use defaults utility at relationships.

[mshima]

Related to #23917.

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[mshima]

This doesn’t fix the problem, but the api for loading relationships is there.
Fetching not ownerSide is only useful for view, unless we are going to cascade, so we probably need to add a flag for write/read.

[hide212131]

I've reviewed the generated code and have a general understanding of the concept.