When using fields with a custom serialization class, Mongoid doesn't seem to cache the object we're assigning, and instead it's mongoizing/demongoizing the value each time it's accessed:

class Document
  include Mongoid::Document

  field :foo, type: Foo
end

doc = Document.new
doc.foo = Foo.new
doc.foo === doc.foo # => false

Note: all built-in field types do not seem to behave this way, and calling doc.foo === doc.foo on them returns true.

Not sure if this is an expected behavior, but it would be cool if it was mentioned somewhere in the docs.