Just a quick post here for something that I see asked (or, more accurately, see being done in the wrong way) way too often.
When you're building your own components or data model using xPDO Schemas, you will probably be working with that data in processors, snippets or external applications as well. At some point you will have related tables, and you will love coming across methods like addOne and addMany - they're awesome!
There's one big caveat when using them, and that is to figure out which one to use.
Oh, you use addOne when you want to add *one* related object, and addMany when you want to add multiple in one go? WRONG. Well, okay - there's a small truth in there. But it is not related to the fact if you're in the process of adding only one related object or an array of objects. If you are using the wrong method you can often see this by the lack of relation being added to the extra object(s), like the relation is not "sticking" for some reason.
So do I use addOne or addMany?
If you've read the Defining Relationships documentation page, which is a great introduction to aggregate and composite relations with xPDO schemas, you should have noticed this thing called the cardinality. This little thing is very important and the key to figuring out when you need addOne, and when you need addMany.
You see, if the cardinality is one, you will need to use addOne. If the cardinality is many, you will need to use addMany. And that's all folks.
Teaser image by Sepehr Ehsani.