Do ORMs normally allow circular relations? If so, how would they handle it?

Circular references aren't a problem in an ORM. ORMS are lazy in their evaluation, meaning that they don't retrieve records until the record is accessed. In this way, there's no infinite loop issue.

The same can go for writes, if you are lazy when you write, then the issues go away.

I believe most ORMs enforce a single unique object in memory. So, the first time you access car, car gets loaded into memory and it can be identified by a unique key. After going to the wheel and back to the car, the ORM realizes is already has that car in memory, so it doesn't load it again.

This approach isn't without downsides, but each ORM works around those downsides in various ways.

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