I have the following classes:

public class State
{
    public long Id                    { get; set; }
    public string Name                { get; set; }
    public string Abbreviation        { get; set; }

    // Navigation Properties
    public virtual Country Country    { get; set; }
}

public class School
{
    public long Id                    { get; set; }
    public string Name                { get; set; }
    public string Abbreviation        { get; set; }

    // Navigation Properties
    public virtual State State        { get; set; }
}

and the following data in my SQL Server

| School|             |               |
|  Id   | Name        |    State      |
|  1    | UCLA        |      1        |
+-------+-------------+---------------+
| State |             |               |
|  Id   | Name        | Abbreviation  |
|  1    | California  |      CA       |

I'm attempting to create a Rest controller that creates an instance of school using the HTTP POST verb using Web API.

public HttpResponseMessage<School> Post( School school )
{
    SchoolService.CreateSchool( school );
    var response = new HttpResponseMessage<School>( school, HttpStatusCode.Created );
    string uri = Url.Route( null, new { id = school.Id } );

    response.Headers.Location = new Uri( Request.RequestUri, uri );
    return response;
}

Web API properly binds my School class's Name and Abbreviation properties from my web form and calls the POST method in the controller, but it doesn't know what to do with the State class. I'm not quite sure how to set that up. I'd like to have a dropdown that is bound to the State class and when I submit the creation of the School, the correct state from my existing data will get assigned to the new school instance.

• asp.net-mvc-4
• asp.net-web-api
• entity-framework-4.1

First of all, your domain model design is a little rough. Your School table should have a FK to State table and you should have that inside your POCO classes as well:

public class School
{
    public long Id                    { get; set; }
    public long StateId               { get; set; }
    public string Name                { get; set; }
    public string Abbreviation        { get; set; }

    // Navigation Properties
    public virtual State State        { get; set; }
}

Then you should have a form field as similar to below:

<select id="StateId" name="StateId">
    <option value="">Select a State</option>
    <option value="310">CA</option>
    <option value="311">NY</option>
</select>

In my current project I have achieved something similar using following approach:

I have a Device class (POCO) and a DeviceCommand class (POCO). Device can have many commands and each command will have exactly one device.

I did not bother with ViewModel and POCO classes are used between layers.

public class Device
{
    [Key]
    public int ID { get; set; }

    public int DeviceID { get; set; }

    public bool IsActive { get; set; }

    // Navigational properties
    public ICollection<DeviceCommand> CommandList { get; set; }
}

And

public class DeviceCommand
{
    public int Id { get; set; }

    public string CommandText { get; set; }
    public string CommandId { get; set; }

    // Navigational properties
    public int DeviceId { get; set; }
    public Device Device { get; set; }
}

And in my DbContext derived class I have following code:

    protected override void OnModelCreating(System.Data.Entity.DbModelBuilder modelBuilder)
    {         
        modelBuilder.Entity<DeviceCommand>()
            .HasRequired(d => d.Device)
            .WithMany(l => l.CommandList)
            .HasForeignKey(b => b.DeviceId)
            .WillCascadeOnDelete();
    }

IMO you will have to setup a foreign key relationship either using annotations or fluent API.

If the parameter in the POST method has no attribute on it, then the decision is made purely on the parameter’s .NET type.“Simple types” uses model binding. Complex types uses the formatters.

Please refer this link to understand web api binding better:

http://blogs.msdn.com/b/jmstall/archive/2012/04/16/how-webapi-does-parameter-binding.aspx

Hope this helps.