add support for snapshot option to Collection#find

lib/mongo/collection.rb

@@ -67,6 +67,11 @@ module XGen
         #          ascending, -1 == descending, or array of field names (all
         #          assumed to be sorted in ascending order).
         # :hint :: See #hint. This option overrides the collection-wide value.
+        # :snapshot :: If true, snapshot mode will be used for this query.
+        #              Snapshot mode assures no duplicates are returned, or
+        #              objects missed, which were preset at both the start and
+        #              end of the query's execution. For details see
+        #              http://www.mongodb.org/display/DOCS/How+to+do+Snapshotting+in+the+Mongo+Database
         def find(selector={}, options={})
           fields = options.delete(:fields)
           fields = nil if fields && fields.empty?
@@ -74,13 +79,14 @@ module XGen
           limit = options.delete(:limit) || 0
           sort = options.delete(:sort)
           hint = options.delete(:hint)
+          snapshot = options.delete(:snapshot)
           if hint
             hint = normalize_hint_fields(hint)
           else
             hint = @hint        # assumed to be normalized already
           end
           raise RuntimeError, "Unknown options [#{options.inspect}]" unless options.empty?
-          @db.query(self, Query.new(selector, fields, offset, limit, sort, hint))
+          @db.query(self, Query.new(selector, fields, offset, limit, sort, hint, snapshot))
         end
 
         # Find the first record that matches +selector+. See #find.

lib/mongo/message/query_message.rb

@@ -62,7 +62,7 @@ module XGen
             end
             sel['$hint'] = query.hint if query.hint && query.hint.length > 0
             sel['$explain'] = true if query.explain
-                               
+            sel['$snapshot'] = true if query.snapshot
           end
           write_doc(sel)
           write_doc(query.fields) if query.fields

lib/mongo/query.rb

@@ -26,7 +26,7 @@ module XGen
       # Mongo documentation for query details.
       class Query
 
-        attr_accessor :number_to_skip, :number_to_return, :order_by
+        attr_accessor :number_to_skip, :number_to_return, :order_by, :snapshot
         # If true, $explain will be set in QueryMessage that uses this query.
         attr_accessor :explain
         # Either +nil+ or a hash (preferably an OrderedHash).
@@ -63,9 +63,9 @@ module XGen
         # hint :: If not +nil+, specifies query hint fields. Must be either
         #                +nil+ or a hash (preferably an OrderedHash). See
         #                Collection#hint.
-        def initialize(sel={}, return_fields=nil, number_to_skip=0, number_to_return=0, order_by=nil, hint=nil)
+        def initialize(sel={}, return_fields=nil, number_to_skip=0, number_to_return=0, order_by=nil, hint=nil, snapshot=nil)
-          @number_to_skip, @number_to_return, @order_by, @hint =
+          @number_to_skip, @number_to_return, @order_by, @hint, @snapshot =
-            number_to_skip, number_to_return, order_by, hint
+            number_to_skip, number_to_return, order_by, hint, snapshot
           @explain = nil
           self.selector = sel
           self.fields = return_fields
@@ -111,7 +111,7 @@ module XGen
         end
 
         def contains_special_fields
-          (@order_by != nil && @order_by.length > 0) || @explain || @hint
+          (@order_by != nil && @order_by.length > 0) || @explain || @hint || @snapshot
         end
       end
     end

tests/test_db_api.rb

@@ -781,6 +781,14 @@ class DBAPITest < Test::Unit::TestCase
     assert_equal 0, b.count()
   end
 
+  # doesn't really test functionality, just that the option is set correctly
+  def test_snapshot
+    @@db.collection("test").find({}, :snapshot => true).to_a
+    assert_raise RuntimeError do
+      @@db.collection("test").find({}, :snapshot => true, :sort => 'a').to_a
+    end
+  end
+
 # TODO this test fails with error message "Undefed Before end of object"
 # That is a database error. The undefined type may go away.