ActiveRecord serialize field and SerializationTypeMismatch


In one of my ActiveRecord models I specified a a serialize field:
serialize :settings MyModelSettings

If I understand the documentation correctly this restricts settings to
the class MyModelSettings. The problem is that this is so restrictive
that I even don't know how to initialize the field.
or even => MyModelSettings)

everything I do leads to

ActiveRecord::SerializationTypeMismatch: settings was supposed to be a
MyModelSettings, but was a String

Where is the correct place to initialize the settings (preferable
inside my model)?