I've run into an issue that I think I've tracked down to accepts_nested_attributes_for and a lopsided has_and_belongs_to_many.

has_and_belongs_to_many :attendees, :class_name => 'Attendee', :inverse_of => nil
accepts_nested_attributes_for :attendees, :allow_destroy => true

It seems like this find query:

def process_attributes(parent, attrs)
return if reject?(parent, attrs)
if id = attrs.extract_id
first = existing.first
converted = first ? convert_id(first.class, id) : id
doc = existing.find(converted) # <============================================
if destroyable?(attrs)
destroy(parent, existing, doc)
else
update_document(doc, attrs)
end
else
existing.push(Factory.build(metadata.klass, attrs)) unless destroyable?(attrs)
end
end

https://github.com/mongoid/mongoid/blob/master/lib/mongoid/relations/builders/nested_attributes/many.rb#L102

is translated into this:

QUERY collection=attendees selector={"$and"=>[{"_id"=>{"$in"=>["5315f25c166093b274000001", "5315f25c166093b274000002"]}}], "_id"=>"5314ba811660930c03000002"}

where the two other ids are for the two other models in the relation. The query will never match anything, since it basically where id == 1 && id == 2.

A wild guess is that normally the query would be scoped based on the association, e.g. "parent_id" => {"$in"... but because of the lopsided HABTM-relation that does not happen and this query is run instead.

I understand that this issue would be more complete with a test-case, but unfortunately I cannot provide that right now. Feel free to close this issue without action if you find it insufficient, but hopefully it's still somewhat useful (and perhaps someone else with a similar issue can chime in).