Validation failed Class must exist

I have been (hours) trouble with associations in Rails. I found a lot of similar problems, but I couldn't apply for my case:

City's class:

class City < ApplicationRecord
  has_many :users
end

User's class:

class User < ApplicationRecord
  belongs_to :city

  validates :name, presence: true, length: { maximum: 80 }
  validates :city_id, presence: true
end

Users Controller:

def create
    Rails.logger.debug user_params.inspect
    @user = User.new(user_params)
    if @user.save!
      flash[:success] = "Works!"
      redirect_to '/index'
    else
      render 'new'
    end
 end

def user_params
  params.require(:user).permit(:name, :citys_id)
end

Users View:

<%= form_for(:user, url: '/user/new') do |f| %>
  <%= render 'shared/error_messages' %>

  <%= f.label :name %>
  <%= f.text_field :name %>

  <%= f.label :citys_id, "City" %>
  <select name="city">
    <% @city.all.each do |t| %>
      <option value="<%= t.id %>"><%= t.city %></option>
    <% end %>
  </select>
end

Migrate:

class CreateUser < ActiveRecord::Migration[5.0]
  def change
    create_table :user do |t|
      t.string :name, limit: 80, null: false
      t.belongs_to :citys, null: false
      t.timestamps
  end
end

Message from console and browser:

ActiveRecord::RecordInvalid (Validation failed: City must exist):

Well, the problem is, the attributes from User's model that aren't FK they are accept by User.save method, and the FK attributes like citys_id are not. Then it gives me error message in browser saying that "Validation failed City must exist".

Thanks

Try the following:

belongs_to :city, optional: true

According to the new docs:

4.1.2.11 :optional

If you set the :optional option to true, then the presence of the associated object won't be validated. By default, this option is set to false.