I'm not sure if this is intended behavior or not.

class User
  include Mongoid::Document
  include Mongoid::Timestamps
  field :name
  field :is_admin, type: Boolean, default: false
  belongs_to :relation
end

User.create(name: 'bob', :is_admin => true)
user = User.only(:id, :relation_id).where(name: 'bob').first
user.update_attribute(:relation_id, some_id) #only expect relation_id and updated_at to be changed

Yields an update along the lines of:

MONGODB db['users'].update(

, {"$set"=>{"is_admin"=>false, "relation_id"=>BSON::ObjectId('4efce81fa393a9823b000001'), "updated_at"=>2012-01-31 16:18:39 UTC}})

As you can see it is setting is_admin back to the default value (false) instead of leaving it alone even though it was not actually "changed". I can see why this might happen since the User loaded with only(:id, :relation_id) has no value set for is_admin but to me this is unexpected behavior. I would expect for an existing model it would set the dirty flag on an attribute only if that field is actually modified after the model is loaded.

P.S. In this bogus scenario above 'only' would not really be needed but in my actual use-case I'm going through lots of documents and trying to minimize memory usage by not fetching unnecessary fields. Is there some other way to force mongoid to ONLY update a single field and ignore its normal behavior?