This aligns better with the relation principal "parent saves child" / "child cannot save parent" and is more conducive to the natural workflow of a coder, ie
// Relation first
$gallery = new Gallery;
$gallery->save();
// Primary model last
$post = new Post;
$post->gallery = $gallery;
$post->save();
Refs https://github.com/octobercms/library/pull/277
b9581332aa