我使用rspec ./spec/models/prediction_spec.rb运行这个rspec文件

require 'rails_helper'
require 'prediction.rb'

RSpec.describe Prediction, type: :model do
  context "validations tests" do
    it "ensures zip presence" do
      prediction = Prediction.new(zip: "zip").save
      expect(prediction).to eq(false)
    end

    it "ensures school presence" do
      prediction = Prediction.new(school: "school").save
      expect(prediction).to eq(false)
    end

    it "should save successfully" do
      prediction = Prediction.new(zip: "zip", school: "school").save
      expect(prediction).to eq(true)
    end
  end

  context "scope tests" do
    let(:params) { { zip: "zip", school: "school" } }
    before(:each) do
      Prediction.new(params).save
      Prediction.new(params).save
      Prediction.new(params).save
    end

    it "should return all predictions" do
      expect(Prediction.count).to eq(3)
    end
  end
end

下面是我的预测模型：

class Prediction < ApplicationRecord
    #belongs_to :user

    def getRequest
        response = HTTParty.get('https://sdp-api.herokuapp.com/api/v1/query/' + @prediction.zip)
        @inch = response["snow"]
        @temp = response["temp"]
    end

    def percentage
        #for our weighing purposes
        weight_1 = inch/5.0
        weight_2 = temp/32.0

        #standardize weights
        if weight_1 >= 1
            weight_1 = 0.7
        else
            weight_1 = weight_1 * 0.7
        end
        
        if weight_2 <= 1
            weight_2 = 0.3
        else
            weight_2 = 0
        end

        return 95 * (weight_1 + weight_2)
    end

    #getters and setterss
    # attr_accessor :inch
    # attr_accessor :temp
end

当我运行它时，在包含.保存的每一行上都得到以下错误：

NoMethodError:
       undefined method `fetch_value' for nil:NilClass

为什么会发生这种情况？其他类似的问题列出了有保留字的数据库列，而我的数据库没有保留字。另外，这似乎与这些错误所暗示的不太接近。