This repo illustrates a bug present in arbre 1.2.0 as mentioned in the following issue.
Setup the vanilla rails app + active admin with standard instructions:
$> bundle install
$> rails db:setup
$> rails s
Then open open your browser and navigate to edit the first Order, ie:
$> open http://localhost:3000/admin/orders/1/edit
Observe that the 2 payments recorded on the Order are shown twice, resulting in 4 rows.