Joseph Wilk

Joseph Wilk

Things with code, creativity and computation.

Cucumber Patterns

At Songkick.com we have developed a number of patterns to make it easier to write Cucumber features. I thought I would start sharing some of those patterns here. So here is the first one:

Implicit Reference Pattern

Make use of implicit references to previously discussed topics to produce scenarios which are easier to read and write. Achieving this while avoiding highly coupled steps.

Problem

Storing and relying on state in a step definition can make it hard to reuse. So often state is avoided. This can lead to scenarios like the following:

Given there is a Artist named "XXs"
 And I visit the page for the Artist named "The XXs"

This leaves us with:

  • Verbose steps which are not natural to read.
  • Extra noise information purely for identity (referencing the name)

Solution

Map implicit references in the language to the objects being discussed.

Accept that we have to store state but do so in an encapsulated way where the feature language is the only thing needed within the step definition to provide a direct mapping to the relevant object from the state.

What we are aiming for is a scenario like this:

Given there is an Artist named "XXs"
 When I visit the page for the Artist

Requirements

This pattern is described in the context of ActiveRecord and Factory Girl.

Identification through a single meaningful name

Key to this pattern is mapping a single identifier (that we would happily talk about in our Features) to a model.

We have a method that provides all the mappings of the models to the field that uniquely identifies them.

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def map_to_id_attribute(type_of_thing)
  {
    :user => :username,
    :concert => :title,
  }[type_of_thing] || :name
end

Domain model class names are meaningful to everyone

In our features when we talk about something in our domain we refer to its class name and we use the correct capitalization.

Non-technical people still understand what the references mean while allowing us to simplify identifying the model in a snippet of feature text.

This leads to steps such as:

Given the Artist
 Given the AdminUser

Implementation

Somewhere to store stuff

We store all the state in a specialised hash. This has a special find_things method which performs some validation and gives us nice error messages if we try and access incorrect types or non-existent objects.

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class StuffContainer < Hash
  def find_thing(opts)
    expected_type = opts[:type]
    name = opts[:name]
    thing = self[name]

    raise("Unable to find any object in stuff[] with the name '#{name}' that you asked for, boss. I could however offer you one of the following: #{self.to_s}") unless thing

    raise("That thing you asked for, it appears to be a #{thing.class.name} when you asked for a #{expected_type.name}") unless thing.is_a?(expected_type)

    thing
  end

  def to_s
    result = ["#{self.length} items in total:"]
    self.each do |key, thing|
      result << "the #{thing.class.name} \"#{key}\""
    end

    result.join("\n")
  end
end

Recording the subjects under discussion

In order to record references to created Models we extend Factory Girl’s ‘Factory’ method which is used for all our model creations.

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def Factory(type_of_thing, attributes = {})
  test_id = attributes[map_to_id_attribute(type_of_thing)]
  new_thing = super(type_of_thing, attributes)
  stuff[test_id] = new_thing if test_id
  new_thing
end

We will record created models in steps like these:

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Given /^there is (?:one|an|a|another) ([^ ]+) named "([^"]+)"$/ do |entity_type, name|
  attributes = {:name => name}
  entity = Factory(entity_type.underscore.to_sym, attributes)
end

Resolving Implicit references

Starting with the step definition:

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When /I (?:view|visit|go to) the page for (#{IDRE})$/ do |entity|
  visit model_path(identified_model(entity))
end

IDRE represents the ID regexp which provides a way of identifying a model and is reused in many steps.

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IDRE = /(?:the(?: first | last | )(?:[^ ]+)|the (?:[^ ]+) "(?:[^"]+)"|"(?:[^"]+)")/

The identified_model method turns an English string into a Model. It provides a number of ways of referencing a model.

the Artist
 the first Artist
 the last Artist
 the Artist "Jude"

The identified_model method: (The key case we are focusing on in this example is where we have no identify just the type of the model – line 11 and 12)

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def identified_model(str)
  case str
  when /^the (first|last) ([^ ]+)$/
    klass = safe_constantize($2)
    return klass.__send__($1.to_sym)
  when /^the ([^ ]+) "(.+)"$/
    klass = safe_constantize($1)
    instance = stuff.find_thing(:type => klass, :name => $2)
    instance.reload
    return instance
  when /^the ([^ ]+)$/
    return implicit_model($1)
  when /^"(.+)"$/
    instance = stuff[$1]
    instance.reload if instance
    return instance
  end
  raise "No such instance: '#{str}'.\n Current stuff: #{stuff.to_s}"
end

The implicit_model method

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def implicit_model(str)
   klass = safe_constantize(str)
   raise "expected only one #{klass.name}" if klass.count > 1
   raise "expected one #{klass.name} to exist" if klass.count == 0
   klass.first
end

Notice to avoid ambiguity we restrict that only one model of the specified type must exist.

Full source

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module StuffManagment
  def map_to_id_attribute(type_of_thing)
    {
      :user   => :username,
      :concert => :title,
    }[type_of_thing] || :name
  end

  class StuffContainer < Hash
    def find_thing(opts)
      expected_type = opts[:type]
      name = opts[:name]
      thing = self[name]
      raise("Unable to find any object in stuff[] with the name '#{name}' that you asked for, boss. I could however offer you one of the following: #{self.to_s}") unless thing
      raise("That thing you asked for, it appears to be a #{thing.class.name} when you asked for a #{expected_type.name}") unless thing.is_a?(expected_type)
      thing
    end

    def to_s
      result = ["#{self.length} items in total:"]
      self.each do |key, thing|
        result << "the #{thing.class.name} \"#{key}\""
      end
      result.join("\n")
    end
  end

  def clear_stuff
    @stuff = StuffContainer.new
  end

  def stuff
    return @stuff if @stuff
    clear_stuff
  end

  SEARCH_MODULES = ['']

  def identified_model(str)
    case str
    when /^the (first|last) ([^ ]+)$/
      klass = safe_constantize($2)
      return klass.__send__($1.to_sym)
    when /^the ([^ ]+) "(.+)"$/
      klass = safe_constantize($1)
      instance = stuff.find_thing(:type => klass, :name => $2)
      instance.reload
      return instance
    when /^the ([^ ]+)$/
      return implicit_model($1)
    when /^"(.+)"$/
      instance = stuff[$1]
      instance.reload if instance
      return instance
    end
    raise "No such instance: '#{str}'.\n Current stuff: #{stuff.to_s}"
  end

  def implicit_model(str)
    klass = safe_constantize(str)
    raise "expected only one #{klass.name}" if klass.count > 1
    raise "expected one #{klass.name} to exist" if klass.count == 0
    klass.first
  end

  def safe_constantize(str)
    begin
      recorded_exception = nil
      SEARCH_MODULES.each do |mod|
        begin
          return "#{mod}::#{str}".constantize
        rescue NameError => e
          recorded_exception = e
        end
      end
      error_message = "\"#{str}\" does not appear to be a valid object in the domain. Did you mean \"#{str.classify}\"?"\
                + "\nDetailed error message:\n#{recorded_exception.message}"
      raise NameError.new(error_message)
    end
  end
end

World(StuffManagment)

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