How do I update a model with has_many :through association in grouped_collection_select

I have the following models:

class User < ApplicationRecord
  self.table_name = 'user.users'
  has_many :user_sources
  has_many :sources, through: :user_sources, foreign_key: :source_code
end

class User::Source < ApplicationRecord
    self.table_name = 'user.sources'
        self.primary_key = 'code'
        has_many :user_sources, foreign_key: :source_code
        has_many :users, through:  :user_sources
end

class User::UserSource < ApplicationRecord
    self.table_name = 'user.user_sources'
    belongs_to :user
    belongs_to :source, foreign_key: :source_code
    belongs_to :category,   class_name: 'User::SourceCategory',   foreign_key:  :category_code, inverse_of: :sources
    validates :user, :source, presence: true
end

class User::SourceCategory < ApplicationRecord
  self.table_name = 'user.source_categories'
  self.primary_key = 'code'
  has_many :sources,   foreign_key: :category_code,  inverse_of: :category
  validates :code, :name, presence: true, uniqueness: true
end

I have the following form (using slim html):

            form_for(user, multipart: true) do |f|
                    ...
                    ....
                  .form-group
                            = f.label :sources, "Source"
                            = f.grouped_collection_select(:sources,  User::SourceCategory.order(name: :asc), :sources, :name, :code, :name, \
                                    {prompt: true}, {class: 'form-control', required: true})    

In controller#new, i also did something like this:

    @user.sources.build
    @user.user_sources.build

I am not sure if I have set the grouped_collection_select correctly, but the @user.sources didn't have any value upon create. And yes I did permit the parameters as well. I am not sure how to configure the form so that upon submit the form, the @user.sources will be updated correctly based on the grouped_collection_select input selected. Not sure if i am missing something. Appreciate if anyone could guide/advise me on the right way to implement this.