Validation failed Class must exist

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!

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.

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!

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.