Can't get 'has_and_belongs_to_many' to work - ruby-on-rails

Question

Made a previous post about this but after a lot of testing and changing it's outdated.
I'm trying to tie users and activities together.(One user can have several activities. An activity can have several users). I'ts pretty much a multi-user agenda thing.
Whenever i create a new activity i can select users by checkbox which will be doing the activity.
I can't get it to work though; The users won't show up in my activity show. Printing #activity.users in my show results in #<ActiveRecord::Associations::CollectionProxy::ActiveRecord_Associations_CollectionProxy_User:0x43d2158> but looping it, or checking my activities.js shows nothing(The activities.js shows "users":[] in the activity. So I'm pretty certain they're not associated properly.
Here's some code:
The activities.js index method
def index
#activities = Activity.all
respond_to do |format|
format.html
format.js {render_json #activities.to_json(:include => [:pictogram ,:users]) }
end
end
The activities' form(Loops all users)
<%= form_for(#activity) do |f| %>
<% if #activity.errors.any? %>
<div id="error_explanation">
<h2><%= pluralize(#activity.errors.count, "error") %> prohibited this activity from being saved:</h2>
<ul>
<% #activity.errors.full_messages.each do |msg| %>
<li><%= msg %></li>
<% end %>
</ul>
</div>
<% end %>
<div class="field">
<%= f.label :title %><br>
<%= f.text_field :title %>
</div>
<div class="field">
<%= f.label :start_date %><br>
<%= f.date_select :start_date %>
</div>
<div class="field">
<%= f.label :end_date %><br>
<%= f.date_select :end_date %>
</div>
<div class="users">
<% for user in User.all %>
<label class="activity">
<%= check_box_tag "activity[user_ids][]", user.id %>
<%= user.name %>
</label>
<% end %>
</div>
<div class="pictograms">
<% for p in Pictogram.all %>
<% #f.fields_for :pictograms do |x| %>
<%= p %>
<% #end %>
<% end %>
</div>
<div class="actions">
<%= f.submit %>
</div>
<% end %>
The schema.rb
ActiveRecord::Schema.define(version: 20130911095113) do
# These are extensions that must be enabled in order to support this database
enable_extension "plpgsql"
create_table "activities", force: true do |t|
t.string "title"
t.date "start_date"
t.date "end_date"
t.integer "pictogram_id"
t.datetime "created_at"
t.datetime "updated_at"
end
add_index "activities", ["pictogram_id"], name: "index_activities_on_pictogram_id", using: :btree
create_table "activities_users", id: false, force: true do |t|
t.integer "activity_id"
t.integer "user_id"
end
add_index "activities_users", ["activity_id"], name: "index_activities_users_on_activity_id", using: :btree
add_index "activities_users", ["user_id"], name: "index_activities_users_on_user_id", using: :btree
create_table "pictograms", force: true do |t|
t.string "title"
t.string "url"
t.datetime "created_at"
t.datetime "updated_at"
end
create_table "users", force: true do |t|
t.string "name"
t.text "avatar"
t.date "birthdate"
t.datetime "created_at"
t.datetime "updated_at"
end
end
Activity.rb
class Activity < ActiveRecord::Base
belongs_to :pictogram
has_and_belongs_to_many :users
accepts_nested_attributes_for :pictogram, allow_destroy: false
accepts_nested_attributes_for :users, allow_destroy: false
end
User.rb
class User < ActiveRecord::Base
has_and_belongs_to_many :activities
validates_presence_of :name, :on => :create
validates_presence_of :birthdate, :on => :create
accepts_nested_attributes_for :activities, allow_destroy: false
end
And the activity params from my activities controller
def activity_params
params.require(:activity).permit(:title,
:start_date,
:end_date,
:users,
:pictogram)
end

Answer 1

Try sth like this:
class Activity < ActiveRecord::Base
belongs_to :pictogram
has_and_belongs_to_many :users
accepts_nested_attributes_for :pictogram, allow_destroy: false
def user_ids=(values)
self.users << User.find(values)
end
end
#controller
def activity_params
params.require(:activity).permit(:title,
:start_date,
:end_date,
{:user_ids =>[]},
:pictogram)
end

Answer 2

I suggest you try some minimalist debugging first,
User.create!(...) # create a valid user record
Activity.create!(...) # same as above
User.all.first.activities << Activity.all.first
User.all.first.activities.any?
See if this works, also keep an eye on your join_table and check that actual records are being persisted. Your schema looks good as well.

Answer 3

I think, none of them read about the strong_parameters with nested attributes :
You can easily define :users_attributes in permit parameters for nested_attributes.
activities controller :
def activity_params
params.require(:activity).permit(:title,
:start_date,
:end_date,
users_attributes: [:id])
end
This is the ideal way to do this. Thanks