Mocking Enumerables in Rspec

Mocking in Rails

Rails puts some really powerful mocking libraries at our disposal. As long as your models are backed by ActiveRecord, ActiveRecord Fixtures and Factory Bot are two very good libraries for mocking. More often than not, these cover the use cases for most applications. But when your models are not backed by ActiveRecord, there’s an excellent test-double framework for rspec, called RSpec Mocks that you can use.

Basic Mocking with RSpec Mocks

Basic mocking using RSpec Mocks is straight forward. Initialize the mock object as below

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@mock_post = double('post')

and then

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allow(@mock_post).to receive(:title) { 'sample title' }

Mocking a chain of methods

For mocking a chain of methods, there are two options.

Lets say that the Post object has a method author, that returns another object of class Author. The Author class has multiple methods, including name and image

  • If your code only ever calls the name method for the post’s author, you can easily setup this mock object as

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    allow(@mock_post).to receive_message_chain(:author, :name) { 'Sample Author' }
  • If you need a more complex mocking, you can have 2 mock objects

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    @mock_author = double('author')
    allow(@mock_author).to receive(:name) { 'Sample Author' }
    allow(@mock_author).to receive(:image) { 'https://sample.image' }

    allow(@mock_post).to receive(:author) { @mock_author }

Mocking Enumerables

In my specific project, I had a model that responded to each, and also had other methods that needed mocking, total, skip, limit etc. My initial instinct was that I could just do something like

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posts = double('post')
allow(posts).to receive(:each) { posts_list }

But unfortunately that doesn’t work, because each expects a block and yields each of the list item to the block. Fortunately, RSpec Mocks has a method and_yields that lets us set up just what we need.

Continuing with the above example, lets say we need to have 10 mock posts. We can set it up as,

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posts_list = []
10.times do |index|
post = double('post', title: "post_#{index}")
posts_list << post
end

posts = double('post', total: posts_list.size, skip: 0, limit: posts_list.size)
iterator = allow(posts).to receive(:each)
posts_list.each do |post|
iterator.and_yield(post)
end

At this point, the posts mock object responds to the each iterator and returns the 10 mock post objects. It also responds to the total, skip and limit methods correctly.

Hope this helps.

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