DRYing out model specs

[Updated 14/3/07: corrected specify_attributes as per Paul’s comment]
[Updated 18/12/07: modified to avoid crazy RSpec errors]

A week or so ago I wrote about writing specs for simple pieces of functionality (particularly those that are arguably just configuration, like Rails validations). I argued that it’s important to test-drive even the simple things – however, the amount of test code can get out of hand.

Here’s a spec for the single validation from the previous post, rewritten using one expectation per example:

def valid_attrs
  {
    :username => 'fred'
  }
end

describe "A user without a username" do
  setup do
    @user = User.new
    @user.attributes = valid_user_attributes.except(:username)
  end
 
  it "should be invalid" do
    @user.should_not_be_valid
  end
 
  it "should have an error on the username attribute" do
    @user.errors.on(:username).size.should == 1
  end
end

And here’s the massive amount of code that satisfies that spec:

class User 
Clearly once you have a few more attributes you end up with a lot of test code, much of which is repetitive.
I decided to try to create a helper method (see below) to reduce the clutter. Here's how to use it to specify that:

The username, first_name and last_name attributes are mandatory
The age attribute must be numeric
The email attribute must be at least three characters long
The username, first_name, last_name and email attributes must be no more than 12, 20, 20 and 200 characters long respectively

def valid_attrs
  {
    :username => 'fred',
    :first_name => 'Fred',
    :last_name => 'Bloggs',
    :email => 'fred@bloggs.com',
    :age => 21
  }
end

specify_attributes User, valid_attrs,
{
  :mandatory => [:username, :first_name, :last_name],
  :numeric => [:age],
  :min_lengths => {:email => 3},
  :max_lengths => {:username => 12, :first_name => 20, :last_name => 20, :email => 200}
}

This automatically executes the following specs:
A User with all attributes set

- should be valid
A User with no username

- should be invalid

- should not allow saving

- should cause exactly one error when save attempted

- should set an error on the username field
A User with no first_name

- should be invalid

- should not allow saving

- should cause exactly one error when save attempted

- should set an error on the first_name field
A User with no last_name

- should be invalid

- should not allow saving

- should cause exactly one error when save attempted

- should set an error on the last_name field
A User with non-numeric age

- should be invalid

- should not allow saving

- should cause exactly one error when save attempted

- should set an error on the age field
A User with email of length 3

- should be valid
A User with email of length 2

- should be invalid

- should not allow saving

- should cause exactly one error when save attempted

- should set an error on the email field
A User with last_name of length 20

- should be valid
A User with last_name of length 21

- should be invalid

- should not allow saving

- should cause exactly one error when save attempted

- should set an error on the last_name field
A User with username of length 12

- should be valid
A User with username of length 13

- should be invalid

- should not allow saving

- should cause exactly one error when save attempted

- should set an error on the username field
A User with first_name of length 20

- should be valid
A User with first_name of length 21

- should be invalid

- should not allow saving

- should cause exactly one error when save attempted

- should set an error on the first_name field
A User with email of length 200

- should be valid
A User with email of length 201

- should be invalid

- should not allow saving

- should cause exactly one error when save attempted

- should set an error on the email field
Here's the code that makes it possible (I added it to spec_helper.rb).
First the ubiquitous except Hash extension:
class Hash
  # Usage { :a => 1, :b => 2, :c => 3}.except(:a) -> { :b => 2, :c => 3}
  def except(*keys)
    self.reject { |k,v|
      keys.include? k.to_sym
    }
  end
end

A couple of local helper methods (I factored out should_cause_error_on from the repetitive specs for validation failure):
def string_of_length(length)
  (1..length).collect {'a'}.join
end

def should_cause_error_on(attr)
  it "should be invalid" do
    @obj.should_not be_valid
  end

  it "should not allow saving" do
    @obj.save.should be_false
  end

  it "should cause exactly one error when save attempted" do
    @obj.save
    @obj.errors.size.should == 1
  end

  it "should set an error on the #{attr} field" do
    @obj.errors_on(attr).size.should == 1
  end
end

And finally the actual specify_attributes method:
def specify_attributes(clazz, attr_list, options = {})
  label = "#{clazz =~ /^aeiouy/i ? 'An' : "A"} #{clazz}"
  describe "#{label} with all attributes set", :type => 'model' do
    setup do
      @obj = clazz.new
      @obj.attributes = attr_list
    end

    it "should be valid" do
      @obj.should be_valid
    end
  end

  if options[:mandatory]
    options[:mandatory].each do |attr|

      describe "#{label} with no #{attr}", :type => 'model' do
        setup do
          @obj = clazz.new
          @obj.attributes = valid_attrs.except attr
        end
        
        should_cause_error_on attr
      end
    end
  end

  if options[:numeric]
    options[:numeric].each do |attr|
      describe "#{label} with non-numeric #{attr}", :type => 'model' do
        setup do
          @obj = clazz.new
          @obj.attributes = valid_attrs.except attr
          eval("@obj.#{attr} = string_of_length valid_attrs[attr].to_s.size")
        end

        should_cause_error_on attr
      end
    end
  end
  
  if options[:min_lengths]
    options[:min_lengths].keys.each do |attr|
      limit = options[:min_lengths][attr]

      describe "#{label} with #{attr} of length #{limit}", :type => 'model' do
        setup do
          @obj = clazz.new
          @obj.attributes = valid_attrs.except attr
          eval("@obj.#{attr} = string_of_length limit")
        end

        it "should be valid" do
          @obj.should be_valid
        end
      end

      describe "#{label} with #{attr} of length #{limit - 1}", :type => 'model' do
        setup do
          @obj = clazz.new
          @obj.attributes = valid_attrs.except attr
          eval("@obj.#{attr} = string_of_length(limit - 1)")
        end

        should_cause_error_on attr
      end
    end
  end

  if options[:max_lengths]
    options[:max_lengths].keys.each do |attr|
      limit = options[:max_lengths][attr]

      describe "#{label} with #{attr} of length #{limit}", :type => 'model' do
        setup do
          @obj = clazz.new
          @obj.attributes = valid_attrs.except attr
          eval("@obj.#{attr} = string_of_length limit")
        end

        it "should be valid" do
          @obj.should be_valid
        end
      end

      describe "#{label} with #{attr} of length #{limit + 1}", :type => 'model' do
        setup do
          @obj = clazz.new
          @obj.attributes = valid_attrs.except attr
          eval("@obj.#{attr} = string_of_length(limit + 1)")
        end

        should_cause_error_on attr
      end
    end
  end
end

It ought to be possible to extend this to other validations too, as well as the various options like :message, :on, :allow_nil and so on, but that's for another day.
[tags]tdd, bdd, agile, rails, rspec, dry[/tags]