Rails link_to new with parameters not saving parameters

So, I have two objects, Floor, which has_many FloorMonsters, and FloorMonster which belongs_to Floor.

I have a link_to new_floor_monster_path in the show page on Floor, and I intend to pass the Floor id as a parameter. The link_to currently looks like this:

= link_to "Add a Monster to this Floor", new_floor_monster_path(floor_id: @floor.id)

In the controller for FloorMonster, the new method is this:

def new
  @floor_monster = FloorMonster.new(floor_id: params[:floor_id])

  respond_to do |format|
    format.html # new.html.erb
    format.json { render json: @floor_monster }
  end
end

When I click the link, the URL shows the parameters:

http://.../floor_monsters/new?floor_id=4

But when I save, the floor_id is nil and the program crashes. What am I missing here? I've tracked down sources that do exactly what I did, but I am getting no succes. The accepted answer here and this blog post are doing exactly what I am doing as far as I can tell, but they managed to get it to work.

Solution

I assume that you are exposing the new FloorMonster in a form to allow setting other attributes and then saving it. In order to pass the floor_id attribute along to the create action, you will need to add a field to your form for it. If you don't want it to be seen or to be editable, use a hidden_field. In your form, add:

= f.hidden_field :floor_id