Today I Learned

A Hashrocket project

Chaining Multiple RSpec Change Matchers

It can be handy to use RSpec’s change matchers to determine if some method or process creates a new record.

expect{ Registration.create(attrs) }.to change{ User.count }.by(1)

But what if we are testing a method that creates a couple different records in the system?

RSpec allows us to chain together change matchers with and. Consider this additional contrived example.

expect {
  Project.generate(attrs)
}.to change{ Project.count }.by(1).and \
     change{ User.count }.by(1)

In addition to keeping our tests tight and concise, this approach gives some pretty nice output on failure.

If we were just beginning our implementation with a failing test, we’d see a multi-part failure like the following.

Failure/Error:
  expect {
    Project.generate(attrs)
  }.to change{ Project.count }.by(1).and \
       change{ User.count }.by(1)

     expected result to have changed by 1, but was changed by 0

  ...and:

     expected result to have changed by 1, but was changed by 0
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