How to create an association between two models with form choice field using Active Admin, Paperclip in Rails 5.0.1

Scenario

I am building a similar App like AirBnB but the difference is that only the Admin can add new Apartments, Rooms, Houses etc. Therefore I created an Admin Panel with the activeadmin gem. I am now adding the image upload system into activeadmin so I am using the paperclip gem. I have two models "room" and "photo". The photo model has an "image" column. The room model has an "listing_name" column.

room.rb (app/models)

class Room < ApplicationRecord
    has_many :photos
end

photo.rb (app/models)

class Photo < ApplicationRecord
  belongs_to :room

  has_attached_file :image, styles: { medium: "300x300>", thumb: "100x100>" }
  validates_attachment_content_type :image, content_type: /\Aimage\/.*\z/
end

If I am not putting a custom form inside "photo.rb (app/admin)" I can associate the image to a room by default. But I can't upload a picture! This looks like this:

So I changed the "photo.rb (app/admin)" file into this:

photo.rb (app/admin)

ActiveAdmin.register Photo do
   permit_params :image

   form :html => {:multipart => true} do |f|
      f.inputs "Project Details" do
          f.input :image, :required => false, :as => :file
      end
      f.actions
   end

   show do |ad|
      attributes_table do
         row :image do
            image_tag(ad.image.url(:thumb))
         end
      end
   end
end

With this form I can upload a picture into the image column but I can't associate it to a room and the edit view does not give me a preview of a picture as the show view does:

Questions:

1. How can I have a preview of the image in the admin edit view?

2. How can build a form to associate an image to a room by choosing the listing_name of the room?

Something like this:

Thank you in advance for your help.