Unable to save Join Table details to database in Rails 4 - ruby-on-rails-4

Question

I've always had trouble with has_many :through relationships on join tables and am stuck again on where I'm going wrong. I think it might be in my controller as I'm still getting to grips with how it all works.
I have three models:
class Role < ActiveRecord::Base
has_many :assignments, inverse_of: :role
has_many :employees, :through => :assignments
accepts_nested_attributes_for :assignments
end
class Employee < ActiveRecord::Base
has_many :assignments, inverse_of: :employee
has_many :roles, :through => :assignments
accepts_nested_attributes_for :assignments
end
class Assignment < ActiveRecord::Base
belongs_to :employee, inverse_of: :assignment
belongs_to :role, inverse_of: :assignment
accepts_nested_attributes_for :employee
accepts_nested_attributes_for :role
end
I'd like to be able to set the pre-created role for an employee when creating or editing an employee. My New Employee form is as follows:
<%= semantic_form_for #employee do |f| %>
<%= render 'shared/error_messages' %>
<%= f.inputs do %>
<%= f.input :first_name %>
<%= f.input :last_name %>
<%= f.input :email %>
<%= f.input :password %>
<%= f.input :password_confirmation %>
<%= semantic_fields_for :roles do |role| %>
<%= role.input :role, :as => :select, :collection => Role.all %>
<%= role.semantic_fields_for :assignments do |assignment| %>
<%= assignment.input :start_date, :as => :date_select %>
<%= assignment.input :end_date, :as => :date_select %>
<%= assignment.input :assignment_no %>
<%= assignment.input :assignment_id %>
<% end %>
<% end %>
<% end %>
<%= f.actions do %>
<%= f.action :submit, :as => :button %>
<%= f.action :cancel, :as => :link %>
<% end %>
<% end %>
and finally my Employee Controller is:
class EmployeesController < ApplicationController
before_action :set_employee, only: [:show, :edit, :update, :destroy]
def index
#employees = Employee.paginate(page: params[:page])
end
def show
end
def new
#employee = Employee.new
role = #employee.roles.build
end
def edit
#employee = Employee.find(params[:id])
end
def create
#employee = Employee.new(employee_params)
if #employee.save
#employee.send_activation_email
flash[:info] = "Please check your email to activate your account."
redirect_to root_url
else
render 'new'
end
end
def update
respond_to do |format|
if #employee.update(employee_params)
flash[:success] = "Profile updated"
format.html { redirect_to #employee, notice: 'Employee was successfully updated.' }
format.json { render :show, status: :ok, location: #employee }
else
format.html { render :edit }
format.json { render json: #employee.errors, status: :unprocessable_entity }
end
end
end
def destroy
Employee.find(params[:id]).destroy
flash[:success] = "Employee deleted"
respond_to do |format|
format.html { redirect_to employees_url, notice: 'Employee was successfully destroyed.' }
format.json { head :no_content }
end
end
private
# Use callbacks to share common setup or constraints between actions.
def set_employee
#employee = Employee.find(params[:id])
end
# Never trust parameters from the scary internet, only allow the white list through.
def employee_params
params.require(:employee).permit(:avatar, :email, :first_name, :last_name, :password, :assignments_attributes => [:employee_id, :role_id, :roles_attributes => [:id, :employee_id, :PID]])
end
end
I appreciate that this subject is always on Stackoverflow, but I can't seem to translate any of the problems into where I am. Any help would be greatly appreciated!
The console output is as follows:
Started POST "/employees" for 127.0.0.1 at 2015-05-16 18:05:34 +0200
source=rack-timeout id=d5933405c94d9c2e8bca3332564528ec timeout=60000ms service=25ms state=active
Processing by EmployeesController#create as HTML
Parameters: {"utf8"=>"✓", "authenticity_token"=>"TgHHELkt2zXUtS474WwGeHcmfKKw/broHxRdhlsF1P4JyGoa+03rchb6mfxbOSXKdrPgcJMeBCyIlCMHqPlQBA==", "employee"=>{"first_name"=>"John", "last_name"=>"Smith", "email"=>"john#smith.com", "password"=>"[FILTERED]", "password_confirmation"=>"[FILTERED]"}, "roles"=>{"role"=>"2", "assignments"=>{"start_date(1i)"=>"2012", "start_date(2i)"=>"7", "start_date(3i)"=>"5", "end_date(1i)"=>"2016", "end_date(2i)"=>"3", "end_date(3i)"=>"1", "assignment_no"=>"4", "assignment_id"=>"A3392822"}}, "button"=>""}
Unpermitted parameter: password_confirmation
(0.2ms) BEGIN
source=rack-timeout id=d5933405c94d9c2e8bca3332564528ec timeout=60000ms service=1113ms state=active
SQL (70.3ms) INSERT INTO "employees" ("email", "first_name", "last_name", "password_digest", "created_at", "updated_at", "activation_digest") VALUES ($1, $2, $3, $4, $5, $6, $7) RETURNING "id" [["email", "john#smith.com"], ["first_name", "John"], ["last_name", "Smith"], ["password_digest", "$2a$10$g3PNppheVZ8AFnnKuWg6secBdGev0NlCXjUx.RXsky03Xl9L3CubO"], ["created_at", "2015-05-16 16:05:35.422312"], ["updated_at", "2015-05-16 16:05:35.422312"], ["activation_digest", "$2a$10$L0yFuvmlJon7fWod6lHj..O7yDRaqwqTlTkJgD7Evqx.dA4pDTlBC"]]
(133.0ms) COMMIT
(I'm working on a very slow computer!)