Issue with setting up multiple form_for in an each loop

trying to wrap my head round something and hoping someone can help.

I have a loop as follows to dislay a list of assets (images) for a property:

- @property.property_assets.each do |asset|

Each asset has an image with a description field, ie:

= asset.description

This works perfectly fine.

I'd like to have a form for each asset so I can update the description, with a submit.

My form_for currently looks like:

= form_for asset , url: property_property_assets_path do |f|

My routes file:

resources :properties do 

    resources :property_assets

    member do
      post :update_general
      post :update_promo_image
      post :update_location

    end
end

My property_assets_controller:

def index
    @property = Property.find(params[:property_id])
end

def update

    if @property_asset.update(property_asset_params)
      respond_to do |format|
        format.html
        format.js
        flash[:notice] = "Updated property asset"
      end
    else
      render 'edit'
    end
  end

The issue

I'm getting the error:

No route matches {:action=>"index", :controller=>"property_assets", :id=>"1"} missing required keys: [:property_id]

Thanks in advance