RUBY-141 raise exception if attempting to overwrite with Grid#put (only in safe mode)

lib/mongo/gridfs/grid.rb

@@ -41,10 +41,11 @@ module Mongo
       @chunks.create_index([['files_id', Mongo::ASCENDING], ['n', Mongo::ASCENDING]], :unique => true)
     end
 
-    # Store a file in the file store.
+    # Store a file in the file store. This method is designed only for writing new files;
+    # if you need to update a given file, first delete it using #Grid#delete.
     #
     # Note that arbitary metadata attributes can be saved to the file by passing
-    # them is as options.
+    # them in as options.
     #
     # @param [String, #read] data a string or io-like object to store.
     #

lib/mongo/gridfs/grid_io.rb

@@ -315,11 +315,18 @@ module Mongo
       @aliases       = opts.delete(:aliases) if opts[:aliases]
       @file_length   = 0
       opts.each {|k, v| self[k] = v}
+      check_existing_file if @safe
 
       @current_chunk = create_chunk(0)
       @file_position = 0
     end
 
+    def check_existing_file
+      if @files.find_one('_id' => @files_id)
+        raise GridError, "Attempting to overwrite with Grid#put. You must delete the file first."
+      end
+    end
+
     def to_mongo_object
       h                = BSON::OrderedHash.new
       h['_id']         = @files_id

test/grid_test.rb

@@ -27,6 +27,12 @@ class GridTest < Test::Unit::TestCase
         assert_equal 'sample', file['filename']
       end
 
+      should "not be able to overwrite an exising file" do
+        assert_raise GridError do
+          @grid.put(@data, :filename => 'sample', :_id => @id, :safe => true)
+        end
+      end
+
       should "return nil if it doesn't exist" do
         assert_nil @grid.exist?(:metadata => 'foo')
       end