Custom fields (demongoize) behavior unintuitive

Current behavior: initialize a new object on read

Right now, specifying a custom field type with

: MyType

calls the

every time the attribute is read, which initializes a new object for the field. So you get unexpected behavior:

class Post
field :author, type: Author
end

post = Post.first
post.author.name = "new name"
post.author.name # < — changes would be lost because the second call to 'post.author' instantiates a new object!

I don't think I'm the only one who found this surprising: see https://github.com/mongoid/mongoid/issues/1604#issuecomment-3547128
and https://github.com/mongoid/mongoid/issues/2951.

Current work around (inconvenient)

There's a solution, which is to assign the field object on the first time and then manipulate that local variable:

post = Post.first
author = post.author
author.name = "new name"
post.save

But this is fairly inconvenient, and if you forget to do so, you can find yourself wondering why changes aren't saved.

Proposed work-around

I hacked together a solution that basically memoizes the object returned from demongoize, and the getter and setter created by field would first look to see if an object is already in memory.

This works alright, but I wanted to see if there's work being done on this, or if there's a better way to handle things I'm completely missing. I'd be happy to take a stab at putting together a more robust solution.