The code below assumes that Post embeds many Comments:

post = Post.create!
10.times do |i|
  post.comments.create!
end
pp Post.collection.find().to_a.first['comments'].size # => 10
post.comments.delete(post.comments.first)
pp Post.collection.find().to_a.first['comments'].size # => 9
post.comments.delete_one(post.comments.first)
pp Post.collection.find().to_a.first['comments'].size # => 9

So, there is a difference in the behaviour of delete (persists changes to the database), and delete_one (does not persist the changes).

Further, there is delete_one method in the EmbedsMany::Proxy class; this method is private. However, it is not being called here; the method of Array extension is called instead.

We should investigate the situation and either prohibit calling delete_one on associations, or document make them behave in accordance with delete, or document the difference.