Rails many-to-many creation fails - ruby-on-rails

Question

I have event and band models, which have a many-to-many relationship through event_bands. I am trying to change my create method to use chosen jQuery like in episode 258 of railscasts. I am not sure how to read the message from my localhost when I try to create an event:
Started POST "/events" for 127.0.0.1 at 2014-03-16 17:11:07 +0900
Processing by EventsController#create as HTML
Parameters: {"utf8"=>"✓", "authenticity_token"=>"Jx1Cm09uCwJcnY8573ZTRKMjH1BHWhlREFCfhij/AB0=", "event"=>{"name"=>"pojpj", "ko_name"=>"", "band_ids"=>["", "110"], "venue_id"=>"", "date(1i)"=>"2014", "date(2i)"=>"3", "date(3i)"=>"16", "time"=>"", "contact"=>"", "facebook"=>"", "ticket"=>"true", "price"=>"", "door_price"=>"", "ticket_url"=>"", "info"=>"", "info_ko"=>""}, "commit"=>"등록", "locale"=>"ko"}
Band Load (0.2ms) SELECT "bands".* FROM "bands" WHERE "bands"."id" = ? LIMIT 1 [["id", 110]]
(0.1ms) begin transaction
Band Exists (1.0ms) SELECT 1 AS one FROM "bands" WHERE ("bands"."name" = '...Whatever That Means' AND "bands"."id" != 110) LIMIT 1
(0.2ms) rollback transaction
Redirected to http://localhost:3000/events/new
It looks like it fails because the Band already exists in the database, but why is it doing that instead of creating the relation?
def new
#event = Event.new
end
def create
#event = Event.new(event_params)
if #event.save
flash[:notice] = "Event Created!"
redirect_to event_path(#event)
else
flash[:notice] = "Event not created!"
redirect_to new_event_path
end
end
private
def event_params
params.require(:event).permit(:name,
:ko_name,
:avatar,
:time,
:facebook,
:ticket,
:door_price,
:ticket_url,
:info_kr,
:contact,
:price,
:info,
:info_ko,
:venue_id,
:date,
band_ids: [])
end
Relevant part of the form:
<p>
<%= f.label "Bands" %><br />
<%= f.collection_select :band_ids, Band.order(:name), :id, :name, {}, {multiple: true} %>
</p>
I have accepts_nested_attributes_for :bands in the model and I think the relation is setup correctly because I can do a = Event.new(name: 'asdf', band_ids: [1,5]) a.save and it is persisted.
update: it seems the problem is coming from the empty item in band_ids. For some reason, rails is setting the param as band_ids: ['',3,5]. I can replicate the SQL message from my server by trying to create a new entry in the console like this: a = Event.create(name: 'asdfasdfasdfasdf2345', band_ids: ['', 3, 2]). But where is the empty first item coming from?
edit2: Disregard the above, it seems the problem is actually that there is no event_id to use in the association?
#messages={:"event_bands.event_id"=>["에 내용을 입력해 주세요"]}