asp.net mvc - Populating a SelectList with Parent/Child Tables -

i trying populate html.dropdownlist in mvc. problem i'm having have child table called "odsessiontopics" has foreign key (id) "odsessiontype" table contain headers topics. here models:

public class odsessiontopic {     public int id { get; set; }     public string description { get; set; }      // foreign key     public int odsessiontypeid { get; set; }      // navigation property     public virtual odsessiontype odsessiontype { get; set; }                }  public class odsessiontype {     public int odsessiontypeid { get; set; }      public string description { get; set; }      // navigation property     public virtual icollection<odsessiontopic> odsessiontopics { get; set; }  } 

i use following code populate viewbag:

viewbag.odsessiontopicid = new selectlist(db.odsessiontopics, "id", "description", "odsessiontypeid", odsession.odsessiontopicid); 

here od session topic data:

    id           description            odsessiontypeid     ---------------------------------------------------     1            internal marketing     1     2            team development       1     3            department retreat     2     4            community service      2 

here od session type data:

    odsessiontypeid           description     -------------------------------------      1                         plan          2                         action 

these results:

    1         internal marketing        team development     2        department retreat        community services 

here results trying achieve:

    plans        internal marketing        team development     actions        department retreat        community services 

the view code is

@html.dropdownlist("odsessiontopicid", null, "-- session type --", htmlattributes: new { @class = "form-control" })  

basically, it's grabbing odsessiontypeid foreign key in odsessiontopic table , grouping value. need grab description odsessiontype table. possible? both topics , types editable , have crud logic attached them how arrived design in first place.

you can use linq .join() , project results anonymous object. assuming var topics = db.odsessiontopics; , var types = db.odsessiontypes; query be

var query = topic in topics             join type in types on topic.odsessiontypeid equals type.odsessiontypeid             select new { group = type.description, id = topic.id, description = topic.description }; 

which output

{ group: "plan", id: 1, description: "internal marketing" } { group: "plan", id: 2, description: "team development" } { group: "action", id: 3, description: "department retreat" } { group: "action", id: 4, description: "community services" } 

and create selectlist

viewbag.odsessiontopicid = new selectlist(query, "id", "description", "group", odsession.odsessiontopicid) 

side note: recommend use typed html helpers generate dropdownlist. viewbag property should not same name property binding to. instead should (say)

viewbag.topiclist = new selectlist(query, "id", "description", "group", null) 

and in view

@html.dropdownlistfor(m => m.odsessiontopicid, (selectlist)viewbag.topiclist, "-- session type --", new { @class = "form-control" })