I am using Entity Framework and having a problem with getting parent and child data to the browser. Here are my classes:
public class Question
{
public int QuestionId { get; set; }
public string Title { get; set; }
public virtual ICollection<Answer> Answers { get; set; }
}
public class Answer
{
public int AnswerId { get; set; }
public string Text { get; set; }
public int QuestionId { get; set; }
public virtual Question Question { get; set; }
}
I am using the following code to return the question and answer data:
public IList<Question> GetQuestions(int subTopicId, int questionStatusId)
{
var questions = _questionsRepository.GetAll()
.Where(a => a.SubTopicId == subTopicId &&
(questionStatusId == 99 ||
a.QuestionStatusId == questionStatusId))
.Include(a => a.Answers)
.ToList();
return questions;
}
On the C# side this seems to work however I notice that the answer objects have references back to the question. When I use the WebAPI to get the data to the browser I get the following message:
The 'ObjectContent`1' type failed to serialize the response body for content type 'application/json; charset=utf-8'.
Self referencing loop detected for property 'question' with type 'Models.Core.Question'.
Is this because the Question has Answers and the Answers have a reference back to Question? All the places I have looked suggest having a reference to the parent in the child so I am not sure what to do. Can someone give me some advice on this.
Is this because the Question has Answers and the Answers have a reference back to Question?
Yes. It cannot be serialized.
EDIT: See Tallmaris's answer and OttO's comment as it is simpler and can be set globally.
GlobalConfiguration.Configuration.Formatters.JsonFormatter.SerializerSettings.ReferenceLoopHandling = ReferenceLoopHandling.Ignore;
Old Answer:
Project the EF object Question
to your own intermediate or DataTransferObject. This Dto can then be serialized successfully.
public class QuestionDto
{
public QuestionDto()
{
this.Answers = new List<Answer>();
}
public int QuestionId { get; set; }
...
...
public string Title { get; set; }
public List<Answer> Answers { get; set; }
}
Something like:
public IList<QuestionDto> GetQuestions(int subTopicId, int questionStatusId)
{
var questions = _questionsRepository.GetAll()
.Where(a => a.SubTopicId == subTopicId &&
(questionStatusId == 99 ||
a.QuestionStatusId == questionStatusId))
.Include(a => a.Answers)
.ToList();
var dto = questions.Select(x => new QuestionDto { Title = x.Title ... } );
return dto;
}