Failing validations in join model when using has_many :through

Question

My full code can be seen at https://github.com/andyw8/simpleform_examples

I have a join model ProductCategory with the following validations:

Answer 1

Specifying inverse_of on your joining models has been documented to fix this issue:

https://github.com/rails/rails/issues/6161#issuecomment-6330795 https://github.com/rails/rails/pull/7661#issuecomment-8614206

Simplified Example:

class Product < ActiveRecord::Base
  has_many :product_categories, :inverse_of => :product
  has_many :categories, through: :product_categories
end

class Category < ActiveRecord::Base
  has_many :product_categories, inverse_of: :category
  has_many :products, through: :product_categories
end

class ProductCategory < ActiveRecord::Base
  belongs_to :product
  belongs_to :category

  validates :product, presence: true
  validates :category, presence: true
end

Product.new(:categories => [Category.new]).valid? # complains that the ProductCategory is invalid without inverse_of specified

Adapted from: https://github.com/rails/rails/issues/8269#issuecomment-12032536