.Net MVC 4 REST Cannot send Object

Question

I have build a .Net Mvc 4 application and now I want to extend it with REST.

I am using the Entity Framework and I have the following problem.

My goal is to have a system where categories have a number of products and where products can belong to multiple categories.

As follows:

public class Categorie
{
    [Key]
    public int Id { get; set; }
    [Required]
    public string Naam { get; set; }
    [Required]
    public string Omschrijving { get; set; }
    public byte[] Plaatje { get; set; }
    private List<Product> producten;
    public virtual List<Product> Producten
    {
        get { return producten; }   
        set { producten = value; }
    }
}

public class Product
    {
        [Key]
        public int Id { get; set; }
        [Required]
        public string Naam { get; set; }
        [Required]
        public string Omschrijving { get; set; }
        [Required]
        public double Prijs { get; set; }
        private List<Categorie> categorien = new List<Categorie>();

        public virtual List<Categorie> Categorien
        {
            get { return categorien; }
            set { categorien = value; }
        }
        [Required]
        public byte[] Plaatje { get; set; }
    }

NOTE: There are virtual properties in there so that my entity framework creates a merging table. Normally it links all the categorie's to the products and vice versa.

And my rest looks like:

// GET api/Rest/5
    public Product GetProduct(int id)
    {
        Product product = db.Producten.Find(id);
        Product newProduct = new Product();
        if (product == null)
        {
            throw new HttpResponseException(Request.CreateResponse(HttpStatusCode.NotFound));
        }
        else
        {
            product.Categorien = null;
        }
        newProduct.Id = product.Id;
        newProduct.Naam = product.Naam;
        newProduct.Omschrijving = product.Omschrijving;
        newProduct.Plaatje = product.Plaatje;
        newProduct.Prijs = product.Prijs;
        newProduct.Categorien = product.Categorien;
        return newProduct;
    }

First problem: I cannot send any product aslong as it has a categorie. I have to make it null. Second problem: I cannot send the original product because of the first problem.

Kevin Junghans · Accepted Answer · 2012-10-29 18:06:44Z

I am assuming your problem is with a circular reference during serialization, since categories reference multiple products and products reference multiple categories. One solution is to use Data Transfer Objects (DTO) instead of returning the straight entities you are using for EF. To make it easy to map your entities to the DTO's I would use AutoMapper. This is essentially what you are doing when you create an instance of newProduct in your REST API method, but AutoMapper takes the hard coding and drudgery out of mapping. Your DTO for a product would look very similar but they would not have the virtual navigation properties or the attributes needed by EF. A DTO for a product would look something like this.

public class Categorie
{
    public int Id { get; set; }
    public string Naam { get; set; }
    public string Omschrijving { get; set; }
    public byte[] Plaatje { get; set; }
}

public class Product
{
    public int Id { get; set; }
    public string Naam { get; set; }
    public string Omschrijving { get; set; }
    public double Prijs { get; set; }
    public List<Categorie> categorien = new List<Categorie>();

    public List<Categorie> Categorien
    {
        get { return categorien; }
        set { categorien = value; }
    }
    public byte[] Plaatje { get; set; }
}

Notice that the DTO for Categorie does not contain a list of products, since in this case you want a listing of products. If you keep the field names the same for your DTO's as your entities AutoMapper will handle the mapping automatically. I usually keep the same class name for the DTO's and just distinguish them from the entities by having a different namespace. Your REST API method would look something like this.

// GET api/Rest/5
public Product GetProduct(int id)
{
    Product product = db.Producten.Find(id);
    return Mapper.Map<Product, Dto.Product>(product);
}

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