[RUBY] [Rails] I studied the difference between new method, save method, build method and create method.

I'm a beginner with "super" who is struggling with Rails every day. Share for organizing your own knowledge.

Isn't there a lot of methods for creating objects?

I didn't notice this fact when I was scanning the text, but the other day "Oh, if you think it's a new method, do you use the create method here ... If you think about it, there's a build ... What ?! " I was in such a state, so I decided to investigate it myself. It's a short article, but please stay with me until the end. image.png

About new method & save method

First, let's start with the basic usage of the new method. By the way, I am writing on the assumption that I will post with the Book model.


def new
  @book = Book.new

I created an empty instance of the Book class in Book.new and assigned it to the @book instance variable. "Everyone knows this" I used to think so, but there was a way to specify the argument with the new method. Here is it


def new
  @book = Book.new(title: "Rashomon", author:"Ryunosuke Akutagawa")
#The title is "Rashomon" and the author is "Ryunosuke Akutagawa".

If you post a new book with this, no matter what characters you enter in the input field, you will not be able to register other than Rashomon (laugh) The new method didn't just pass an empty instance. I'm sorry for those who knew it.

After the new method, use the save method in the create action to save the data. Here is it


def create
  @book = Book.new(book_params)

Finally, I was able to newly register in the Book database. In the case of the save method, it can be saved in the database together with the new method.

About the create method

This is where the rumored create method comes in. The steps saved using the new method and save method described above can be completed in just one line with the create method.

What does that mean?


def create
  Book.create(title: "Rashomon", author:"Ryunosuke Akutagawa")

With this alone, I was able to save it in the database. Not only can you save the data, but you can also instantiate the model at the same time. Mecha easy. Easy.

Basic syntax

Model name.create(Column name: "value")

Again, the create method is a method that is doing new and save at the same time.

Then why not just use the create method? It seems that a shallow idea will come up, but it is not so sweet.

How to use properly

If you want to do something with the created instance, you can't use the create method, so you'll have to write a new method and a save method. Specifically

  1. When you want to create a conditional branch using if
  2. When you want to get an error message with render And so on, which will be explained below.

❶ Conditional branching using if


def create
  @user = User.new(user_params) #Create an instance variable
  if @user.save
    redirect_to @user, notice: 'User creation completed'
    render :new #Render destination when save could not be done

In this way, when conditional branching occurs, such as using instance variables to divide into success patterns and failure patterns, the new method and save method should be used.

❷ I want to issue an error message with render

For example, on the screen for making a new post (new action), the validation check works and the post cannot be made. At that time, if an error message appears above the post column, it is very easy to understand. I will explain in detail.


def new
  @book = Book.new

def create
  @book = Book.new(params[:id])
  if  @book.save
      redirect_to book_path(@book.id)
      render 'layouts/errors'


<h1>New Book</h1>
<%= render 'layouts/errors', obj: @book %>


<% if obj.errors.any? %>  #to [email protected] book
  <div id="error_explanation">
    <h3><%= pluralize(obj.errors.count, "error") %> prohibited this obj from being saved:</h3>

    <% obj.errors.full_messages.each do |message| %>
      <li><%= message %></li>
    <% end %>
<% end %>

As mentioned above, even when using an instance variable containing the new method and issuing an error message, it is not possible with the create method alone. I often have to think about it ...

About the build method

It's finally the last method. However, I think it is the least important. In current rails, the new and build methods work almost the same. In older versions of rails, the difference was whether or not a foreign key was automatically added when the association was made, but now both the new method and the build method are set with a foreign key. Depending on the workplace, there are cases where build is recommended in specific situations, but if not, it will not be a problem to unify it into a new method.

in conclusion

This time too, I learned a lot. The more you dig deeper, the more you will discover new things, even if you think, "I know this well." I will continue to devote myself to studying.

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