Ruby on Rails SQLite3::ConstraintException: NOT NULL constraint failed: - ruby-on-rails

Question

I am developing a simple app where a user can add a subject to a cart. Before I add the authentication I was able to add a subject to the cart but as I want a user to be able to has access to just his/her cart I used Devise to create User with authentication. Now, when I click on the button to add a subject to the cart I have the following error:
This is a snapshot of the error I get: 1
SQLite3::ConstraintException: NOT NULL constraint failed: carts.user_id: INSERT INTO "carts" ("created_at", "updated_at") VALUES (?, ?)
class ApplicationController < ActionController::Base
# Prevent CSRF attacks by raising an exception.
# For APIs, you may want to use :null_session instead.
protect_from_forgery with: :exception
private
def current_cart
Cart.find(params[:user_id])
rescue ActiveRecord::RecordNotFound
cart = Cart.create
params[:user_id] = cart.id
cart
end
end
class User < ActiveRecord::Base
# Include default devise modules. Others available are:
# :confirmable, :lockable, :timeoutable and :omniauthable
devise :database_authenticatable, :registerable,
:recoverable, :rememberable, :trackable, :validatable
has_one :cart
end
class Cart < ActiveRecord::Base
belongs_to :user
has_many :line_items, dependent: :destroy
scope :user_carts, ->(user) { where(['user_id= ?', user.id]) }
end
class AddUsersToCarts < ActiveRecord::Migration
def up
add_reference :carts, :user, index: true
Cart.reset_column_information
user = User.first
Cart.all.each do |cart|
cart.user_id = user.id
cart.save!
end
change_column_null :carts, :user_id, false
add_foreign_key :carts, :users
end
def down
remove_foreign_key :carts, :users
remove_reference :carts, :user, index: true
end
end
Edit: I added the schema.rb below:
ActiveRecord::Schema.define(version: 20151210213408) do
create_table "carts", force: :cascade do |t|
t.datetime "created_at", null: false
t.datetime "updated_at", null: false
t.integer "user_id", null: false
end
add_index "carts", ["user_id"], name: "index_carts_on_user_id"
create_table "line_items", force: :cascade do |t|
t.integer "subject_id"
t.integer "cart_id"
t.datetime "created_at", null: false
t.datetime "updated_at", null: false
end
create_table "subjects", force: :cascade do |t|
t.string "title", null: false
t.string "code", null: false
t.text "description", null: false
t.integer "credits", null: false
t.string "lecturer", null: false
t.datetime "created_at", null: false
t.datetime "updated_at", null: false
end
add_index "subjects", ["title"], name: "index_subjects_on_title", unique: true
create_table "users", force: :cascade do |t|
t.string "email", default: "", null: false
t.string "encrypted_password", default: "", null: false
t.string "reset_password_token"
t.datetime "reset_password_sent_at"
t.datetime "remember_created_at"
t.integer "sign_in_count", default: 0, null: false
t.datetime "current_sign_in_at"
t.datetime "last_sign_in_at"
t.string "current_sign_in_ip"
t.string "last_sign_in_ip"
t.datetime "created_at", null: false
t.datetime "updated_at", null: false
end
add_index "users", ["email"], name: "index_users_on_email", unique: true
add_index "users", ["reset_password_token"], name: "index_users_on_reset_password_token", unique: true
end

Answer 1

You current_cart method does not make much sense.
You cannot find the user's cart by calling Cart.find(params[:user_id]), because that looks for a cart by an id (not by an user_id).
Cart.create fails, because you do not provide an user_id that is required (your database migrations says that the filed cannot be null).
Furthermore, params[:user_id] = cart.id changes the params hash, but not the new cart.
Change that method to something like this (using find_or_create_by) and use the current_user.id instead of params[:user_id]:
def current_cart
Cart.find_or_create_by(user_id: current_user.id)
end