RAILS - User has more than one category - ruby-on-rails

Question

I have a rails app that I'm trying to make an association with USERS (there are many) and Categories (there are 3, which I seeded). Each category has a name and an ID...
When I try to get users to edit their category, the form comes up blank.
The code I'm using for users to select their category is as follows :
<%= form_for (#user) do |f| %>
<%= f.select :categories, #user.categories.all, {multiple: true} %>
(i have also tried Category.all.collect)
Here is what my SCHEMA looks like
create_table "categories", force: :cascade do |t|
t.string "catname"
t.datetime "created_at", null: false
t.datetime "updated_at", null: false
t.integer "user_id"
t.index ["user_id"], name: "index_categories_on_user_id"
end
create_table "specialties", force: :cascade do |t|
t.string "specname"
t.text "description"
t.datetime "created_at", null: false
t.datetime "updated_at", null: false
t.integer "category_id"
end
create_table "users", force: :cascade do |t|
t.string "name"
t.string "email"
t.integer "category_id"
and here are my Models,
class User < ApplicationRecord
has_many :categories
class Category < ApplicationRecord
validates :catname, presence: true
has_many :specialties
belongs_to :user

Answer 1

You can try something like that.
<%= hidden_field_tag "user[category_ids][]", nil %>
<% Category.all.each do |category| %>
<%= check_box_tag "user[category_ids][]", category.id, #user.category_ids.include?(category.id), id: dom_id(category) %>
<%= label_tag dom_id(category), category.catname %> <br>
<% end %>
or
create a field categories in user, then:
<%= form.select :categories, Category.all.collect {|x| [x.name, x.id]}, {}, :multiple => true%>