My airport model has

accepts_nested_attributes_for :city, :country

Then I do:

>> foo = Airport.first
MOPED: 127.0.0.1:27017 QUERY database=gocheap_development collection=places selector={"$query"=>{"deleted_at"=>nil, "_type"=>{"$in"=>["Airport"]}}, "$orderby"=>{:_id=>1}} flags=[:slave_ok] limit=-1 skip=0 batch_size=nil fields=nil (41.4884ms)
#<Airport _id: 4eda60208792904be4002443 ... >

>> foo.save
foo.save
MOPED: 127.0.0.1:27017 QUERY database=gocheap_development collection=places selector={"$query"=>{"deleted_at"=>nil, "_id"=>"4eda5f2c8792904be40000fc", "_type"=>{"$in"=>["Country"]}}, "$orderby"=>{:_id=>1}} flags=[:slave_ok] limit=-1 skip=0 batch_size=nil fields=nil (1.5063ms)
MOPED: 127.0.0.1:27017 QUERY database=gocheap_development collection=places selector={"$query"=>{"deleted_at"=>nil, "_id"=>"4eda5f2e8792904be40001b3", "_type"=>{"$in"=>["City"]}}, "$orderby"=>{:_id=>1}} flags=[:slave_ok] limit=-1 skip=0 batch_size=nil fields=nil (0.7651ms)
true

So it gets every relation listed in accepts_nested_attributes_for despite the fact, that I didn't change anything in the object. These requests look like unnecessary.