_grid_io_opts(filename))
-      del  = opts.delete(:delete_old) && mode == 'w'
+      if mode == 'w'
+        versions = opts.delete(:versions)
+        if opts.delete(:delete_old) || (versions && versions < 1)
+          versions = 1
+        end
+      end
       file = GridIO.new(@files, @chunks, filename, mode, opts)
       return file unless block_given?
       result = nil
       begin
         result = yield file
       ensure
         id = file.close
-        if del
+        if versions
           self.delete do
-            @files.find({'filename' => filename, '_id' => {'$ne' => id}}, :fields => ['_id'])
+            @files.find({'filename' => filename, '_id' => {'$ne' => id}}, :fields => ['_id'], :sort => ['uploadDate', -1], :skip => (versions -1))
           end
         end
       end

test/grid_file_system_test.rb

@@ -165,6 +165,18 @@ class GridFileSystemTest < Test::Unit::TestCase
            assert_equal 0, @db['fs.chunks'].find({'files_id' => {'$in' => @ids}}).count
          end
 
+         should "delete all versions which exceed the number of versions to keep specified by the option :versions" do
+           @versions = 1 + rand(4-1)
+           @grid.open('sample', 'w', :versions => @versions) do |f|
+             f.write @new_data
+           end
+           @new_ids = @db['fs.files'].find({'filename' => 'sample'}).map {|file| file['_id']}
+           assert_equal @versions, @new_ids.length
+           id = @new_ids.first
+           assert !@ids.include?(id)
+           assert_equal @versions, @db['fs.files'].find({'filename' => 'sample'}).count
+         end
+
          should "delete old versions on write with :delete_old is passed in" do
            @grid.open('sample', 'w', :delete_old => true) do |f|
              f.write @new_data