Rails fields_for checkbox to create new record

You don't need nested attributes to simply assign associations to a record.

To start with you want to alter your tables and associations to create a normalization table:

class User < ApplicationRecord
  has_many :user_roles, dependent: :destroy
  has_many :roles, through: :user_roles
end

# remove user_id from the roles table
class Role < ApplicationRecord
  validates_uniqueness_of :name
  has_many :user_roles, dependent: :destroy
  has_many :users, through: :user_roles
end

# rails g model user_role user:references role:references
class UserRole < ApplicationRecord
  validates_uniqueness_of :role_id, scope: :user_id
  belongs_to :user
  belongs_to :role
end

This means the definition name of a role is only defined once in the roles table instead of being duplicated for every role applied to a user. And to assign the role to a user you're just adding a role id and user id in a join table which can be indexed much more efficiently.

Since we don't need to set that duplicate name column for every user role we can just use role_ids= to set the roles of a user with a checkbox.

<%= form_for(@user) do |f| %>
  <%= f.collection_check_boxes(:role_ids, Role.all, :id, :name) %>
  ...
<% end %>

class UsersController < ApplicationController

  # ...

  def create
    @user = User.new(user_params)
    # ...
  end

  # ...

  private
    def user_params
      params.require(:user)
            .permit(:foo, :bar, role_ids: [])
    end
end

https://guides.rubyonrails.org/form_helpers.html#nested-forms

The example given there is :

<%= form_for @person do |f| %>
  Addresses:
  <ul>
    <%= f.fields_for :addresses do |addresses_form| %>
      <li>
        <%= addresses_form.label :kind %>
        <%= addresses_form.text_field :kind %>

        <%= addresses_form.label :street %>
        <%= addresses_form.text_field :street %>
        ...
      </li>
    <% end %>
  </ul>
<% end %>

Adapting that to have I'm assuming you've setup your models:

<%= form_for @user do |f| %>
  <%= f.input :first_name %>
  <%= f.input :last_name %>
  Role:
  <ul>
    <%= f.fields_for :roles do |role_f| %>
      <li>
        <%= role_f.check_box :it %>
        <%= role_f.check_box :accounting %>
        <%= role_f.check_box :sales%>
      </li>
    <% end %>
  </ul>
<% end %>

From there you can see how the params come through and create the roles as needed