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  • If a child object can't do anything useful without a parent, having a child object start without a parent will require it to have a SetParent method which will either need to support changing the parentage of an existing child (which may be hard to do and/or nonsensical) or will only be callable once. Modeling such situations as aggregates (as Mike Brown suggests) can be much better than having children start without parents. Commented Dec 4, 2014 at 4:30
  • If a use case calls for something even once, the design must allow for it always. Then, restricting that capability to special circumstances is easy. Adding such a capability afterwards however is usually impossible. The best solution is Option 3. Probably with a third, "Relationship" object between the parent and child such that [Parent] ---> [Relationship(Parent owns Child)] <--- [Child]. This also allows multiple [Relationship] instances such as [Child] ---> [Relationship(Child is owned by Parent)] <--- [Parent]. Commented Jan 8, 2016 at 9:02