My form doesn't save my category_id foreign key

Question

I'm new to rails and I have a form that will save the supplier unit with a select input of associated category in categories model. but when I try to hit save all the fields where save but the category_id didn't. At first the error was unpermitted parameters but when I try put the category_id attribute in strong params the error was remove but unfortunately it didn't save agian.

I already searched anything here in stackoverflow and google but no solution yet i try solves it.

supplierunit mode

belongs_to :category
belongs_to :supplierprofile

Category model

has_many :supplierunits

Supplierprofile mode

belongs_to :supplier
has_many   :supplierunits

_form.html.erb

<%= simple_form_for([@supplier, @unit]) do |f| %>
  <%= f.association :category, collection: Category.all.collect{ |p| [p.category_name, p.id] } %>
  <%= f.input       :unit_name %>
  <%= f.input       :unit_address      %>
  <%= f.text_area   :description  %>
  <%= f.button :submit %>
<% end %>

Supplierunit controller

    before_action :set_unit , only: [:new, :create]

    def new
        @unit = @supplier.supplierunits.new
    end

   def create
        @unit = @supplier.supplierunits.new(unit_params)

        if @unit.save
            flash[:notice] = "Successfully Added!"
        else
            render 'new'
        end
   end

    private

    def set_unit
        @supplier = Supplierprofile.find_by_supplier_id(session[:user_id])
    end

    def unit_params
        params.require(:supplierunit).permit(:unit_name, :unit_address, :description, category_id: :category_id)
    end
end

Thank you!