I also don't quite understand the mutation

addToItemOnBrand(itemsItemId: ID!, brandBrandId: ID!)

.. I would expect something like

addItemToBrand

or something like that.. 😄 and the mutation arguments are named really strangely too.

@kudlajz I think that relation names are auto-generated But I believe you can change the relation name by clicking Set Relation Name when creating the relation