Studing about relationship in the phalcon documentation I saw that the modeling relationship made between Robots and parts used the many to many approach.
It allows two robots have the same specific part like this. For example Robot A and Robot B will share the Part A. A mother can have many children, but a children have just one specific mother. One robot have many parts(children), but I cant use one specific part(children) for two robots.
So, If I'm not wrong the correct conceptual relationship is one-to-many.
https://docs.phalcon.io/en/latest/reference/model-relationships.html
