ruby-on-rails – 用于nil的未定义方法`fetch_value’：使用Roo

我收到错误：

undefined method `fetch_value' for nil:NilClass

并且该错误在行引用了我的data_point.rb文件(参见下面的完整代码摘录)：

data_point.save!

“应用程序跟踪”说：

app/models/data_point.rb:29:in `block in import'
app/models/data_point.rb:19:in `import'
app/controllers/data_points_controller.rb:65:in `import'

我很困惑,因为我的整个应用程序中的“查找所有”都没有显示fetch_value的实例

这是我的应用程序中的其他代码：

在我的模型中,data_point.rb：

class DataPoint < ActiveRecord::Base
attr_accessor :annual_income,:income_percentile,:years_education

def initialize(annual_income,income_percentile,years_education)
    @annual_income = annual_income
    @income_percentile = income_percentile
    @years_education = years_education
end

def self.import(file)
    spreadsheet = open_spreadsheet(file)
    header = spreadsheet.row(1)
    (2..11).each do |i| 
        annual_income = spreadsheet.cell(i,'A')
        income_percentile = spreadsheet.cell(i,'B')
        years_education = spreadsheet.cell(i,'C')
        data_point = DataPoint.new(annual_income,years_education)
        data_point.save!
    end
end 

def self.open_spreadsheet(file)
    case File.extname(file.original_filename)
    when ".xlsx" then Roo::Excelx.new(file.path)
    else raise "Unknown file type: #{file.original_filename}"
    end
end
end

在我的控制器中,我添加了data_points_controller.rb,除了标准的rails框架：

def import
    DataPoint.import(params[:file])
    redirect_to root_url,notice: "Data points imported."
end

在我正在使用的Excel文件中,标题行的列名与上面提到的DataPoints的3个属性完全相同：annual_income,years_education

附：我已经看过RailsCast 396：导入CSV和Excel,并多次阅读评论.我认为我正在努力将示例代码转换为Rails 4和/或我对各个属性的分配(与RailsCast中使用的方法相比).

在此先感谢您的帮助！