Are associations cached with MemoryStore?

Having trouble with MemoryStore cache. I store an object with
association:

post = Post.find_by_id(id), :includes => :tags
Post.cache.write(key, post)

When I retrieve this Rails.cache.read(key), the model is certainly
there, but the SQL dump indicates the associations (tags) still loaded
from the database. Anyone know how I can store and retrieve the
associations?