Using navigation properties in entity framework code first

Question

Context:

• Code First, Entity Framework 4.3.1;
• User ---- Topic, 1 to Many relation;
• User with public virtual ICollection<Topic> CreatedTopics Navigation Property(Lazy Loading);
• Topic with public virtual User Creator Navigation Property;
• DataServiceController : DbDataController<DefaultDbContext>, Web API beta, ASP.NET MVC 4 Beta , Single Page Application;
• System.Json for Json serialization;

• Web API Action:

  public IQueryable<Topic> GetTopics()
  {
      // return DbContext.Topics;                   // OK
      return DbContext.Topics.Include("Creator");   //With Exception
  }
  

• Result: "an unhandled microsoft .net framework exception occurred in w3wp.exe"

The Problem here seems to be: I should not Add Navigation Property in both Entities(Cause Circular Reference?), and if I delete the CreatedTopics Navigation Property in User Class, It will be OK again.

So, In a similar Context like listed above, Here are my questions:

1. How to deal with Navigation Properties in the situation of 1 to Many relation;
2. Further more, how about a Many to Many relation, do i have to divide it into two 1 to Many relations;
3. What is the Best Practices and Precautions of using Navigation Properties?

I Have read many related posts, but still not clear enough :(,

Thanks for any help!

Dean

Answer 1

This is not a problem of code first or EF - it is a problem of serialization. Simply the serializer used to convert your object graph to some representation passed in a Web API message is not able to work with circular references by default. Depending on the message format you want to use Web API uses different serializers by default - here is more about default serializers used by Web API and about the way how to change it. The following text suppose that you are using DataContractJsonSerializer or DataContractSerializer (should be default for XML serialization) but the same is possible for JSON.NET (should be default for JSON serialization - JSON serialization can be switched to DataContractJsonSerializer but the default serializer is better).

So what you can do? You can tell the serializer that it should track those circular references by marking your classes with DataContract(IsReference = true) and each passed property with DataMember attribute (check the linked article for description how to achieve it with JSON.NET). This will allow serializer correctly recognizing cycles and the serialization will in theory succeed. In theory because this also demands to not using lazy loading. Otherwise you can serialize much more data than you expected (in some catastrophic scenarios it can lead to serializing whole content of your database).

When you serialize entity graph with lazy loading enabled you serailze a Topic and its Creator but serialization will also visit CreatedTopics property => all related topics are lazy loaded and processed by serialization and serialization continues to visit Creator of all newly loaded topics! This process continues until there is no other object to lazy load. Because of this you should never use lazy loading when serializing entities.

Other option is to exclude back reference from serialization. You just need to serialize Creator. You don't need to serialize CreatedTopics so you can mark the property with IgnoreDataMember attribute (JsonIgnore for JSON.NET). The problem is that if you also have Web API action for returning User with all his CreateTopics this will not work because of the attribute.

The last option is not using entities. This option is usually used in web services where you create special DTO objects satisfying requirements for specific operation and you handle conversion between entities and DTOs inside the operation (possible with help of some tool like AutoMapper).

There is no difference between handling one-to-one, one-to-many, or many-to-many relations. If you have navigation properties on both sides you must always deal with this problem.