RSpec: Define negated matcher

Posted Over 2 years ago. Visible to the public. Repeats.

You can use RSpec::Matchers.define_negated_matcher to define a negated version of an existing matcher.

This is particularly useful in composed matcher expressions or to create more expressive and meaningful matchers.

Examples 

RSpec::Matchers.define_negated_matcher :not_change, :change

describe 'A negated matcher' do
  it 'can be used to chain negated changes' do
    expect { subject.maybe_change(object) }
      .to not_change(object, :attr_1)
      .and not_change(contract, :attr_2)
  end
end

RSpec::Matchers.define_negated_matcher :an_array_excluding, :include

describe 'A negated matcher' do
  it 'can be used in a composed matcher expression' do
    expect { list.delete(5) }.to change { list }.to(an_array_excluding(5))
  end
end

RSpec::Matchers.define_negated_matcher :exclude, :include

describe 'A negated matcher' do
  it 'can be used for creating more expressive and meaningful matchers' do
    expect(list.difference([5])).to exclude(5)
  end
end
Julian
Say thanks2
Last edit
Over 2 years ago
Julian
License
Source code in this card is licensed under the MIT License.

Related cards:

RSpec: Composing a custom matcher from existing matchers

When you find similar groups of expect calls in your tests, you can improve readability by extracting the group into its own matcher. RSpec makes this easy by allowing matchers to call other matchers.

Example

The following test checks that ...

Rspec: Scope your custom matchers to example groups

When you find yourself in the situation that you would like to define a custom matcher in your specs, but you do not want to define a global matcher since you need it only for your specific test, there are two ways to do it:

Custom matcher for...

Defining custom RSpec matchers

There are three ways to define your own RSpec matchers, with increasing complexibility and options:

1) Use RSpec::Matchers.define

RSpec::Matchers.define :be_a_multiple_of do |expected|
  match do |actual|
    actual % expected == 0
 ...

RSpec matcher to compare two HTML fragments

The RSpec matcher tests if two HTML fragments are equivalent. Equivalency means:

  • Whitespace is ignored
  • Types of attribute quotes are irrelevant
  • Attribute order is irrelevant
  • Comments are ignored

You use it like this:

html = ...
...

RSpec matcher "be_sorted" 

RSpec::Matchers.define :be_naturally_sorted do
  match do |array|
    array == array.natural_sort
  end
end

See RSpec: Where to put custom matchers and other support code. To use natural_sort, see [this card](https://...

Chaining Capybara matchers in RSpec

You can chain multiple Capybara matchers on the page or any element:

expect(page)...

RSpec matcher to check if an ActiveRecord exists in the database

The attached RSpec matcher exist_in_database checks if a given record still exists in the database and has not been destroyed:

describe Ticket do
  describe '.purge_expired' do
    fresh_ticket = Ticket.create(:expiry => Date.tomorr...

RSpec: Leverage the power of Capybara Finders and Matchers for view specs

View specs are a powerful tool to test several rendering paths by their cases instead of using a more costing feature spec. This is especially useful because they become quite co...

RSpec: Expect one of multiple matchers to match

RSpec let's you chain a matcher with .or. The expectation will then pass if at least one matcher matches:

expect(color).to eq("red").or eq("green")

Real-world example

A real-world use case would be to test if th...

Where to put custom RSpec matchers

When you write a custom RSpec matcher a good place to store them is to create one file per matcher in spec/support/matchers:

spec/support/matchers/be_same_numbers_as.rb

...

Posted by Julian to makandra dev (2021-12-15 14:16)
This website uses short-lived cookies to improve usability.
or learn more