title: Active admin custom index with instant method column
body: <p>i have a user model</p>
  
  <pre><code>class User &lt; ApplicationRecord
  has_many :payments, dependent: :destroy
  has_many :debts, dependent: :destroy
  
  def balance
  @balance = self.payments.sum(:amount) - self.debts.sum(:amount)
  end
  
  end
  </code></pre>
  
  <p>and i have index in user controller</p>
  
  <pre><code>index do 
  selectable_column
  column :id
  column :name
  column :identity_card
  column :address
  column :contact
  column :balance
  column :code
  column :created_at
  column :updated_at
  actions
  end
  </code></pre>
  
  <p>how to display the balance on the index with my instance method on the user model?</p>