Foreign key not saving rails 5 - ruby-on-rails

Question

I am trying to save a product which belongs_to category with foreign_key :category_id. The failed product.save is show below:
Processing by ProductsController#create as HTML
Parameters: {"utf8"=>"✓", "authenticity_token"=>"2V3zVrgU3SoGAwyHDLYQuyDFWq9rI7U4GdDeZNDRwrLTwsGloio4MIXUOdU/ckvnTsMGF9B9TL4tuNWKSqZpVg==", "product"=>{"name"=>"trek"}, "commit"=>"Save Product", "category_id"=>"1"}
Everytime I try to save the product it says "category must exist". I understand I have my models and migrations a little off, but can't figure out how. Below are the files.
product.rb:
class Product < ApplicationRecord
belongs_to :category
end
category.rb: empty
create_categories.rb:
class CreateCategories < ActiveRecord::Migration[5.2]
def change
create_table :categories do |t|
t.string :name
t.timestamps
end
end
end
create_products.rb:
class CreateProducts < ActiveRecord::Migration[5.2]
def change
create_table :products do |t|
t.string :name
t.belongs_to :category, index: {unique: true}, foreign_key: true, null: $
t.timestamps
end
end
end
I can also post my product_controller.rb however it is pretty standard with just #product.save and a redirect_to products_path or 'products#index'.
Thank you.
EDIT: products_controller.rb:
def create
#product = Product.new(product_params)
respond_to do |format|
if #product.save
format.html { redirect_to category_products_path([#category, #product])$
format.json { render :show, status: :created, location: #product }
else
format.html { render :new }
format.json { render json: #product.errors, status: :unprocessable_enti$
end
end
end
def product_params
params.require(:product).permit( :name, :category_id)
end

Answer 1

It seems like you want to do something like:
class ProductsController < ApplicationController
def create
#category = Category.find(params[:category_id])
#product = #category.products.new(product_params)
if #product.save
format.html { redirect_to category_products_path([#category, #product]) }
format.json { render :show, status: :created, location: #product }
else
format.html { render :new }
format.json { render json: #product.errors, status: :unprocessable_entity }
end
end
private
def product_params
params.require(:product).permit(:name, :category_id)
end
end
This assumes, naturally, that you have something like:
class Category < ApplicationRecord
has_many :products
end
class Product < ApplicationRecord
belongs_to :category
end
As you currently have it written, there are at least a couple of problems.
First, you say:
def product_params
params.require(:product).permit( :name, :category_id)
end
But, params[:product] doesn't have category_id.
Second, you say:
format.html { redirect_to category_products_path([#category, #product])}
But, you don't define #category.

Answer 2

You should find the category from the category id, then under that category create product.
class ProductsController < ApplicationController
def create
#category = Category.find(params[:category_id])
#product = #category.products.new(product_params)
if #product.save
format.html { redirect_to category_products_path([#category, #product]) }
format.json { render :show, status: :created, location: #product }
else
format.html { render :new }
format.json { render json: #product.errors, status: :unprocessable_entity}
end
end
private
def product_params
# as your category_id is outside of product hash, doesn't need to permit in product hash though
params.require(:product).permit(:name)
end
end
In your product/_form.html.erb create the form_for with nested resources, then you'll get the category_id under params[:product] hash.