rails 4 semantic ui multi-select show user selections on edit - ruby-on-rails-4

Question

I am trying to implement multi-select in a form in rails 4, using Semantic UI. I want users to be able to select multiple categories for each post. So far, I am able to display the dropdown select field which pulls all the categories from the database as follows:
<%= form_for #post, :html => {:class => "ui form bk-form group"} do |f| %>
<label>Post title:</label><br />
<%= f.text_field :title, autofocus: true %>
<label>Choose a category:</label><br />
<%= f.select(:category_ids,options_for_select(Category.all.collect{|x| [x.name,x.id,class:'item']}),{prompt:'Select categories'}, multiple: true, class:'ui fluid dropdown selection multiple')%>
<% end %>
With this, I am able to create and save posts and the data is inserted in the database. However, when I try to edit an article, the pre-selected categories do not show. I have tried to set the value: #post.categories option in the select field and still cannot get the existing categories to show. Thanks in advance for your thoughts.
UPDATED
Models are as follows:
class Post < ActiveRecord::Base
has_many :post_categories
has_many :categories, through: :post_categories
end
class Category < ActiveRecord::Base
has_many :post_categories
has_many :posts, through: :post_categories
end
class PostCategory < ActiveRecord::Base
belongs_to :post
belongs_to :category, :counter_cache => :posts_count
end
Then my posts_controller.rb
class PostsController < ApplicationController
before_action :set_post, only: [:edit, :update, :show]
def index
#posts = Post.all
end
def new
end
def create
#post = Post.new(post_params)
#post.user = current_user
if #post.save
flash[:notice] = "Post was successfully created"
redirect_to user_posts_path
else
flash[:alert] = "Oh Snap!! We could not save your post"
render 'new'
end
end
def edit
end
def update
if #post.update(post_params)
flash[:notice] = "Post was successfully updated"
redirect_to user_posts_path
else
flash[:alert] = "Oh Snap!! We could not update your post"
render 'edit'
end
end
private
def post_params
params.require(:post).permit(:title, :description, :published, :tag_list, category_ids: [])
end
def set_post
#post = Post.find(params[:id])
end
end