вынесли последовательность в модели

SUBD_Car_rent/database/ImplementationMongoDB.cs

@@ -12,11 +12,7 @@ namespace database
 		private IMongoCollection<Branch> _branchCollection;
 		private IMongoCollection<Status> _statusCollection;
 		private IMongoCollection<BodyType> _bodyTypeCollection;
-		private class Sequence
-		{
-			public string Id;
-			public int count;
-		}
+		
 		private IMongoCollection<Sequence> _sequenceCollection;
 
 		public ImplementationMongoDB()

SUBD_Car_rent/database/models.cs

@@ -78,4 +78,10 @@ namespace database
         public string Text { get; set; }
         public int Id { get; set; }
     }
+	public class Sequence
+	{
+		[BsonRepresentation(BsonType.String)]
+		public string Id { get; set; }
+		public int count { get; set; }
+	}
 }