Complex C# Object Cascading Update

Question

I have the following:

// Day Class
public class Day {
    [Key]
    [JsonProperty("name")]
    public string name { get; set; }

    [JsonProperty("playlists")]
    public virtual List < Playlist > playlists { get; set; }
}

// Schedule Class
public class Schedule {
    [Key]
    public int id { get; set; }

    public virtual List < Day > days { get; set; }

    [Required]
    public string hash { get; set; }
}

// Playlist Class
public class Playlist {
    [Key]
    public int id { get; set; }

    [Required]
    [JsonProperty("name")]
    public string name { get; set; }

    [JsonProperty("media")]
    public virtual List < Media > media { get; set; }

    // Media class
    public class Media {
        ...
    }

I am using Web API, SQL / EF to create CRUD operations on the above classes. However, my issue is that when I POST to /schedules/ I'd like all the related properties to update. I am POSTING a raw JSON object and I can parse it properly. The problem is that if I have a media object in the database with a unique key hash or any other object in the db the POST fails with a key duplicate exception since it is trying to re-add the object. I'd like to update the relationships in that scenario... What should I be doing to correct this?

Please ask if you need any other info. Thanks in advance.

EDIT

  [ResponseType(typeof(Schedule))]
        public async Task<IHttpActionResult> PostSchedule([FromBody]Object model)
        {
            var jsonSchedule = model.ToString();
            try
            {
                var _schedule = JsonConvert.DeserializeObject<Schedule>(jsonSchedule);

                db.Schedules.Add(_schedule);
                await db.SaveChangesAsync();

                return Ok();
            }