Rails中nil：NilClass的未定义方法`+'

Question

In my user.rb file under models , I defined a method full-name , and called this method in my views. It raises this error that I do not know how to solve.

NoMethodError in Statuses#index
Showing app/views/layouts/application.html.erb where line #24 raised:
undefined method `+' for nil:NilClass
Extracted source (around line #10):

def full_name
  first_name + " " + last_name
end

Rails.root:

Application Trace | Framework Trace | Full Trace
app/models/user.rb:10:in `full_name'
app/views/layouts/application.html.erb:24:in `_app_views_layouts_application_html_erb__1801360389632919652_70105866374460'
<li>
  <%= link_to current_user.full_name, edit_user_registration_path %>
</li>

Answer 1

Your first_name attribute contains nil .

Since NilClass does not implement + method, it raises an error. One way to handle that would be to cast values to string:

first_name.to_s + " " + last_name.to_s

But the better, idiomatic way is to use string interpolation:

def full_name
  "#{first_name} #{last_name}"
end

To see the actual value that is contained in a variable you could use

puts first_name

But, since the nil value would become an empty string when printed, it would look the same as "" , a blank string. To see if the value is actually nil when printing, you can use

puts first_name.nil?
# => true

or

puts first_name.class
# => NilClass

Answer 2

Your code is right, just you have one (or more) record with empty value in first_name (and/or empty last_name). As Nic explained Ruby does not have automatic cast from nil to string, but I suppouse you want to some value for those fields, so I suggest you to use a proper validation for them:

class User < ActiveRecord::Base
  validates :first_name, :last_name, presence: true
end