ApplicationDbContext.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Threading.Tasks;

namespace SportsAdministrationApp.Models
{
    public class ApplicationDbContext : IdentityDbContext
    {
        public ApplicationDbContext(DbContextOptions<ApplicationDbContext> options)
            : base(options)
        {

        }
        public DbSet<User> User { get; set; }
        //public DbSet<AthleteData> AthleteData { get; set; }
        //public DbSet<PRs> PRs { get; set; }
        public DbSet<Team> Teams { get; set; }
        public DbSet<PersonalRecord> PersonalRecord { get; set; }
        public DbSet<AthleteData> AthleteData { get; set; }
        protected override void OnModelCreating(ModelBuilder modelBuilder)
        {
            base.OnModelCreating(modelBuilder);
            modelBuilder.Seed();
        }
    }
}

AthleteData.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Threading.Tasks;

namespace SportsAdministrationApp.Models
{
    public class AthleteData
    {
        public int Id { get; set; }
        public string Location { get; set; }
        public int FromPersonalRecordId { get; set; }
        public decimal Time { get; set; }
        public PersonalRecord PersonalRecord { get; set; }
        public int PersonalRecordId { get; set; }

    }
}

CoachController.cs

        public IActionResult Index()
        {
            var user = userManager.Users.Include(u => u.PersonalRecord).ThenInclude(p => p.AthleteData).ToList();
            return View(user);
        }
        
        
        [HttpGet]
        public async Task<IActionResult> AddTime(string id)
        {
            var user = await userManager.FindByIdAsync(id);
            return View(user);
        }
        [HttpPost]
        public async Task<IActionResult> AddTime(string id, string time, string Location)
        {
            var user = await userManager.FindByIdAsync(id);
            PersonalRecord r = new PersonalRecord();
            AthleteData d = new AthleteData() { Location = location, Time = Convert.ToDecimal(time) };
            user.PersonalRecord.AthleteData.Add(d);
            var result = await userManager.UpdateAsync(user);
            if (!result.Succeeded)
            {
                //fix this
                return View(user);
            }
            return RedirectToAction("UserListWithData");
        }

PersonalRecord.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Threading.Tasks;

namespace SportsAdministrationApp.Models
{
    public class PersonalRecord
    {
        public int Id { get; set; }
        public decimal PR { get; set; }
        public List<AthleteData> AthleteData { get; set; }

        public PersonalRecord()
        {
            AthleteData = new List<AthleteData>();
        }
    }
}