Adding and listing has_many through associations in Active Admin - ruby-on-rails

Question

I have the following setup:
class User < ActiveRecord::Base
has_many :device_ownerships, :dependent => :destroy
has_many :devices, :through => :device_ownerships
end
class device < ActiveRecord::Base
has_one :device_ownership, :dependent => :destroy
has_one :user, :through => :device_ownership
end
class deviceOwnership < ActiveRecord::Base
belongs_to :user
belongs_to :device
validates_uniqueness_of :device_id, :scope => :user_id
validates_uniqueness_of :user_id, :scope => :device_id
end
I am trying to achieve the following in Active Admin:
In Edit screen
1) List all devices that belong to user with an option to delete the device or destroy the deviceOwnership that connects device to user
2) have an option to create a new pairing user-device from existing devices (by creating new DeviceOwnership).
3) have an option to create new device and add it to user via new DeviceOwnership.
I listed my problems with what I have now in comments below:
ActiveAdmin.register User do
permit_params :email, :password, :password_confirmation, :role,
device_ownerships_attributes: [:device_id, :user_id],
devices_attributes: [:device_identifier]
index do |user|
user.column :email
user.column :current_sign_in_at
user.column :last_sign_in_at
user.column :sign_in_count
user.column :role
actions
end
filter :email
form do |f|
f.inputs "User Details" do
f.input :email
f.input :password
f.input :password_confirmation
f.input :role, as: :radio, collection: {Editor: "editor", Moderator: "moderator", Administrator: "administrator"}
#This one allows to create new devices but also lists all existing devices with option to modify their device_identifier column which I don't want
f.has_many :devices, :allow_destroy => true, :heading => 'Themes', :new_record => true do |cf|
cf.input :device_identifier
end
#This one lists all the devices but no option to remove any of them.
f.input :devices
#This one shows dropdownw with existing devices but allows to swap them
f.has_many :devices, :allow_destroy => true do |device_f|
device_f.input :device_identifier, :as => :select, :collection => device.all.map{ |device| [device.device_identifier] }, include_blank: false,
end
f.actions
end
end
end

Answer 1

The line
f.has_many :devices, :allow_destroy => true, :heading => 'Themes', :new_record => true do |cf|
cf.input :device_identifier
end
looks like it might do the job. You could check whether the cf.object is a new record and only let the user change the device_identifier in that case.
cf.input :device_identifier if cf.object.new_record?
or something like cf.input :device_identifier, input_html: { readonly: !cf.object.new_record? }
What do you think?