has_many_active_admin.rb

  # model/product.rb
  belongs_to :product, optional: true

  # model/variation.rb
  has_many :variations, dependent: :destroy
  accepts_nested_attributes_for :variations, :allow_destroy => true
  
  # admin/products.rb
  permit_params :name, :description,
                variations_attributes: [
                  :id,
                  :name,
                  :price,
                  :_destroy, 
                  :_edit
                ]

form do |f|
    f.semantic_errors *f.object.errors.keys
    f.inputs "Product" do
      f.input :name
      f.input :status
    end

    f.has_many :variations,
      new_record: 'Add item',
      remove_record: 'Remove item',
      allow_destroy: true, 
      class: "mb-5" do |b|
        b.input :id, as: :hidden
        b.input :name
        b.input :price
    end
    f.actions do 
      if resource.persisted?
        f.action :submit, label: 'Update'
      else
        f.action :submit, label: 'Create'
      end
      f.action :cancel, label: 'Cancel'
    end
  end