Ruby spec stubbing a block using mocha

importer.rb

class Importer
  def parse_csv_file(csv_file)
    File.open(csv_file) do |file|
      ActiveRecordModel.transaction do
        import_csv_stream(file)
      end
    end
  end
end

importer_spec.rb

describe Importer do
  describe '#parse_csv_file' do
    let(:csv_file) { "#{Rails.root}/tmp/expected_import.csv" }
    let(:importer) { Import.new }
    let(:opened_file) { mock('file') }
    let(:data) { "line|value\n2|12345\n" }
    let(:data_stream) { StringIO.new(data) }

    def do_invoke
      importer.parse_csv_file(csv_file)
    end

    it 'opens the received file' do
      File.expects(:open).with(csv_file)
      do_invoke
    end

    it 'performs an ActiveRecordModel.transaction' do
      File.stubs(:open).yields(opened_file).returns(data_stream)
      ActiveRecordModel.expects(:transaction)
      do_invoke
    end

    it 'passes the open file to import_csv_stream' do
      File.stubs(:open).yields(opened_file).returns(data_stream)
      importer.expects(:import_csv_stream).with(opened_file)
      do_invoke
    end
  end
end