C# driver 2.0 Mongodb UpdateOneAsync

Question

public class Student
{
    public long StudentId {get; set;}
    public string Fname {get; set;}
    public string Lname {get; set;}
    public List<ObjectId> CoursesList {get; set;}
    public int IQ {get;set;}
}

public class Courses
{
    [BsonId]
    public ObjectId Id { get; set; }
    public string CourseNumber{get; set;}
    public string CourseName{get; set;}
}

How do I add/append a courser Id to Course list(which may be null for the first time) of a Student object

PS: I know how to set a field using the below command. I am hoping it is on similar lines for the above problem

await StudentCollection.UpdateOneAsync(a => a.StudentId == studentId, Builders<Student>.Update.Set( a => a.IQ,90));

Answer 1

As you've already discovered, the C# code to use $addToSet is:

var filter = Builders<Student>.Filter.Eq(s => s.StudentId, studentId);
var update = Builders<Student>.Update.AddToSet(s => s.CoursesList, courseId);
var result = await collection.UpdateOneAsync(filter, update);

However, $addToSet is not going to work if the CourseList member has been stored in the collection as a null. The server requires that the existing value for $addToSet be an array (it can be an empty array).

The easiest solution is to just store an empty list for CoursesList instead of a null when there are no courses.