Devise and Simple_form error in association - ruby-on-rails

Question

I'm having trouble creating Sign up to my rails app, I'm using Devise and Simple_form, I have 2 models (User and department), users belongs_to :department and department has_many :users, i get an error when i try to sign up saying that department must exits.
devise/registrations/new.html.erb
<h2>Sign up</h2>
<%= simple_form_for(resource, as: resource_name, url: registration_path(resource_name)) do |f| %>
<%= f.error_notification %>
<div class="form-inputs">
<%= f.input :username, required: true, autofocus: true %>
<%= f.input :email, required: true %>
<%= f.input :password, required: true, hint: ("#{#minimum_password_length} characters minimum" if #minimum_password_length) %>
<%= f.input :password_confirmation, required: true %><br>
<%= f.association :department %>
</div>
<div class="form-actions">
<%= f.button :submit, "Sign up" %>
</div>
<% end %>
<%= render "devise/shared/links" %>
user.rb :
class User < ApplicationRecord
# Include default devise modules. Others available are:
# :confirmable, :lockable, :timeoutable and :omniauthable
devise :database_authenticatable, :registerable,
:recoverable, :rememberable, :trackable, :validatable
validates_uniqueness_of :email, :username
belongs_to :department
has_and_belongs_to_many :courses
end
department.rb :
class Department < ApplicationRecord
has_many :users
has_many :courses
end
I populated the departments table using seeds.rb and checked through mysql console.
schema.rb :
ActiveRecord::Schema.define(version: 20180502071349) do
create_table "courses", force: :cascade, options: "ENGINE=InnoDB DEFAULT CHARSET=latin1" do |t|
t.string "name"
t.text "description"
t.bigint "department_id"
t.string "instructor_name"
t.integer "credit_hours"
t.datetime "created_at", null: false
t.datetime "updated_at", null: false
t.index ["department_id"], name: "index_courses_on_department_id"
end
create_table "departments", force: :cascade, options: "ENGINE=InnoDB DEFAULT CHARSET=latin1" do |t|
t.string "name"
t.text "description"
t.datetime "created_at", null: false
t.datetime "updated_at", null: false
end
create_table "enrollments", force: :cascade, options: "ENGINE=InnoDB DEFAULT CHARSET=latin1" do |t|
t.bigint "user_id"
t.bigint "courses_id"
t.datetime "created_at", null: false
t.datetime "updated_at", null: false
t.index ["courses_id"], name: "index_enrollments_on_courses_id"
t.index ["user_id"], name: "index_enrollments_on_user_id"
end
create_table "users", force: :cascade, options: "ENGINE=InnoDB DEFAULT CHARSET=latin1" do |t|
t.string "email", default: "", null: false
t.string "encrypted_password", default: "", null: false
t.string "username", default: "", null: false
t.bigint "department_id"
t.string "reset_password_token"
t.datetime "reset_password_sent_at"
t.datetime "remember_created_at"
t.integer "sign_in_count", default: 0, null: false
t.datetime "current_sign_in_at"
t.datetime "last_sign_in_at"
t.string "current_sign_in_ip"
t.string "last_sign_in_ip"
t.datetime "created_at", null: false
t.datetime "updated_at", null: false
t.index ["department_id"], name: "index_users_on_department_id"
t.index ["email"], name: "index_users_on_email", unique: true
t.index ["reset_password_token"], name: "index_users_on_reset_password_token", unique: true
t.index ["username"], name: "index_users_on_username", unique: true
end
add_foreign_key "courses", "departments"
add_foreign_key "enrollments", "courses", column: "courses_id"
add_foreign_key "enrollments", "users"
add_foreign_key "users", "departments"
end
migration files:
class CreateDepartments < ActiveRecord::Migration[5.1]
def change
create_table :departments do |t|
t.string :name
t.text :description
t.timestamps
end
end
end
# frozen_string_literal: true
class DeviseCreateUsers < ActiveRecord::Migration[5.1]
def change
create_table :users do |t|
## Database authenticatable
t.string :email, null: false, default: ""
t.string :encrypted_password, null: false, default: ""
t.string :username, null: false, default: ""
t.references :department, foreign_key: true
## Recoverable
t.string :reset_password_token
t.datetime :reset_password_sent_at
## Rememberable
t.datetime :remember_created_at
## Trackable
t.integer :sign_in_count, default: 0, null: false
t.datetime :current_sign_in_at
t.datetime :last_sign_in_at
t.string :current_sign_in_ip
t.string :last_sign_in_ip
## Confirmable
# t.string :confirmation_token
# t.datetime :confirmed_at
# t.datetime :confirmation_sent_at
# t.string :unconfirmed_email # Only if using reconfirmable
## Lockable
# t.integer :failed_attempts, default: 0, null: false # Only if lock strategy is :failed_attempts
# t.string :unlock_token # Only if unlock strategy is :email or :both
# t.datetime :locked_at
t.timestamps null: false
end
add_index :users, :email, unique: true
add_index :users, :reset_password_token, unique: true
add_index :users, :username, unique: true
# add_index :users, :confirmation_token, unique: true
# add_index :users, :unlock_token, unique: true
end
end
class CreateCourses < ActiveRecord::Migration[5.1]
def change
create_table :courses do |t|
t.string :name
t.text :description
t.references :department, foreign_key: true
t.string :instructor_name
t.integer :credit_hours
t.timestamps
end
create_table :enrollments do |t|
t.references :user, foreign_key: true
t.references :courses, foreign_key: true
t.timestamps
end
end
end
P.S., I'm just starting out with rails and thanks for your help.
Error screenshot:

Answer 1

Devise doesn't know anything about your none standard department_id field and filters it as unpermitted parameter.
Create your own registrations controller (which extends Devise) and then customize these methods:
def sign_up_params
params.require(:user).permit(:first_name, :last_name, :email, :password, :password_confirmation, :department_id)
end
This is just a sample. Feel it in with your real field names

Answer 2

Rails since 5.1 or so has a required belongs_to validation with newly generated apps.
You can disable that by:
class User < ApplicationRecord
belongs_to :department, optional: true
end
In this way, you can create users with an empty department first.