ASP.NET Core Web API GET 请求进阶（三）｜Owner、Review、Reviewer 接口实战

1. 简介

在本系列的前两篇文章中，我们已经为 Pokemon、Category、Country 资源构建了丰富的 GET 接口。本文将继续介绍如何为 Owner、Review、Reviewer 三个模型设计和实现多样的 GET 请求，涵盖列表查询、关联查询、存在性校验等常见场景，并额外讲解如何解决二对二/多对多关联导致的 JSON 循环引用问题。

2. 环境与项目结构回顾

技术栈：.NET 6+、C#、Entity Framework Core、AutoMapper
依赖注入：在 Program.cs 中已注册 DbContext、AutoMapper 及各仓储 AddScoped < …, … >()

项目结构：

/Interfaces

IOwnerRepository.cs

IReviewRepository.cs

IReviewerRepository.cs

…  

/Repositories

OwnerRepository.cs

ReviewRepository.cs

ReviewerRepository.cs

…  

/DTOs

OwnerDto.cs

ReviewDto.cs

ReviewerDto.cs

/Helpers

MappingProfiles.cs  

/Controllers

OwnerController.cs

ReviewController.cs

ReviewerController.cs

/Data

AppDbContext.cs

3. Owner GET 接口实现

3.1 接口层（IOwnerRepository）

定义三种方法，满足关联查询与存在性校验：

public interface IOwnerRepository
{
    IReadOnlyCollection < Pokemon > GetPokemonsByOwner(int ownerId); // 获取某 Owner 的所有 Pokemon
    bool OwnerExists(int ownerId);                                // 判断 Owner 是否存在
}

3.2 仓储层（OwnerRepository）

在 Repositories/OwnerRepository.cs 中实现：

public class OwnerRepository : IOwnerRepository
{
    private readonly AppDbContext _context;
    public OwnerRepository(AppDbContext context) = > _context = context;

    public IReadOnlyCollection < Pokemon > GetPokemonsByOwner(int ownerId) = >
        _context.PokemonOwners
                .Where(po = > po.Owner.Id == ownerId)      // 过滤关联表
                .Select(po = > po.Pokemon)                  // 投影到 Pokemon
                .ToList();

    public bool OwnerExists(int ownerId) = >
        _context.Owners.Any(o = > o.Id == ownerId);
}

3.3 DTO 与映射（OwnerDto）

DTOs/OwnerDto.cs：

public class OwnerDto
{
  public int Id { get; set; }
  public string FirstName { get; set; }
  public string LastName { get; set; }
}

在 Helpers/MappingProfiles.cs 中添加：
```
CreateMap < Pokemon, PokemonDto > ();
```

3.4 控制器层（OwnerController）

[ApiController]
[Route("api/[controller]")]
public class OwnerController : ControllerBase
{
    private readonly IOwnerRepository _repo;
    private readonly IMapper _mapper;

    public OwnerController(IOwnerRepository repo, IMapper mapper)
    {
        _repo = repo;
        _mapper = mapper;
    }

    // GET api/owner/{id}/pokemons
    [HttpGet("{id}/pokemons")]
    public ActionResult < IReadOnlyCollection < PokemonDto >  >  GetPokemonsByOwner(int id)
    {
        if (!_repo.OwnerExists(id))
            return NotFound($"Owner {id} 未找到。");

        var pokemons = _repo.GetPokemonsByOwner(id);
        return Ok(_mapper.Map < IReadOnlyCollection < PokemonDto >  > (pokemons));
    }
}

4. Review GET 接口实现

4.1 接口层（IReviewRepository）

public interface IReviewRepository
{
    IReadOnlyCollection < Review >  GetReviews();                  // 全部 Review
    Review GetReview(int reviewId);                            // 单条 Review
    IReadOnlyCollection < Review > GetReviewsByPokemon(int pokemonId); // 某 Pokemon 的所有 Review
    bool ReviewExists(int reviewId);                           // Review 存在性校验
}

4.2 仓储层（ReviewRepository）

public class ReviewRepository : IReviewRepository
{
    private readonly AppDbContext _context;
    public ReviewRepository(AppDbContext context) = >  _context = context;

    public IReadOnlyCollection < Review > GetReviews() = >
        _context.Reviews.ToList();

    public Review GetReview(int reviewId) = >
        _context.Reviews.FirstOrDefault(r = > r.Id == reviewId);

    public IReadOnlyCollection < Review > GetReviewsByPokemon(int pokemonId) = >
        _context.Reviews
                .Where(r = > r.Pokemon.Id == pokemonId)
                .ToList();

    public bool ReviewExists(int reviewId) = >
        _context.Reviews.Any(r = > r.Id == reviewId);
}

4.3 DTO 与映射（ReviewDto）

DTOs/ReviewDto.cs：

public class ReviewDto
{
  public int Id { get; set; }
  public string Title { get; set; }
  public string Text { get; set; }
  public int Rating { get; set; }
}

MappingProfiles 中：
```
CreateMap < Review, ReviewDto > ();
```

4.4 控制器层（ReviewController）

[ApiController]
[Route("api/[controller]")]
public class ReviewController : ControllerBase
{
    private readonly IReviewRepository _repo;
    private readonly IMapper _mapper;

    public ReviewController(IReviewRepository repo, IMapper mapper)
    {
        _repo = repo;
        _mapper = mapper;
    }

    // GET api/review
    [HttpGet]
    public ActionResult < IReadOnlyCollection < ReviewDto > > GetReviews()
        = > Ok(_mapper.Map < IReadOnlyCollection < ReviewDto > >(_repo.GetReviews()));

    // GET api/review/{id}
    [HttpGet("{id}")]
    public ActionResult < ReviewDto > GetReview(int id)
    {
        if (!_repo.ReviewExists(id))
            return NotFound($"Review {id} 未找到。");
        return Ok(_mapper.Map < ReviewDto > (_repo.GetReview(id)));
    }

    // GET api/review/pokemon/{pokemonId}
    [HttpGet("pokemon/{pokemonId}")]
    public ActionResult < IReadOnlyCollection < ReviewDto > >  GetReviewsByPokemon(int pokemonId)
    {
        var reviews = _repo.GetReviewsByPokemon(pokemonId);
        return Ok(_mapper.Map < IReadOnlyCollection < ReviewDto > > (reviews));
    }
}

5. Reviewer GET 接口实现

5.1 接口层（IReviewerRepository）

public interface IReviewerRepository
{
    IReadOnlyCollection < Reviewer > GetReviewers();               // 全部 Reviewer
    Reviewer GetReviewer(int reviewerId);                       // 单条 Reviewer
    IReadOnlyCollection < Review > GetReviewsByReviewer(int reviewerId); // 某 Reviewer 的所有 Review
    bool ReviewerExists(int reviewerId);                        // Reviewer 存在性校验
}

5.2 仓储层（ReviewerRepository）

public class ReviewerRepository : IReviewerRepository
{
    private readonly AppDbContext _context;
    public ReviewerRepository(AppDbContext context) = > _context = context;

    public IReadOnlyCollection < Reviewer > GetReviewers() = >
        _context.Reviewers.Include(r = > r.Reviews).ToList();    // Include 加载导航属性

    public Reviewer GetReviewer(int id) = >
        _context.Reviewers.FirstOrDefault(r = > r.Id == id);

    public IReadOnlyCollection < Review > GetReviewsByReviewer(int id) = >
        _context.Reviews.Where(r = > r.Reviewer.Id == id).ToList();

    public bool ReviewerExists(int id) = >
        _context.Reviewers.Any(r = > r.Id == id);
}

5.3 DTO 与映射（ReviewerDto）

DTOs/ReviewerDto.cs：

public class ReviewerDto
{
  public int Id { get; set; }
  public string FirstName { get; set; }
  public string LastName { get; set; }
}

MappingProfiles 中：
```
CreateMap < Reviewer, ReviewerDto > ();
```

5.4 控制器层（ReviewerController）

[ApiController]
[Route("api/[controller]")]
public class ReviewerController : ControllerBase
{
    private readonly IReviewerRepository _repo;
    private readonly IMapper _mapper;

    public ReviewerController(IReviewerRepository repo, IMapper mapper)
    {
        _repo = repo;
        _mapper = mapper;
    }

    // GET api/reviewer
    [HttpGet]
    public ActionResult < IReadOnlyCollection < ReviewerDto > >  GetReviewers()
        = > Ok(_mapper.Map < IReadOnlyCollection < ReviewerDto >  >(_repo.GetReviewers()));

    // GET api/reviewer/{id}
    [HttpGet("{id}")]
    public ActionResult < ReviewerDto > GetReviewer(int id)
    {
        if (!_repo.ReviewerExists(id))
            return NotFound($"Reviewer {id} 未找到。");
        return Ok(_mapper.Map < ReviewerDto > (_repo.GetReviewer(id)));
    }

    // GET api/reviewer/{id}/reviews
    [HttpGet("{id}/reviews")]
    public ActionResult < IReadOnlyCollection < ReviewDto > >  GetReviewsByReviewer(int id)
    {
        var reviews = _repo.GetReviewsByReviewer(id);
        return Ok(_mapper.Map < IReadOnlyCollection < ReviewDto > > (reviews));
    }
}

6. JSON 循环引用问题处理

对于多对多或双向导航属性，默认序列化会出现循环依赖。解决方法：在 Program.cs 中替换默认 JSON 序列化设置为 ReferenceHandler.IgnoreCycles：

builder.Services.AddControllers()
    .AddJsonOptions(options = >
        options.JsonSerializerOptions.ReferenceHandler = ReferenceHandler.IgnoreCycles);