Entity Framework Core 代码优先方法一对多关系返回 null

Question

我是 Entity Framework Core 的新手，在我的代码中我有许多 Questions 类对象，以及这些问题的选项列表。我的Choices 查询列表总是返回 Null。

这是我的课程：

public class Questions
{
    [Key]
    public int QuestionID { get; set; }
    public string  Question { get; set; }

    public ICollection<Choices> choices { get; set; }
}

public class Choices
{
    [Key]
    public int ChoicesId { get; set; }
    public int QuestionId { get; set; }
    public Questions questions  { get; set; }
    public string Choice { get; set; }
}

public class SqlUserRepository : IUserRepository
{
    private readonly AppDbContext context;

    public SqlUserRepository(AppDbContext context)
    {
        this.context = context;
    }

    public List<Questions> getAllQuestions()
    {
        return context.questions.ToList();
    }
}

我收到了这个 O/p。选择总是返回 null

[
    { "questionID":1, "question":"which is your fav.heroien", "choices":null },
    { "questionID":2, "question":"which is your fav.hero", "choices":null  } 
]

我期待这个输出

[{"questionID":1,"question":"which is your fav.heroien","choices":[{"choicesId":5,"questionId":1,"questions":null,"choice":"kajol Agarwal"},{"choicesId":6,"questionId":1,"questions":null,"choice":"Thammanna Bhatia"},{"choicesId":7,"questionId":1,"questions":null,"choice":"Anushka Shetty"},{"choicesId":8,"questionId":1,"questions":null,"choice":"Keerthi Suresh"}]},{"questionID":2,"question":"which is your fav.hero","choices":[{"choicesId":1,"questionId":2,"questions":null,"choice":"upendera"},{"choicesId":2,"questionId":2,"questions":null,"choice":"real star"},{"choicesId":3,"questionId":2,"questions":null,"choice":"Super star"},{"choicesId":4,"questionId":2,"questions":null,"choice":"Only star"}]}]

Answer 1

根据EF Core-Loading Related Data 文档，您必须使用.Include 来加载相关数据，如下所示：

public List<Questions> getAllQuestions()
{
    return context.questions.Include(q => q.choices).ToList();
}

Answer 2

public List<Questions> getAllQuestions()
  {
    return context.questions.Include(q => q.choices).ToList();
  }

Include() 允许您指示应从数据库中读取哪些相关实体作为同一查询的一部分。

调用 Include("Choices") 的最终语句可能如下所示：

SELECT *
FROM questions JOIN Choices ON questions.Id = Choices.questionsId;

https://stackoverflow.com/a/26661930/8006943

Answer 3

如何使用代码优先方法和关系数据构建您的应用程序，link 将逐步指导您应用代码优先方法。