Association implies symmetry; both classes know each other and can message each other.

A Product class object may need to know the last sale that it was involved with. We could put a pointer to a Sale class object inside the private part of Product.

A Sale class object may need to know all of the Products involved with the Sale. We can put a pointer to a pointer (double pointer) in Sale that will let us store zero or more Products associated with a Sale.

hence this is not uncommon

$stock->product("Book")->lastSale()->undo()

and affects both objects

update

a maybe better example I found is the part_of association of Apartment <-> Room

A $room can only belong to one containing $apartment and has a (weakened) back-reference

Like this if $room->burns() than $apartment->burns()

> but I don't believe that has any bearing on method chaining.

$street->apartment("5a")->room("living")->burns()

HTH! :)