How to get multiple fields form into one Json column using ActiveAdmin Active Admin Ruby on Rails and MySQL?

Question

Hi I'm new to ActiveAdmin, it's inherent Formtastic and Ruby (2.6) on Rails (6.1) and need to make a modification on an existing system.

To make it short, I have a Prices model, which is filled with data on a prices.html.erb form.

I need to add key/value pairs (eg: {"fee_1": 10, "fee_2": 20,...}) to be stored in a JSON "fees_list" column, in a MySQL database.

I'm having issues about how to modify the form to input the values.

Amongst many attempts so far, in the prices.html.erb Formtastic form I've tried (Edit: Thanks to Lam Phan for typo fix):

<%= f.inputs, for: => :fees_list do |j|  %>
  <%= j.input :fee_1, require: false, input_html: { value: resource.fees_list['fee_1'] } %>
  <%= j.input :fee_2, require: false, input_html: { value: resource.fees_list['fee_2'] } %>
<%= end %>

Additional information is that I have a @prices reference available in the prices.html.erb file.

But the resulting page does not display the price inputs. Do I need some Gem to edit/show the JSON data in the DB?

Could someone help please? Thanks a lot to this great community in advance. Best regards!