Existing entity constraints

Action methods of controllers usually perform constraints on incoming data. A common practice is to centralize that kind of logic in filters. Let's take, for example, OrderController, which we discussed in the previous chapter:

using System;using System.Collections.Generic;using System.Linq;using Microsoft.AspNetCore.JsonPatch;using Microsoft.AspNetCore.Mvc;using SampleAPI.Filters;using SampleAPI.Models;using SampleAPI.Repositories;using SampleAPI.Requests;namespace SampleAPI.Controllers{    [Route("api/order")]    [ApiController]    public class OrderController : ControllerBase    {        private readonly IOrderRepository _orderRepository;        public OrderController(IOrderRepository ordersRepository)        { _orderRepository = ordersRepository; ...

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