问题
I am receiving the following error on a project of mine: First argument in form cannot contain nil or be empty. I am trying to create an edit page for my code. Fairly new with Rails and trying to learn without scaffolding.
Controller:
class BooksController < ApplicationController
def new
@book = Book.new
@authors = Author.all
end
def edit
@book = Book.find(params[:id])
end
def show
#Notice how the @books is plural here.
@books = Book.all
@authors = Author.all
#@books = Book.where(id: params[:id])
end
#Create method will save new entries
def create
@book = Book.new(book_params)
@authors = Author.all
if @book.save
flash[:success] = "Book Added to Databse!"
redirect_to @book
else
render 'new'
end
end
private
#Note that this method will go up into the create method above.
def book_params
params.require(:book).permit(:title, :pub_date, :publisher, :author_id)
end
end
Model Page: (For Book)
class Book < ActiveRecord::Base
validates :title, :pub_date, :publisher, presence: true
validates :title, uniqueness: true
belongs_to :author
end
Model Page: (For Author)
class Author < ActiveRecord::Base
validates :name, presence: true
validates :name, uniqueness: true
has_many :books
end
Edit page:
<h1>Update a book entry</h2>
<div class="row">
<div class="col-md-6 col-md-offset-3">
<%= form_for(@book) do |f| %> **ERROR SEEMS TO BE RIGHT HERE!!!**
<%= render 'form' %>
<div class="form-group">
<%= f.label :title %>
<%= f.text_field :title, class: 'form-control' %>
</div>
<div class="form-group">
<%= f.label :pub_date %>
<%= f.text_field :pub_date, class: 'form-control' %>
</div>
<div class="form-group">
<%= f.label :publisher %>
<%= f.text_field :publisher, class: 'form-control' %><br />
</div>
<div class="form-group">
<%= f.select(:author_id,
@authors.collect {|a| [ a.name, a.id ]},
{:include_blank => 'Please select an author'},
class: "form-control") %><br />
</div>
<%= f.submit 'Save Changes', class: "btn btn-primary" %>
<% end %>
</div>
</div>
Render form page (_form.html.erb)
<% if @book.errors.any? %>
<div id="error_explanation">
<div class="alert alert-danger">
<h2><%= pluralize(@book.errors.count, "error") %>
prohibited this entry from being saved:</h2>
<ul>
<% @book.errors.full_messages.each do |msg| %>
<li><%= msg %></li>
<% end %>
</ul>
</div>
</div>
<% end %>
SHOW PAGE:
<div class="move">
<h1>Showing Book Titles:</h1>
</div><br />
<div class="row">
<% @books.each do |book| %>
<div class="col-md-4">
<div class="panel panel-default">
<div class="panel-body">
<h2><%= book.title %></h2>
<h2><%= book.publisher %></h2>
<h2><%= book.pub_date %></h2>
<h2><%= book.author.name %></h2>
<h2><%= link_to "Edit", edit_path, class: "btn btn-primary" %>
</div>
</div>
</div>
<% end %>
Here is my Log telling me what is wrong:
Started GET "/edit" for ::1 at 2015-08-14 16:49:17 -0400
Processing by BooksController#edit as HTML
Rendered books/edit.html.erb within layouts/application (2.2ms)
Completed 500 Internal Server Error in 9ms (ActiveRecord: 0.0ms)
ActionView::Template::Error (First argument in form cannot contain nil or be empty):
3: <div class="row">
4: <div class="col-md-6 col-md-offset-3">
5:
6: <%= form_for(@book) do |f| %>
7: <%= render 'form' %>
8:
9: <div class="form-group">
app/views/books/edit.html.erb:6:in `_app_views_books_edit_html_erb___525891009649529081_70260522100960'
I will say that I have deleted the first 14 books from my data base and so the first book start on ID 14. Not sure if that matters.
Finally, I have tried adding all of these different instance variables to my controller in the edit method:
#@book = Book.where(id: params[:id])
#@book = Book.find_by_id(params[:id])
#@book = Book.all
#@book = Book.find_by_id(params[:id])
#book = Book.new(book_params)
#When I use the two below lines,
there are no error pages but create a new entry.
#@book = Book.new
#@authors = Author.all
Any Help will be appreciated! Thank you for your time!!!
回答1:
This error means that the first argument to form_for is a nil value (in this case @book). Most of the times I've seen this, it's due to malformed controller actions, but that doesn't look to be the case here. From what I can tell, it's one of two things:
You're trying to edit a Book that doesn't exist. Do a
.nil?check on it before deciding to render the form, and render an error message (or redirect) instead.Your routes are broken, and the
editaction is not rendering theeditview. This is most likely not the case.
EDIT:
After updating with your template for show, this looks like your problem:
<%= link_to "Edit", edit_path, class: "btn btn-primary" %>
I see two problems with this (though I'll need to see the output of rake routes to verify). Firstly, you need to pass an argument to the edit path (without them, where would your params come from?). Secondly, the default route for this would be edit_book_path.
Try this:
<%= link_to "Edit", edit_book_path(book), class: "btn btn-primary" %>
回答2:
Assuming book ID 14 is in your database, you should be able to navigate to localhost:3000/books/14/edit if you created it with something like resources :books (documentation here). If this doesn't work, either your routes are not defined correctly or book with ID 14 does not exist in your database.
On your show view, change the link_to line to:
<%= link_to 'Edit', edit_book_path(book), class: "btn btn-primary" %>
So two changes:
- Again, assuming book is a Restful resource, when you run
rake_routes, you should see the path to edit isedit_book_path. - You need to pass the
bookinstance with the path so Rails knows which object you wish to edit.
I found the correct syntax here.
Hope this helps.
来源：https://stackoverflow.com/questions/32018830/rails-4-receiving-the-following-error-first-argument-in-form-cannot-contain-ni