abp/framework/test/Volo.Abp.TestApp/Volo/Abp/TestApp/Domain/City.cs

using System;
using System.Collections.Generic;
using System.Linq;
using Volo.Abp.Domain.Entities;

namespace Volo.Abp.TestApp.Domain;

public class City : AggregateRoot<Guid>
{
    public string Name { get; set; }

    public ICollection<District> Districts { get; set; }

    private City()
    {

    }

    public City(Guid id, string name)
        : base(id)
    {
        Name = name;
        Districts = new List<District>();
    }

    public int GetPopulation()
    {
        return Districts.Select(d => d.Population).Sum();
    }
}