Dynamic classes

Verifying instance doubles do not support methods which the class reports to not exist since an actual instance of the class would be required to verify against. This is commonly the case when method_missing is used.

There are a few ways to work around this. If the object has already been loaded you may consider using an object_double, but that cannot work if you are testing in isolation. Alternatively you could implement the methods directly (calling super to return the method_missing definition).

Some of these classes may have methods to define these methods on the objects at runtime. (For example, ActiveRecord does this to define methods from database columns.) For these cases we provide an API which can be used to customise verifying doubles on creation. We use this ourselves in rspec-rails to set up some niceties for you.

These types of methods are supported at class level (with class_double) however, since respond_to? can be queried directly on the class.

Background

Given a file named "lib/fake_active_record.rb" with:

class FakeActiveRecord
  COLUMNS = %w[name email]

  def respond_to_missing?(method_name)
    COLUMNS.include?(method_name.to_s) || super
  end

  def method_missing(method_name, *args)
    if respond_to?(method_name)
      instance_variable_get("@#{method_name}")
    else
      super
    end
  end

  def self.define_attribute_methods
    COLUMNS.each do |name|
      define_method(name) { instance_variable_get("@#{name}") }
    end
  end
end

Given a file named "spec/user_spec.rb" with:

require 'user'

RSpec.describe User do
  it 'can be doubled' do
    instance_double("User", :name => "Don")
  end
end

Fails with method missing

Given a file named "lib/user.rb" with:

require 'fake_active_record'

class User < FakeActiveRecord
end

When I run rspec spec/user_spec.rb

Then the output should contain "1 example, 1 failure".

Workaround with explicit definitions

Given a file named "lib/user.rb" with:

require 'fake_active_record'

class User < FakeActiveRecord
  def name;  super end
  def email; super end
end

When I run rspec spec/user_spec.rb

Then the examples should all pass.

Workaround using callback

Given a file named "lib/user.rb" with:

require 'fake_active_record'

class User < FakeActiveRecord
end

And a file named "spec/fake_record_helper.rb" with:

RSpec.configuration.mock_with(:rspec) do |config|
  config.before_verifying_doubles do |reference|
    reference.target.define_attribute_methods
  end
end
#
# or you can use:
#
# RSpec::Mocks.configuration.before_verifying_doubles do |reference|
#   reference.target.define_attribute_methods
# end

When I run rspec -r fake_record_helper spec/user_spec.rb

Then the examples should all pass.