Saving Calculated Data when using rails_admin gem

In my project i have product model and stock_in model, using rails_admin gem how can i update a quantity of a product automatically when new stock_in is created for that product?

Without seeing any code, it's hard to give you anything more than general advice.

You don't need to do this through rails_admin because Rails already has hooks.

Assuming your Product and StockIn models are related:

class Product
  has_many :stock_ins
end
class StockIn
  belongs_to :product
end

You can just use an after_create hook on StockIn:

class StockIn
  belongs_to :product
  after_create :update_product_quantity
  ...
  private
  def update_product_quantity
    product.update(quantity: self.quantity)
  end
end