Validation failed Class must exist

问题

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

回答1:

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.

回答2:

This comes a bit late but this is how to turn off this by default in rails 5:

config/initializers/new_framework_defaults.rb

Rails.application.config.active_record.belongs_to_required_by_default = false

In case you don't want to add optional: true to all your belongs_to.

I hope this helps!

回答3:

You need to add the following to the end of the belongs_to relationship statement:

optional: true

It is possible to set this on a global level so that it works in the same way as older versions of rails, but I would recommend taking the time to manually add it to the relationships that really need it as this will cause less pain in the future.

回答4:

I found out a solution to the problem "Validation failed: Class must exist" and it's better than use:

belongs_to :city, optional: true

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.

cause you still make a validation in application level. I solve the problem making my own validation in create method and changing user_params method:

def create

  @city = City.find(params[:city_id])

  Rails.logger.debug user_params.inspect
  @user = User.new(user_params)

  @user.city_id = @city.id

  if @user.save!
    flash[:success] = "Works!"
    redirect_to '/index'
  else
    render 'new'
  end
end

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

I didn't test this code, but it works in another project mine. I hope it can help others!

回答5:

Rails 5

If you have a belongs_to relationship to :parent then you have to pass an existing parent object or create a new one then assign to children object.

回答6:

belongs_to :city, required: false

来源：https://stackoverflow.com/questions/38983666/validation-failed-class-must-exist