class ActiveAdmin::ResourceDSL

end
['COALESCE(NULLIF(last_name, ''), first_name), first_name', order_clause.order].join(' ')
order_by(:full_name) do |order_clause|
# by last_name but in the case that there is no last name, by first_name.

end
[order_clause.to_sql, 'NULLS LAST'].join(' ') if order_clause.order == 'desc'
order_by(:age) do |order_clause|
# nulls last

For example:

Redefine sort behaviour for column

def order_by(column, &block)
  config.ordering[column] = block
end