Find documents in mongodb by Date field + some date value

Question

I Need some help with C# Mongo Driver and mongodb.

C# class example:

 public class Document
 {
    public string Id { get; set; }

    public DateTime StartTime { get; set; }

    public TimeSpan CustomPeriod { get; set; }
 }

I need a way to find documents in mongodb by StartTime field + some TimeSpan value, like this predicate:

Expression<Func<Document, bool>> customExpression = x 
    => x.StartTime.Add(x.CustomPeriod) <= DateTime.UtcNow;

These predicates are not working and I am getting an error when executing Collection.Find() query now:

{document}{StartTime}.Add({document}{CustomPeriod}) is not supported.

Answer 1

The issue is the query language used by the underlying find command at the MongoDB instance does not support comparing one field in a document with another, or performing operations on these fields before comparison.

The $expr operator permits using aggregation expressions, but you usually forfeit the ability to use an index for that portion of the query.

In the mongo shell that might look like:

db.collection.find({
      $expr:{
         $gte:[ 
            new Date(), 
            {$add: [ "$StartTime", "$CustomPeriod"]}
         ]
      }
})

I'm not familiar with C#, so I don't know how to express that using the .net driver