Saving an array of ids into a foreign key field - Rails 4

I'm having an issue with my ROR 4 application. Here's a quick background:

The application has several classes, Users, Training_Events and Mission_Notes.

A Training event can be associated with multiple users from a multi-select drop-down which builds an array of user_ids which are then saved to the Training_Event, whilst Mission_Notes can only be associated with one User and one Training_Event. Models below:

MissionNote.rb

 class MissionNote < ActiveRecord::Base
    belongs_to :training_event
    belongs_to :user
 end

User.rb

class User < ActiveRecord::Base
attr_accessor :remember_token, :activation_token, :reset_token
before_save   :downcase_email
before_create :create_activation_digest

belongs_to :group
has_many :ranks
has_many :mission_notes
has_and_belongs_to_many :training_events
validates :username, presence: true
validates :email, presence: true
VALID_EMAIL_REGEX = /\A[\w+\-.]+@[a-z\d\-.]+\.[a-z]+\z/i
validates :email, presence: true, length: { maximum: 255 },
                format: { with: VALID_EMAIL_REGEX },
                uniqueness: { case_sensitive: false }
validates :group_id, presence: true
has_secure_password
validates :password, length: { minimum: 6 }, allow_blank: true
end

TrainingEvent.rb

class TrainingEvent < ActiveRecord::Base
has_and_belongs_to_many :users
has_many :mission_notes



validates :title, presence: true
validates :date, presence: true
validates :mission, presence: true
validates_format_of :video, :with => /\A(https\:\/\/)?((www\.)?youtube\.com|youtu\.?be)\/.+$\Z/, :allow_blank => true, :message => "must be a valid YouTube URL"
validates_format_of :date, :with => /\A((19|20)\d\d+)-(0[1-9]|1[012]+)-(0[1-9]|[12][0-9]|3[01])\Z/

end

What I then want it to do is on the user's profile display a list of the events that the particular user has been associated with and the mission_notes for each event. The issue I have is when I save the training event the user_id field is not saved in the database however if I do TrainingEvent.all.each{|x| x.user_ids} then I get an array of the user_ids which were saved.

Can someone help point out what I am doing wrong here and maybe help clarify while the single user_id finds nothing but user_ids returns at least an array of items.

------------------- Edit ------------------------------------ Training_Events_Controller.rb

class TrainingEventsController < ApplicationController
  before_action :set_training_event, only: [:show, :edit, :update, :destroy]
  before_action :admin_user, only: [:new, :edit, :update]

  # GET /training_events
  # GET /training_events.json
  def index
    @training_events = TrainingEvent.all
  end

  # GET /training_events/1
  # GET /training_events/1.json
  def show
    @user_ids = @training_event.user_ids
    @user = User.find(@user_ids)
    @mission_notes = MissionNote.find_by(user_id: @user)
    byebug
  end

  # GET /training_events/new
  def new
    @training_event = TrainingEvent.new
    @user_options = User.all.map{|u| [ u.username, u.id ] }
  end

  # GET /training_events/1/edit
  def edit
     @user_options = User.all.map{|u| [ u.username, u.id ] }
  end

  # POST /training_events
  # POST /training_events.json
  def create
      @training_event = TrainingEvent.new(training_event_params)

      respond_to do |format|
        if @training_event.save
          format.html { redirect_to @training_event, notice: 'Training event was successfully created.' }
          format.json { render :show, status: :created, location: @training_event }
        else
          format.html { render :new }
          format.json { render json: @training_event.errors, status: :unprocessable_entity }
        end
      end
    end

# PATCH/PUT /training_events/1
# PATCH/PUT /training_events/1.json
 def update
 respond_to do |format|
   if @training_event.update(training_event_params)
     format.html { redirect_to @training_event, notice: 'Training event was successfully updated.' }
    format.json { render :show, status: :ok, location: @training_event }
  else
    format.html { render :edit }
    format.json { render json: @training_event.errors, status: :unprocessable_entity }
  end
end
end

# DELETE /training_events/1
# DELETE /training_events/1.json
def destroy
  @training_event.destroy
  respond_to do |format|
    format.html { redirect_to training_events_url, notice: 'Training event was successfully destroyed.' }
    format.json { head :no_content }
  end
end

private
  # Use callbacks to share common setup or constraints between actions.
  def set_training_event
    @training_event = TrainingEvent.find(params[:id])
  end

# Never trust parameters from the scary internet, only allow the white list through.
def training_event_params
  params.require(:training_event).permit(:title, :date, :training_objective, :mission, :video, :user_ids => [])

end
end

Also as an additional can someone suggest the best way to then put this on the view in the way I described above, I realise this part is a little vague but it has sent me round the bend as I seem to get a repeating list of events and notes. Will attach view code when I have second