Entity Framework not updating foreign key when entity updates

Question

I'm using Entity Framework with Web API 2. I have a boat entity with properties like name, price etc. Those simple properties update fine when sent by Put to the web api controller. The boat entity also has a many to one relationship with an entity called BoatType. Boat types are "Yacht", "Motor Yacht" etc.

When a boat entity is updated in the controller the foreign key for boat type in the database doesn't get updated. Do I have to somehow manually update the child entity value or is there a way to get EF to do this automatically?

Here's an example PUT request sent to web API:

{
  "$id":"1",
  "Images":[],
  "BoatType": {
    "$id":"3",
    "Boat":[],
    "Id":1,
    "DateCreated":"2015-09-15T13:14:39.077",
    "Name":"Yacht"
  },
  "Id":2,
  "Name":"Schooner",
  "Description":"A harmless schooner",
  "DateCreated":"2015-09-15T17:59:37.8",
  "Price":65000
}

Here's the update function in web API:

[ResponseType(typeof(void))]
public async Task<IHttpActionResult> Put(int id, Boat boat)
{
    if (id != boat.Id)
    {
        return BadRequest();
    }

    _db.Entry(boat).State = EntityState.Modified;

    try
    {
        await _db.SaveChangesAsync();
    }
    catch (DbUpdateConcurrencyException)
    {
        if (!BoatExists(id))
        {
            return NotFound();
        }
        else
        {
            throw;
        }
    }
    return StatusCode(HttpStatusCode.NoContent);
}

I've looked at similar questions like Entity Framework Code First Update Does Not Update Foreign Key , Entity Framework does not update Foreign Key object and Update foreign key using Entity Framework but none seem to have quite the same scenario (or the answers didn't help me understand my issue).

Here's the Boat and BoatType model classes (auto-generated by EF designer).

public partial class Boat
{
    [System.Diagnostics.CodeAnalysis.SuppressMessage("Microsoft.Usage", "CA2214:DoNotCallOverridableMethodsInConstructors")]
    public Boat()
    {
        this.Images = new HashSet<Image>();
    }

    public int Id { get; set; }
    public string Name { get; set; }
    public string Description { get; set; }
    public System.DateTime DateCreated { get; set; }
    public Nullable<double> Price { get; set; }

    [System.Diagnostics.CodeAnalysis.SuppressMessage("Microsoft.Usage", "CA2227:CollectionPropertiesShouldBeReadOnly")]
    public virtual ICollection<Image> Images { get; set; }
    public virtual BoatType BoatType { get; set; }
} 

public partial class BoatType
{
    [System.Diagnostics.CodeAnalysis.SuppressMessage("Microsoft.Usage", "CA2214:DoNotCallOverridableMethodsInConstructors")]
    public BoatType()
    {
        this.Boat = new HashSet<Boat>();
    }

    public int Id { get; set; }
    public System.DateTime DateCreated { get; set; }
    public string Name { get; set; }

    [System.Diagnostics.CodeAnalysis.SuppressMessage("Microsoft.Usage", "CA2227:CollectionPropertiesShouldBeReadOnly")]
    public virtual ICollection<Boat> Boat { get; set; }
}

Answer 1

Ok, I figured out what the problem was. Using SQL Server Profiler to look at the SQL Update statement I saw that the foreign key for Boat.BoatType wasn't even in there - so I figured my model must be screwed up somewhere. When I created the model in the designer, I mistakenly set the relationship between Boat and BoatType as one to one. I later realised the mistake and changed the association to one (BoatType) to many (Boats) but that must have been AFTER I generated the database. D'oh! Something about the way EF handles associations meant that simply changing the association type in the diagram wasn't enough - I should have dropped/recreated the database constraint at that time.

Since I only had test data in the database what worked for me was to recreate the database using the "Generate database from model..." option in the designer.

Once I got the PUT working correctly the other thing I had to solve (which is not really on topic for this question but it's been discussed above so just in case it's useful to someone) was that Web API gave the error "A referential integrity constraint violation occurred: The property value(s) of 'Boat.BoatTypeId' on one end of a relationship do not match the property value(s) of 'Boat.BoatType.Id' on the other end.". The select list that allows the user to change the boat type is bound on the client using AngularJS to Boat.BoatType. So in the PUT data, Boat.BoatType had been updated to new values but Boat.BoatTypeId hadn't changed - hence the "referential integrity" error. So I just manually set the value of Boat.BoatTypeId to Boat.BoatType.Id before sending the PUT and all works as expected now.