Running total of a field in child records

What is the best way to get the running total of a number field in child
records. Let say I have an Order model, which has_many :line_items, and
LineItem has an amount field. I would like to get the grand total of a
specific Order. Is it a good idea to add a ‘total’ column in the
table and update it every time a line_item is created/updated/deleted?

see Counter Cache section as one of the solutions

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