select_check_boxes with an has many through relation and :checked option - ruby-on-rails

Question

I am using collection_check_boxes to create object in a has_many through relation;
here some models:
#emotion.rb
class Emotion < ActiveRecord::Base
has_many :emotional_states
has_many :reports, :through => :emotional_states
end
#report.rb
class Report < ActiveRecord::Base
has_many :emotional_states
has_many :emotions, :through => :emotional_states
[... other "irrelevant" stuff here ...]
end
#emotional_states.rb
class EmotionalState < ActiveRecord::Base
belongs_to :report
belongs_to :emotion
end
As you may understand when I create a Report I also select with a collection_check_box a list of Emotions I want to bind to that report (through the model EmotionalState); Everything works on create (I retrieve the hash values and if #report.save I also create EmotionalStates with the #report.id and #emotion.id.)
But when it cames to edit the Report I would like to edit also the associated EmotionalStates (this means creating new EmotionalStates or deleting old one).
How can I populate the select_check_boxes with ALL the available Emotions having checked that emotions that are alredy associated through the EmotionalStates bojects?
If I write something like:
<%= collection_check_boxes(:report, :emotion_id, #report.emotional_states.map{|e| e.emotion}, :id, :name) %>
I'll get a unchecked checkbox for every alredy associated Emotion.
<%= collection_check_boxes(:report, :emotion_id, Emotion.all, :id, :name, :checked => #report.emotional_states.map{|e| e.emotion}) %>
While this code will correctly returns Emotion.all, but will not check the emotions alredy associated to #report through #report.emotional_states.
I've searched all around the wheb for examples on the usage of :checked options for collection_select_boxes without any results...
any hint?

Answer 1

I did the same once in this way.you can also try :
Emotions:
<% Emotion.all.each do |emotion| %>
<%= check_box_tag 'report[emotion_ids][]' , emotion.id, #report.emotion.include?(emotion) %><%= emotion.name %><br/>
<% end %>
In Controller add :emotion_ids=>[] into strong parameters.
And one line into controller update method:
params[:report][:emotion_ids] ||= []

Answer 2

After coming back to this bug I discovered that the problem was an incorrect use of the .map method, mapping a whole object (e.emotion) instead its id (e.emotion.id).
This easily fixed my problem:
<%= collection_check_boxes(:report, :emotion_id, Emotion.all, :id, :name, :checked => #report.emotional_states.map{|e| e.emotion.id}) %>
Thank you for your help!