Mock constants in Ruby tests

#ruby #test #minitest

Here's a quick and dirty way to mock class constants in unit tests without using any gems.

Given this class:

module Query
   class Paginator
     PAGE_SIZE = 1000

     def paginate(page:)
         offset = if page > 1
           per_page * (page.to_i - 1)
         else
           0
         end
         Person.order(:created_at)
               .offset(offset)
               .limit(PAGE_SIZE)
     end
   end
end

Given PAGE_SIZE is set to 1000 records, I think it's a bit of a lift to run a test with that much records so why not set the constant with a lower value in our tests?

module Query
   class PaginatorTest < ActiveSupport::TestCase
     setup do
        # We change the constant value
        Query::Paginator.send(:const_set, 'PAGE_SIZE', 1)
     end

     teardown do
       # We restore the original value to avoid issues with other tests
       Query::Paginator.send(:remove_const, "PAGE_SIZE")   
     end

     ...
   end
end

You can set and unset the constant value in the test case body as well.