advanced query injection

Program.cs

using MongoDB.Entities;
using System;
using System.Text;
using System.Threading.Tasks;

namespace TestApplication
{
    public class UserActivity : Entity
    {
        public string RawUrl { get; set; }
        public string StartTime { get; set; }
        public string EndTime { get; set; }
    }

    public class Result
    {
        public double AvgPerDayMins { get; set; }
    }

    public static class Program
    {
        private static async Task Main()
        {
            await DB.InitAsync("test");

            var searchWord = "edit";

            var query = new Template<UserActivity, Result>(@"
            [
                {
                    $match: {
                        RawUrl: <search_word>,
                        EndTime: { $ne: null }
                    }
                },
                {
                    $set: {
                        EndTime: { $toDate: '$EndTime' },
                        StartTime: { $toDate: '$StartTime' }
                    }
                },
                {
                    $set: {
                        Duration: { $divide: [{ $subtract: ['$EndTime', '$StartTime'] }, 60000] }
                    }
                },
                {
                    $group: {
                        _id: null,
                        Total: { $sum: '$Duration' },
                        MinDate: { $min: '$StartTime' },
                        MaxDate: { $max: '$StartTime' }
                    }
                },
                {
                    $set: {
                        TotalDays: {
                            $add: [
                                {
                                    $round: [
                                        {
                                            $divide: [
                                                { $subtract: ['$MaxDate', '$MinDate'] }, 86400000]
                                        }, 0]
                                }, 1]
                        }
                    }
                },
                {
                    $project: {
                        _id: 0,
                        AvgPerDayMins: { $divide: ['$Total', '$TotalDays'] }
                    }
                }
            ]")
            .Tag("search_word", $"/{searchWord}/i");

            var result = await DB.PipelineAsync(query);

            Console.WriteLine("Average Per Day Minutes: " + result[0].AvgPerDayMins);
            Console.ReadLine();
        }
    }
}