How to debug simple RoR application - ruby-on-rails

Question

So far, to a very experienced Java EE developer with years of experience in many different languages, I am having real difficulties with Ruby on Rails. I am using: ruby 2.3.1p112 (2016-04-26 revision 54768) [x86_64-darwin15] and Rails 5.0.0. I am following a very simple on-line tutorial on building a private library web application, BUT, in order to learn something, instead of having Books with a linked table of Subjects, I changed Subjects to Authors since many books have the same authors. I am using SQLLite for development and MySQL for production( haven't gotten there yet! ). I find that when you follow exactly the directions in most tutorials, you end up with whatever application you were building. But, IF you deviate in any fashion, things just don't work and it's very hard to figure out what happened. You get error messages ( sometimes ) in the logs that you've got an undefined variable or constant. Normally, you would search for where that variable is used, then be sure you define it or spell it correctly. However, in RoR, that constant doesn't appear anywhere except in the log, if there. RoR, due to its conventions, has either created or assumed that you had such a variable, when in fact, you may have named a "view" folder in the singular instead of the plural. It "invented" a variable to point to that, but it didn't match the pattern, so it fails with very poor error messages.
The server doesn't complain, just does a rollback, and goes on. The log has some unmeaningful message, as per above. I end up spending hours trying different patterns for routes suggested by people, or renaming things, but it's all guesswork.
I enjoy working with frameworks and systems where I understand them. This seems to be a collection of different pieces which parse in yml, yaml, erb, rb, sass, haml, etc. I've tried logging, but to no avail. How do you located simple mistakes?
Here is my "books_controller.rb":
class BooksController < ApplicationController
def list
#books = Book.all
end
def show
#book = Book.find(params[:id])
end
def new
#book = Book.new
#authors = Author.all
end
def create
#book = Book.new(book_params)
if #book.save
logger.debug 'Redirecting to list'
redirect_to :action => 'list'
else
#authors = Author.all
render :action => 'new'
end
end
def edit
#book = Book.find(params[:id])
#authors = Author.all
end
def update
#book = Book.find(params[:id])
if #book.update_attributes(book_params)
redirect_to :action => 'show', :id => #book
else
#authors = Author.all
render :action => 'edit'
end
end
def delete
Book.find(params[:id]).destroy
redirect_to :action => 'list'
end
def show_authors
#author = Author.find(params[:id])
end
def book_params
params.require(:books).permit(:title, :description, :author_id)
end
end
The new.html.erb under app/views/books is:
<h1>Add new book</h1>
<%= form_tag :action => 'create' do %>
<p><label for = "book_title">Title</label>:
<%= text_field 'books', 'title' %></p>
<p><label for = "book_author_id">Author</label>:
<%= collection_select(:book, :author_id, #authors, :id, :name, prompt: true) %></p>
<p><label for = "book_description">Description</label><br/>
<%= text_area 'books', 'description' %></p>
<%= submit_tag "Create" %>
<% end -%>
<%= link_to 'Back', {:action => 'list'} %>
routes.rb is:
Rails.application.routes.draw do
# For details on the DSL available within this file, see http://guides.rubyonrails.org/routing.html
resources :books
#get 'books/list'
#post 'books/create'
#get 'books/new'
#patch 'books/update'
#get 'books/show'
#get 'books/edit'
#get 'books/delete'
get 'books/show_authors'
get 'authors/list'
post 'authors/create'
get 'authors/new'
patch 'authors/update'
get 'authors/show'
get 'authors/edit'
root :to => 'books#list'
end
When I try to add a new book, I enter the title, select an author, and put in a description and click "Create". It then just returns to the new screen. The console has:
Started GET "/books/new" for ::1 at 2016-08-04 17:18:22 -0400
Processing by BooksController#new as HTML
Rendering books/new.html.erb within layouts/application
Author Load (0.1ms) SELECT "authors".* FROM "authors"
Rendered books/new.html.erb within layouts/application (5.4ms)
Completed 200 OK in 26ms (Views: 21.6ms | ActiveRecord: 0.5ms)
Started POST "/books" for ::1 at 2016-08-04 17:18:28 -0400
Processing by BooksController#create as HTML
Parameters: {"utf8"=>"✓", "authenticity_token"=>"noRmEq8rHE6RLs0cPNrlZoQXq//2sr+SAOSHEFc0U3zqbSJZOSKDmdgwpdm5/nVswItHp4Ken0mjggt47ph46Q==", "books"=>{"title"=>"sdfasdf", "description"=>"asdfasdf"}, "book"=>{"author_id"=>"2"}, "commit"=>"Create"}
(0.1ms) begin transaction
(0.1ms) rollback transaction
Rendering books/new.html.erb within layouts/application
Author Load (0.1ms) SELECT "authors".* FROM "authors"
Rendered books/new.html.erb within layouts/application (2.0ms)
Completed 200 OK in 24ms (Views: 20.3ms | ActiveRecord: 0.2ms)
and the development log has:
Started GET "/books/new" for ::1 at 2016-08-04 17:18:22 -0400
Processing by BooksController#new as HTML
Rendering books/new.html.erb within layouts/application
[1m[36mAuthor Load (0.1ms)[0m [1m[34mSELECT "authors".* FROM "authors"[0m
Rendered books/new.html.erb within layouts/application (5.4ms)
Completed 200 OK in 26ms (Views: 21.6ms | ActiveRecord: 0.5ms)
Started POST "/books" for ::1 at 2016-08-04 17:18:28 -0400
Processing by BooksController#create as HTML
Parameters: {"utf8"=>"✓", "authenticity_token"=>"noRmEq8rHE6RLs0cPNrlZoQXq//2sr+SAOSHEFc0U3zqbSJZOSKDmdgwpdm5/nVswItHp4Ken0mjggt47ph46Q==", "books"=>{"title"=>"sdfasdf", "description"=>"asdfasdf"}, "book"=>{"author_id"=>"2"}, "commit"=>"Create"}
[1m[35m (0.1ms)[0m [1m[36mbegin transaction[0m
[1m[35m (0.1ms)[0m [1m[31mrollback transaction[0m
Rendering books/new.html.erb within layouts/application
[1m[36mAuthor Load (0.1ms)[0m [1m[34mSELECT "authors".* FROM "authors"[0m
Rendered books/new.html.erb within layouts/application (2.0ms)
Completed 200 OK in 24ms (Views: 20.3ms | ActiveRecord: 0.2ms)
Yes, the transaction was rolled back. WHY? How can I get information on what caused the database to "rollback"? The two tables in the database are:
class CreateBooks < ActiveRecord::Migration[5.0]
def change
create_table :books do |t|
t.string :title
t.integer :author_id
t.string :description
t.timestamp :created
t.timestamps
end
end
end
class CreateAuthors < ActiveRecord::Migration[5.0]
def change
create_table :authors do |t|
t.string :name
t.timestamps
end
end
end
class Book < ApplicationRecord
belongs_to :author
validates_presence_of :title
end
class Author < ApplicationRecord
has_many :books
end
I can create a book in rails console as:
b=Book.create :title=>'Test', :author_id=>1, :description=>'Desc'
(0.1ms) begin transaction
Author Load (0.1ms) SELECT "authors".* FROM "authors" WHERE "authors"."id" = ? LIMIT ? [["id", 1], ["LIMIT", 1]]
SQL (0.3ms) INSERT INTO "books" ("title", "author_id", "description", "created_at", "updated_at") VALUES (?, ?, ?, ?, ?) [["title", "Test"], ["author_id", 1], ["description", "Desc"], ["created_at", 2016-08-04 20:17:40 UTC], ["updated_at", 2016-08-04 20:17:40 UTC]]
(2.4ms) commit transaction
=> #<Book id: 1, title: "Test", author_id: 1, description: "Desc", created: nil, created_at: "2016-08-04 20:17:40", updated_at: "2016-08-04 20:17:40">
I would appreciate input and especially help on understanding why what happened actually happened. It seems that a very simple error is being made, but I can't see it.
------------------ Added after several answers and "guesses" by me.
I changed the form_tag to a form_for as I'll show below.
----new.html.erb------
<%= form_for(#book) do |f| %>
Title: <%= f.text_field :title %><br/>
Author: <%= select("book", "author_id", Author.all.collect{|p| [p.name,p.id]}, prompt: 'Select') %><br/>
Description: <%= f.text_area :description %><br/>
<%= f.submit "Create" %>
<% end -%>
<%= link_to 'Back', {:action => 'list'} %>
I get in the browser:
Validation failed: Author must exist, Title can't be blank
Extracted source (around line #18):
16
17
18
19
20
21
def create
#book = Book.new
if #book.save!
redirect_to :action => 'list'
else
#authors = Author.all
Rails.root: /Users/woo/Development/rails/library
Application Trace | Framework Trace | Full Trace
app/controllers/books_controller.rb:18:in `create'
Request
Parameters:
{"utf8"=>"✓",
"authenticity_token"=>"gi+wVGV3MIlkJsRjO8Ig1cS3YV/OIADSevFJg7ItBesokIiHFDThycTO8/kob+2E1fuPFquFUK+b7bGksWRZGQ==",
"book"=>{"title"=>"Book", "author_id"=>"2", "description"=>"test"},
"commit"=>"Create"}
As far as I can see, book does have a title, and an author_id, and a description. Why "Author must exist, Title can't be blank"?

Answer 1

Try using form_for instead of form_tag in your books/new view. This is the Rails way to create forms for model object.
Check a handy guide on form_for here.

Answer 2

How to debug ....
There are several tools to help debug a Rails application. One you have already discovered: the log file in log/development.log.
Another is Byebug. Add the gem to your Gemfile and insert the following in the create action after the 'else':
require 'byebug'
byebug
then post the form again. The development server will bring up a Byebug console where you can inspect local variables, instance variables, and the stack trace. Try inspecting:
#book.errors
When an ActiveRecord model fails to save it is usually because validations failed. Errors encountered when saving are added to the errors object on the model instance.
The reason for the failure is probably that the form is not passing the expected parameters. By convention, Rails expects attributes for the model to be in a hash where the key is the model name, so params[:book][:title], not params[:title]. See the documentation for the form_for helper for more info.

Answer 3

Thanks for the help. With your suggestions and a lot of guessing from ready various Google sites, I combined them and in the new.html.erb put form_for(#book), and then in the create method of books_controller.rb, I put #book - Book.new(book_params), where book_params is:
def book_params
params.require(:book).permit(:title, :author_id, :description)
end
I'm guessing that this is to handle the strong attribution required by Rails 4 and up. Before I had books as the first argument and since it existed, but book was not filled, I got the weird error. After using form_for with #book as an argument, that set the values from the form into the book hash. Then the params.require with :book as the first argument, looked in that hash to extract title, author_id, and description.
Again, many thanks for the help and I learned about bye bug and save! and so forth. I find information is very sketchy and ofter the version is not mentioned, thus leading one astray many times.