dereference helper method

lib/mongo/db.rb

@@ -372,6 +372,11 @@ module XGen
           raise "Error with count command: #{doc.inspect}"
         end
 
+        # Dereference a DBRef, getting the document it points to.
+        def dereference(dbref)
+          collection(dbref.namespace).find_first("_id" => dbref.object_id)
+        end
+
         # Evaluate a JavaScript expression on MongoDB.
         # +code+ should be a string or Code instance containing a JavaScript
         # expression. Additional arguments will be passed to that expression

tests/test_db_api.rb

@@ -544,6 +544,20 @@ class DBAPITest < Test::Unit::TestCase
     assert_equal 1, test.group([], {"a" => {"$gt" => 1}}, {"count" => 0}, "function (obj, prev) { prev.count++; }")[0]["count"]
   end
 
+  def test_deref
+    @@coll.clear
+
+    assert_equal nil, @@db.dereference(DBRef.new("test", ObjectID.new))
+    obj = {"x" => true}
+    key = @@coll.insert(obj)["_id"]
+    assert_equal true, @@db.dereference(DBRef.new("test", key))["x"]
+
+    assert_equal nil, @@db.dereference(DBRef.new("test", 4))
+    obj = {"_id" => 4}
+    @@coll.insert(obj)
+    assert_equal obj, @@db.dereference(DBRef.new("test", 4))
+  end
+
 # TODO this test fails with error message "Undefed Before end of object"
 # That is a database error. The undefined type may go away.