I have a simple (hopefully) scenario.
- Seats table
- Computers table
- SeatComputers table (since a seat can have multiple computers assigned)
I have a strongly typed “Edit” view for “Seats”. I have managed to get a multi select list on that page in order to assign or unassign computers (jquery to add/remove items).
However, when I submit the form, the contents of select list are not posted to the controller action.
I assume this is because “computers” select list is not a model property.
Is there anyway to POST the additional data to the controller outside of model’s properties?
My tables look something like this:

You don’t need to post this list as you already have it stored into the database and you even have a repository to fetch it, haven’t you? So the only thing that needs to be posted is the user selection as this is the only thing you don’t know. In the POST action reconstruct the list in your view model using a repository, the same way you did in the GET action that rendered the form.
Sure, simply include them as input fields so that their values are sent along the POST and in your controller action:
But I insist once again: you don’t need this in your case.