使用has_many：through关系为现有子级创建新的父级记录

Question

I am working on a Ruby on Rails API (version 4.0) to create and update invoices. The relationship between invoices and products is a has_many trough: relationship. Imagine I have product 1, 2, & 3. I am having trouble creating a new invoice that contains product 1 & 3. . When I run the code below I get the error:

Unknown primary key for table invoices_products in model InvoicesProduct.

This error doesn't really make sense to me since InvoicesProduct is a join table and shouldn't require a primary key.

One tricky part about the design is that it needs to track which employee added which products to the invoice, which is why invoices_product has employee_id . It does not seem to be the cause of the problem at the moment. Here is the DB design of the tables in questions:

InvoicesController

This is the code I currently have in the controller. The error message occurs on the first line of create:

def create
  invoice = Invoice.new(create_invoice_params)
  invoice.created_by = @current_user
  # eventually need to set employee_id on each invoice_product,
  # but just need to get it working first
  # invoice.invoices_products.map!{|link| link.employee = @current_user }
  invoice.save
  respond_with invoice
end

def create_invoice_params
  params.fetch(:invoice).permit(:customer_id, :status_code, :payment_method_code, product_ids: [])
end

Invoice

# /app/models/invoice.rb
class Invoice < ActiveRecord::Base
  validates_presence_of :customer
  validates_presence_of :created_by

  belongs_to :customer, inverse_of: :invoices
  belongs_to :created_by, inverse_of: :invoices, class_name: 'Employee'
  has_many :invoices_products, inverse_of: :invoice
  has_many :products, through: :invoices_products
end

InvoicesProduct

class InvoicesProduct < ActiveRecord::Base
  validates_presence_of :invoice
  validates_presence_of :product
  validates_presence_of :employee

  belongs_to :product, inverse_of: :invoices_products
  belongs_to :invoice, inverse_of: :invoices_products
  belongs_to :employee, inverse_of: :invoices_products
end

Product

# /app/models/product.rb
class Product < ActiveRecord::Base
  validates :name, presence: true, length: { maximum: 100 }

  has_many :invoices_products, inverse_of: :product
  has_many :invoices, through: :invoices_products
end

Request

This is what I've got in mind for a working request, the solution doesn't need to match this, but its what I've been trying

{
  "invoice": {
    "customer_id": "1",
    "product_ids": ["1", "5", "8"]
  }
}

Answer 1

I was able to fix the relationship by adding a primary key to the invoices_products . For some reason I was under the impression that join tables did not require a primary key for has_many :through relationships. However, looking at the example on the Rails guide site , the example join table does have a primary key.

Answer 2

That is because you are using has_many :through. If you don't want id (or any other additional field) in the join table, use has_many_and_belongs_to instead