Test case

https://runkit.com/lehni/objection-has-one-relation-with-upsert

Description

I'm experiencing something strange with my upsert() method here. When I run it on an empty has-one relation through model.$relatedQuery('relation').upsert(…), I get the error "Cannot read property '$id' of null", from InsertAndFetchOperation.js:26.

But when I replace the call of upsert() with a direct call of insertAndFetch() (which is used internally in upsert() also, since results === 0), this works.

I've started debugging, and found that while in the direct call, InsertAndFetchOperation.onAfter2(builder, inserted) receives an array with one element in inserted, but when I use the approach of upsert() above, then inserted is just the inserted model, not wrapped in an array. super.onAfter2(builder, inserted) then wrongly thinks it's an array and tries to unwrap it (inserted[0]), resulting in this error. I don't understand why this is happening.

there is already a check for arrays here, but the problem happens before.
If I add the same check before this line then it starts working as expected:

onAfter2(builder, inserted) {
  const modelClass = builder.modelClass();
  const insertedArray = Array.isArray(inserted) ? inserted : [inserted];
  const maybePromise = super.onAfter2(builder, insertedArray);

This change solves the issue and doesn't break any existing tests, but I cannot tell if this is right or not : )