authorization with cancan

After looking at the ability.rb. I have allowed the admins to manage
everything (that part works) but how do I allow the user to just, view
and edit their own Logg using cancan? At the moment the users cannot
view anything at all, not even their own created logg. But admins can do
everything.

    class Logg < ActiveRecord::Base
    has_and_belongs_to_many :user
    end

      class User < ActiveRecord::Base
        devise :database_authenticatable, :registerable,
         :recoverable, :rememberable, :trackable, :validatable
          ROLES = %w[admin moderator author banned]
       has_and_belongs_to_many :logg
   end

I have no User controller. I have the loggs controller:

    class LoggsController < ApplicationController

     before_action :set_logg, only: [:show, :edit, :update, :destroy]
     load_and_authorize_resource

      respond_to :html

     def index
     @loggs = Logg.all
    respond_with(@loggs)
     end

    def show
    respond_with(@logg)
    end

     def new
    @logg = Logg.new
    respond_with(@logg)
     end

    def edit

    end

     def create
    @logg = Logg.new(logg_params)
    @logg.save
    respond_with(@logg)
     end

     def update
    @logg.update(logg_params)
    respond_with(@logg)
    end

    def destroy
     @logg.destroy
    respond_with(@logg)
    end

     private
     def set_logg
       @logg = Logg.find(params[:id])
     end

     def logg_params
      params.require(:logg).permit(:name, :date, :time,
:whats_gone_well_this_week, :whats_not_gone_well_this_week,
:learnt_anything_new, :what_would_you_like_to_improve, :anything_else)
    end
end

    class Ability
    include CanCan::Ability
    def initialize(user)

       if user.nil?
      cannot :read, Logg
      elsif user.admin?
      can :manage, Logg
     else
      can :create, Logg, :user_id => user.id
      can :update, Logg, :user_id => user.id
      end
      end
    end