How can I get my Rspec unit test to pretend that the output of another method is XYZ

Question

Using Rspec, I am writing unit tests for @survey.description:

class Survey < ActiveRecord::Base
  def description
    if self.question.try(:description).present? && self.selected_input.present?
      return self.question.try(:description).gsub("{{product-name}}", self.selected_input.name)
    else
      return self.question.try(:description)
    end
  end    
  def selected_input
    @matches = Input.all.select{|input| self.goods.to_a.matches(input.goods) && self.industries.to_a.matches(input.industries) && self.markets.to_a.matches(input.markets)}
    @selection = @matches.select{|input| input.in_stock(self.competitor) == true}
    if @selection.empty? || @selection.count < self.iteration || @selection[self.iteration-1].try(:name).nil?
      return false
    else
      return @selection[self.iteration-1]
    end
  end    
end

At a minimum, I'd like to write a test case for when @survey.selected_input.present? is true and one for when it is false.

But I don't want to write line upon line of code creating an @input, setting other values elsewhere to ensure the @input is selected for the @survey, etc etc, just to set @survey.selected_input.present? to true. Is there some way I can do something like:

describe "description" do
  it "should do something when there is a selected input" do
      just_pretend_that @survey.selected_input = "apples"
      @survey.description.should == "How's them apples?"
  end
end

I've tagged this post mocking and stubbing as I've never consciously used either technique but I think one of them may hold the answer.

Answer 1

In general it's not a good idea to stub methods for the object under test. Though, since you asked about the syntax, what you are looking for is RSpec Mocks:

describe Survey do
  subject(:survey) { Survey.new(...) }

  context 'requesting the description' do
    it 'contains product name when it has input' do
      survey.stub(selected_input: 'apples')
      expect(survey.description).to eq "How's them apples?"
    end
  end
end