“存在？”方法不起作用-在保存到数据库之前尝试检查用户是否存在

Question

I'm new to Rails, and am trying to create a signup form for my site. It works fine, with one exception- it saves users to the database without checking first to see if they exist. How can I check to make sure a user exists before I save it?

User controller:

class UsersController < ApplicationController
  def new
    @user = User.new
  end

  def create 
    @user = User.new(user_params)
    if @user.exists?(:username) == false && @user.save(user_params)
      session[:id] = @user.id
      redirect_to posts_path
    else
      redirect_to '/signup'
    end
  end

  def edit

  end

  def update
  end

  def show
  end

  def destroy
  end

    private

  def user_params
    params.require(:user).permit(:username, :jabber_id, :password)
  end
end

Answer 1

A very simple validation would be something like this:

# in your user.rb
validates :username, uniqueness: true

You might want to ignore upper and lower case:

# in your user.rb
validates :username, uniqueness: { case_sensitive: false }

And change your create method to something like this:

def create 
  @user = User.new(user_params)

  if @user.save 
    session[:id] = @user.id
    redirect_to posts_path
  else
    render :new
  end
end

Furthermore, I suggest having a unique index on the database level too:

# in a migration
add_index :users, :username, unique: true

Answer 2

在模型文件中使用这样的文件，例如

validates_uniqueness_of :username #=> or specify :email or :name etc...

Answer 3

As suggested, you should use model validation for something like this. However, here's why your code doesn't work.

The exists? method is to be used on the model class, not an instance of it for example:

Person.exists?(5)
Person.exists?(:name => "David")

The above is from the exists? docs

Answer 4

As almost people suggested you above and this is the best way to use uniqueness validation.

But, If you want to give it a try with exists? You can do like,

User.exists?(username: params[:username])