Test that rails helper renders a partial

Question

Given the following helper method, how would I properly test this using rspec?

  def datatable(rows = [], headers = [])
    render 'shared/datatable', { :rows => rows, :headers => headers }
  end

  def table(headers = [], data = [])
    render 'shared/table', headers: headers, data: data
  end

I've tried the following but I get the error: can't convert nil into String

describe 'datatable' do
  it 'renders the datatable partial' do
    rows = []
    headers = []
    helper.should_receive('render').with(any_args)
    datatable(rows, headers)
  end
end

Rspec Output

Failures:

  1) ApplicationHelper datatable renders the datatable partial
     Failure/Error: datatable(rows, headers)
     TypeError:
       can't convert nil into String
     # ./app/helpers/application_helper.rb:26:in `datatable'
     # ./spec/helpers/application_helper_spec.rb:45:in `block (3 levels) in <top (required)>'

./app/helpers/application_helper.rb:26

render 'shared/datatable', { :rows => rows, :headers => headers }

views/shared/_datatable.html.haml

= table headers, rows

views/shared/_table.html.haml

%table.table.dataTable
  %thead
    %tr
      - headers.each do |header|
        %th= header
  %tbody
    - data.each do |columns|
      %tr
        - columns.each do |column|
          %td= column

Answer 1

if you just want to test that your helper calls the right partial with the correct parameters you can do the following:

describe ApplicationHelper do

  let(:helpers) { ApplicationController.helpers }

  it 'renders the datatable partial' do
    rows    = double('rows')
    headers = double('headers')

    helper.should_receive(:render).with('shared/datatable', headers: headers, rows: rows)

    helper.datatable(rows, headers)
  end

end

note that this won't call the actual code in your partial.

Answer 2

The argument of should_receive should be a symbol instead of string. At least I have not seen string is used in doc(https://www.relishapp.com/rspec/rspec-mocks/v/2-14/docs/message-expectations)

So, instead of

helper.should_receive('render').with(any_args)

Use this

helper.should_receive(:render).with(any_args)

Not sure if this could solve the problem but at least this is an error which cause your error message probably.