How to prevent more than a max size children entities from being added to a parent entity, per parent entity in Entity Framework Core?

Question

Problem

If I have 2 or more students trying to register for the last remaining seat available for a classroom lesson and they save simultaneously, entering my RegisterStudent() controller method, the only certainty I have that the parent (YogabandEvent) entity property MaxSize will be maintained is when I fetch the parent, and I check the number of registrants already added is < MaxSize and if true, I add the new registrant.

But I have a concurrency problem where if 2+ students have fetched the parent (YogabandEvent) entity at the same time, getting a MaxSize of 9, and then add a new registrant I could end up with more than MaxSize (ex. 11+). So I have a concurrency issue!

Currently, I can either fetch the parent entity including the children (Registrants) add the new child (Registrant) to the parent and save or just insert a new Registrant entity into the Registrant table directly. But with either scenario, I could end up with a situation, stated above, where more students than the MaxSize are registered.

Question

How can I restrict the child (Registrant) entities to the MaxSize in the parent (YogabandEvent) entity using EF Core?

Here is my code - some properties left out for verbosity:

public class YogabandEvent : BaseEntity
{
    public YogabandEvent() 
    {
        Registrants = new List<Registrant>();
    }

    public DateTime Date { get; set; }
    public DateTime DateUtc { get; set; }
    public MaxSize MaxSize { get; set; }

    public ICollection<Registrant> Registrants { get; set; }
 }

 public class Registrant : BaseEntity
 {
     public RegistrantType RegistrantType { get; set; }
     public int UserId { get; set; }
     public User User { get; set; }
     public int EventId { get; set; }
     public YogabandEvent Event { get; set; }
 }

 public enum MaxSize
 {
     Five = 5,
     Ten = 10,
     Fifteen = 15,
     Twenty = 20,
     ThirtyFive = 35
 }

Answer 1

First thing is enabling concurrency conflict detection on YogabandEvent entity.

Here you have the official documentation for it:

https://learn.microsoft.com/en-us/ef/core/saving/concurrency?tabs=data-annotations

In this case I'm going to propose you to use a concurrency token, it is just an integer property called Version than will increment whenever YogabandEvent entity is updated.

To detect concurrency conflicts when you add a new Registrant you need to update YogabandEvent, so add a new property to YogabantEvent called RegistrantCount that you will increment when adding new registrants.

public class YogabandEvent : BaseEntity
{
    public YogabandEvent() {
        Registrants = new List<Registrant>();
    }
    public DateTime Date { get; set; }
    public DateTime DateUtc { get; set; }
    public MaxSize MaxSize { get; set; }
    [ConcurrencyCheck]
    public int Version {get; set;}
    public int RegistrantCount {get; set;}
    public ICollection<Registrant> Registrants { get; set; }
 }

In your controller write the following:

var yogabandEvent = ReadYogabantEventFromContext();
yogabandEvent.RegistrantCount++;
yogabandEvent.Version++;
var registrant = CreateNewRegistrant();
addRegistrantToContext(registrant);

while (true)
{
    if (yogabandEvent.RegistrantCount >= yogabandEvent.MaxSize) 
    {
       throw new InvalidOperationException("Event max size reached");
    }
    try 
    {
       context.SaveChanges();
       break;
    }
    catch (DbUpdateConcurrencyException) 
    {
        ReloadYogabantEvent();
        yogabandEvent.RegistrantCount++;
        yogabandEvent.Version++;
    }
}