Using mongoid 2.4.6 on rails 3.2.2. The documentation for referenced 1-n relationships states that if a relationship has :dependent => :nullify (the default option), then all calls to delete on the relationship will nullify the relationship not destroy. Calling delete_all on the has_many relationship seems to ignore this option and call delete on all of the related documents regardless. Consider this setup:

document Calendar
include Mongoid::Document

has_many :days # :dependent => :nullify is the default here so it's not necessary to declare
end

document Day
include Mongoid::Document

belongs_to :calendar
end

Now say you have a calendar document with 3 related day documents. Calling clear on the relationship like so:

calendar.days.clear

or setting the days relationship to an empty array like so:

calendar.days = []

will nullify the related days as expected. However, if you call delete_all, like so:

calendar.days.delete_all

instead of nullifying the related days as expected, it appears to call delete on each individual day, and they all then get removed from the database.