I seem to run into a problem related to this with custom types (ruby 1.9.4, mongoid 3.0.9):

In a document, I have a field :practice, type: Address

When reading that value, I get the correct behaviour (typecasted) with:

doc.practice #=> Address

but I get the wrong behaviour (the serialized version of the object) with:

doc.read_attribute("practice") #=> [...]
doc.attributes #=> all attributes, practice: [...]
doc["practice"] #=> [...]