c# - EF 4.1 code first - How to update/delete many to many join table entries automatically -

i have 2 entities, let's say, trip , activity. relationship between them many many join table created automatically ef.

entity trip attributes:
-id (pk) generated database
-name
-description
-property1
-property2
-property3

entity activity attributes (this entity contains fixed records -read only-, no records inserted here on performing inserts):
-id (pk) generated database
-name
-description
-cost

join table contains 2 columns, is, ids of above entities, primary , foreign keys @ same time.

i have no problems inserting entries automatically ef creates join table tripactivities , add entries it. entries added entity trip , leaves unchanged entity activity.

my problem on updating entries, example, - suppose user can modify information related trip gui - take info gui , perform following steps update existing trip:

   trip trip = context.trips.find(id); // search appropriate trip update id     trip.name = obtainnamefromgui();    trip.description = obtaindescriptionfromgui();    trip.property1 = obtainproperty1fromgui();    trip.property2 = obtainproperty2fromgui();    trip.property3 = obtainproperty3fromgui();    trip.activities = new list<activity>();     // gui user selects checkbox list activities associated trip    // read ids , ids fetch database activities obtain    // info related each activity selected in gui. done inside    // below method.    list<activity> activities = this.obtainactivitiesselectedfromgui();     // if no activites selected (=null) want ef automatically deletes entries in    // joined table trip. , of course, if there activities selected, ef     // should update respectives entries in joined table trip new    // ones.    if (activites != null)    {        activities.foreach(a =>                     {                         trip.activities.add(a);                     });    }     context.trips.add(trip);    context.savechanges();<br><br> 

by doing want ef updates entities related (except activity has fixed entries, must kept unchanged), is, trip , joined table automatically not work: new trip created , more entries in joined table (the thing working entity activity kept unchanged want).

how achieve this? have spent lot of hours trying without success...

thanks in advance.

edit:
have removed line:
context.trips.add(trip);

now results are:
-entity trip correctly updated, no new records added ok.
-entity activity kept unchanged ok.
-join table: old records current trip being updated not updated, instead new records inserted current trip not correct.

i have used different approach similar scenario faced, works detached entities. ended finding out entities added , ones deleted comparing gui(detached entity) values database values. here sample code have used. entities in play relayconfig , standardcontact have many many relationship

    public void update(relayconfig relayconfig, list<standardcontact> exposedcontacts) {         relayconfig dbrelayconfig = context.relayconfigs.include(r => r.standardcontacts)                                            .where(r => r.id == relayconfig.id).singleordefault();         context.entry<relayconfig> (dbrelayconfig).currentvalues.setvalues(relayconfig);          list<standardcontact> addedexposedcontacts =              exposedcontacts.where(c1 => !dbrelayconfig.standardcontacts.any(c2 => c1.id == c2.id)).tolist();         list<standardcontact> deletedexposedcontacts =              dbrelayconfig.standardcontacts.where(c1 => !exposedcontacts.any(c2 => c2.id == c1.id)).tolist();          standardcontact dbexposedcontact = null;         addedexposedcontacts.foreach(exposedcontact => {             dbexposedcontact = context.standardcontacts.singleordefault(sc => sc.id == exposedcontact.id);             dbrelayconfig.standardcontacts.add(dbexposedcontact);         });         deletedexposedcontacts.foreach(exposedcontact => { dbrelayconfig.standardcontacts.remove(exposedcontact);});