Rails 4访问模型属性

Question

我正在学习rails (我主要是一个java开发人员)，从我的问题中可以明显看出，并且在从rspec测试中访问我的模型上的方法时遇到了困难。我基本上是在跟踪本教程，但将它应用到我自己的项目中。我试图将graduation_year定义为一种基于学生成绩和每年当前时间的方法。当我实例化该对象并在rails控制台中调用该方法时，它如我所期望的那样工作，但是当我试图在测试中执行它时，我得到了如下结果：

undefined method `graduation_year'

student.rb

class Student < ActiveRecord::Base
  validates :school_id, presence: true, uniqueness: true
  validates :first_name, presence: true, length: { maximum: 30 }
  validates :last_name, presence: true, length: { maximum: 40 }
  validates :gender, presence: true, inclusion: { :in => %w(m f M F) }
  validates :grade, presence: true, numericality: { greater_than_or_equal_to: 1 , less_than_or_equal_to: 12 }

  def graduation_year
    year = Date.today.year + ( 12 - grade )
  end
end

student_spec.rb

require 'spec_helper'

describe Student do
  before { @student = Student.new(
                        school_id: 3, 
                        first_name: "Michael", 
                        last_name: "Green", 
                        gender: "M", 
                        grade: 6)}

  subject { @student }

  it { should respond_to(:school_id) }
  it { should respond_to(:first_name) }
  it { should respond_to(:last_name) }
  it { should respond_to(:gender) }
  it { should respond_to(:grade) }
  it { should respond_to(:graduation_year) }

  it { should be_valid }

  describe "when school_id is not present" do
    before { @student.school_id = " " }
    it { should_not be_valid }
  end
  .
  .
  .  
  describe "when grade is 7 graduation_year should be calculated correctly" do
    before { @student.grade = 7 }
    expect(@student.graduation_year).to eq(Date.today.year+5)
  end
end

错误消息

~/dev/rails_apps/spec/models/student_spec.rb:96:in `block (2 levels) in <top (required)>': undefined method `graduation_year' for nil:NilClass (NoMethodError)
from ~/dev/.rvm/gems/ruby-2.0.0-p247/gems/rspec-core-2.13.1/lib/rspec/core/example_group.rb:242:in `module_eval'
from ~/dev/.rvm/gems/ruby-2.0.0-p247/gems/rspec-core-2.13.1/lib/rspec/core/example_group.rb:242:in `subclass'
from ~/dev/.rvm/gems/ruby-2.0.0-p247/gems/rspec-core-2.13.1/lib/rspec/core/example_group.rb:228:in `describe'
from ~/dev/rails_apps/spec/models/student_spec.rb:94:in `block in <top (required)>'
from ~/dev/.rvm/gems/ruby-2.0.0-p247/gems/rspec-core-2.13.1/lib/rspec/core/example_group.rb:242:in `module_eval'
from ~/dev/.rvm/gems/ruby-2.0.0-p247/gems/rspec-core-2.13.1/lib/rspec/core/example_group.rb:242:in `subclass'
from ~/dev/.rvm/gems/ruby-2.0.0-p247/gems/rspec-core-2.13.1/lib/rspec/core/example_group.rb:228:in `describe'
from ~/dev/.rvm/gems/ruby-2.0.0-p247/gems/rspec-core-2.13.1/lib/rspec/core/dsl.rb:18:in `describe'
from ~/dev/rails_apps/spec/models/student_spec.rb:3:in `<top (required)>'
from ~/dev/.rvm/gems/ruby-2.0.0-p247/gems/rspec-core-2.13.1/lib/rspec/core/configuration.rb:819:in `load'
from ~/dev/.rvm/gems/ruby-2.0.0-p247/gems/rspec-core-2.13.1/lib/rspec/core/configuration.rb:819:in `block in load_spec_files'
from ~/dev/.rvm/gems/ruby-2.0.0-p247/gems/rspec-core-2.13.1/lib/rspec/core/configuration.rb:819:in `each'
from ~/dev/.rvm/gems/ruby-2.0.0-p247/gems/rspec-core-2.13.1/lib/rspec/core/configuration.rb:819:in `load_spec_files'
from ~/dev/.rvm/gems/ruby-2.0.0-p247/gems/rspec-core-2.13.1/lib/rspec/core/command_line.rb:22:in `run'
from ~/dev/.rvm/gems/ruby-2.0.0-p247/gems/rspec-core-2.13.1/lib/rspec/core/runner.rb:80:in `run'
from ~/dev/.rvm/gems/ruby-2.0.0-p247/gems/rspec-core-2.13.1/lib/rspec/core/runner.rb:17:in `block in autorun'

而且，我不认为这是问题，但我最初将graduation_year作为一个属性放在了Student表中，但是删除了它，并且这个测试应该验证它的计算是否正确。

我还在测试中尝试过puts first_name，并且在first_name上得到了相同的未定义方法错误。我想我只是错过了一些关于ruby/rails工作方式的东西。

Answer 1

expect语句没有包装在'it‘子句中。试着写作

describe "when grade is 7" do
  before { @student.grade = 7 }
  it 'calculates the graduation year correctly' do
    expect(@student.graduation_year).to eq(Date.today.year+5)
  end
end

顺便说一句，用这种方式使用“学生”并不是惯用的。你最好用一个叫let的语句来定义学生。因此，您可以使用Student.new代替最初的like块，编写如下所示的内容：

let(:student) { Student.new(
                    school_id: 3, 
                    first_name: "Michael", 
                    last_name: "Green", 
                    gender: "M", 
                    grade: 6)}

你随后提到的“学生”将被替换为“学生”。这确保了描述块内(或外)没有泄漏。

let语句将回溯对象，并且只有在规范中引用了学生时才会实例化该对象。

Answer 2

不能在describe块中直接设置期望，就像行中的情况一样：

expect(@student.graduation_year).to eq(Date.today.year+5)

您没有说明所得到的具体错误(即，有未定义方法的对象)，但此时@student将是nil，因为所有未定义的实例变量都是nil，因此它不会有graduation_year方法。但是，如果@student.graduation_year是一个有效的表达式，那么您就会在expect调用上得到一个未定义的方法。