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7stud --

2011-Jul-25 11:12 UTC

What does using a lambda in rspec tests accomplish?

Here is the code in question:


describe UsersController do
  render_views
  …
  …

  describe "POST to ''create'' action" do
    describe "failure" do
      before(:each) do
        @attr = { :name => '''', :email =>
'''', :password => '''',
                         :password_confirmation => '''' }
      end

      it "should not create a user" do
        lambda {
          post :create, :user => @attr
        }.should_not change(User, :count)
      end

      it "should have the right title" do
        post :create, :user => @attr
        response.should have_selector(:title, :content => ''Sign
up'')
      end




Comparing the last two tests, it looks to me like:



       lambda {
          post :create, :user => @attr
        }.should_not change(User, :count)

should be equivalent to:

        post :create, :user => @attr
        response.should_not change(User, :count)


But when I make that change, the test fails with this rror:



Failures:

  1) UsersController Post create failure should not create a user
     Failure/Error: response.should_not change(User, :count)
     NoMethodError:
       undefined method `call'' for
#<ActionController::TestResponse:0x00000100f3d3a0>
     # ./spec/controllers/users_controller_spec.rb:62:in `block (4
levels) in <top (required)>''

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John Feminella

2011-Jul-25 12:51 UTC

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Re: What does using a lambda in rspec tests accomplish?

First, the test fails because it expects a Proc -- it''s trying to
invoke #call. So you can''t do what you''re proposing without
writing a
new definition for how #change works.

As a deeper answer, your "shouldn''t these be equivalent?"
statement is
not accurate. RSpec, like any piece of Ruby code, needs to know when
it should start paying attention for the change(). In other words, it
needs a "before" to start with and an "after" to compare to.
With a
lambda, this is accomplished by measuring the thing you want to check
before the lambda is run, and then again after it''s run. If the
difference matches what you expected, the test passes. If it doesn''t
it fails.

Your example is asking RSpec to do this:

before.val = 5
before.val = 6
before.should change(:val).from(5).to(6)

How can it know that `before.val` used to be 5? Answer: it can''t. By
the time you reach the third line, the old value of `before.val` is
gone forever.

~ jf
--
John Feminella
Principal Consultant, BitsBuilder
LI: http://www.linkedin.com/in/johnxf
SO: http://stackoverflow.com/users/75170/



On Mon, Jul 25, 2011 at 07:12, 7stud --
<lists-fsXkhYbjdPsEEoCn2XhGlw@public.gmane.org>
wrote:> Here is the code in question:
>
>
> describe UsersController do
>  render_views
>  …
>  …
>
>  describe "POST to ''create'' action" do
>    describe "failure" do
>      before(:each) do
>        @attr = { :name => '''', :email =>
'''', :password => '''',
>                         :password_confirmation => '''' }
>      end
>
>      it "should not create a user" do
>        lambda {
>          post :create, :user => @attr
>        }.should_not change(User, :count)
>      end
>
>      it "should have the right title" do
>        post :create, :user => @attr
>        response.should have_selector(:title, :content => ''Sign
up'')
>      end
>
>
>
>
> Comparing the last two tests, it looks to me like:
>
>
>
>       lambda {
>          post :create, :user => @attr
>        }.should_not change(User, :count)
>
> should be equivalent to:
>
>        post :create, :user => @attr
>        response.should_not change(User, :count)
>
>
> But when I make that change, the test fails with this rror:
>
>
>
> Failures:
>
>  1) UsersController Post create failure should not create a user
>     Failure/Error: response.should_not change(User, :count)
>     NoMethodError:
>       undefined method `call'' for
> #<ActionController::TestResponse:0x00000100f3d3a0>
>     # ./spec/controllers/users_controller_spec.rb:62:in `block (4
> levels) in <top (required)>''
>
> --
> Posted via http://www.ruby-forum.com/.
>
> --
> You received this message because you are subscribed to the Google Groups
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> To post to this group, send email to
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>
>
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7stud --

2011-Jul-25 22:40 UTC

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Re: What does using a lambda in rspec tests accomplish?

John Feminella wrote in post #1012869:> In other words, it
> needs a "before" to start with and an "after" to
compare to. With a
> lambda, this is accomplished by measuring the thing you want to check
> before the lambda is run, and then again after it''s run.
>
Okay, I sort of sussed that out myself.  But the question remains, how 
can should_not() expect a lambda in one case, and then successfully 
operate on a String in another case, like here:


@user.encrypted_password.should_not be_blank



 Does the definition of should_not check the type of the receiver before 
executing?  I can''t find any docs for the definition of should_not().

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David Chelimsky

2011-Jul-25 23:18 UTC

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Re: What does using a lambda in rspec tests accomplish?

On Jul 25, 6:40 pm, 7stud --
<li...-fsXkhYbjdPsEEoCn2XhGlw@public.gmane.org>
wrote:> John Feminella wrote in post #1012869:
>
> > In other words, it
> > needs a "before" to start with and an "after" to
compare to. With a
> > lambda, this is accomplished by measuring the thing you want to check
> > before the lambda is run, and then again after it''s run.
>
> Okay, I sort of sussed that out myself.  But the question remains, how
> can should_not() expect a lambda in one case, and then successfully
> operate on a String in another case, like here:
>
> @user.encrypted_password.should_not be_blank
>
>  Does the definition of should_not check the type of the receiver before
> executing?  I can''t find any docs for the definition of
should_not().
should and should_not are both added to every object, and pass self
(the receiver) to the matches?() method on the matcher passed to
should[_not]. e.g.

  obj.should matcher # => matcher.matches?(obj)

It is the matcher that cares what the object is, not the should[_not]
method. And the matchers do not, generally, do type checking on the
object - they just assume that you''re doing the right thing, and let
Ruby handle errors if you do the wrong thing (e.g. undefined method
''call'').

HTH,
David

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7stud --

2011-Jul-26 08:08 UTC

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Re: What does using a lambda in rspec tests accomplish?

David Chelimsky wrote in post #1013001:> On Jul 25, 6:40pm, 7stud --
<li...-fsXkhYbjdPsEEoCn2XhGlw@public.gmane.org> wrote:
>>
>> @user.encrypted_password.should_not be_blank
>>
>> Does the definition of should_not check the type of the receiver before
>> executing? I can''t find any docs for the definition of
should_not().
>
> should and should_not are both added to every object, and pass self
> (the receiver) to the matches?() method on the matcher passed to
> should[_not]. e.g.
>
>   obj.should matcher # => matcher.matches?(obj)
>
> It is the matcher that cares what the object is, not the should[_not]
> method.
>
Thanks for that.  I was able to find some more details based on your
post.  The lambda construct in rspec can be simplified like this:

a_function.should_not( change(User, :count )

The change() method simply returns a Change ''matcher'' object:

def change(a_class, message)  #(User, :count)
  Change.new(a_class, message)  #(User, :count)
end

where the Change class is defined something like this:

class Change
  def initialize(a_class, message) #(User, :count)
    @receiver = a_class
    @msg = message
  end

  def matches?(callable)  #(a_function)
    before = @receiver.send(msg)  #same as calling User.count()
    callable.call        #execute a_function()
    after = @receiver.send(msg) #same as calling User.count()

    before == after
  end

  ...
  ...
end

should_not() is defined something like this:

def should_not(obj)
  result = obj.matches?(self)
  ...
  ...
end

For references, see:

1) http://rspec.rubyforge.org/rspec/1.1.9/classes/Spec/Expectations.html

===Spec::Expectations adds two methods to Object:

  should(matcher=nil)
  should_not(matcher=nil)

Both methods take an optional Expression Matcher (See Spec::Matchers).

When should receives an Expression Matcher, it calls matches?(self)**.
If it returns true, the spec passes and execution continues. If it
returns false, then the spec fails with the message returned by
matcher.failure_message.

Similarly, when should_not receives a matcher, it calls matches?(self).
If it returns false, the spec passes and execution continues. If it
returns true, then the spec fails with the message returned by
matcher.negative_failure_message.

RSpec ships with a standard set of useful matchers, and writing your own
matchers is quite simple. See Spec::Matchers for details.
====
**That can be made more clear:

When should() receives an Expression Matcher, it calls
expr_matcher.matches?(self):

def should(expr_matcher)
  result = expr_matcher.matches?(self)
  …
  …
end

self is the object calling the should() method.

If matches?() returns true, the spec passes and execution continues. If
it returns false, then the spec fails with the message returned by
expr_matcher.failure_message.

2)
http://www.robertsosinski.com/2008/12/10/up-and-running-with-custom-rspec-matchers/

However, that tutorial contains this passage:

==3. Within the heart of our matcher is the matches? method. This method
takes one parameter, the actual value of the object that the should or
should_not method is passed to within your RSpec test.
==
That sentence is faulty ruby speak, and therefore doesn''t make any
sense.  It should say,

==3.
…
…
This method takes one parameter, the actual value of the object that the
should or should_not *message* is passed to within your RSpec test.
==
But in ruby, that simply means one thing:

==This method takes one parameter: the receiver of the should() or
should_not method call() (where the receiver is the ''thing''
that is
calling the method).
==
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7stud --

2011-Jul-26 08:30 UTC

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Re: What does using a lambda in rspec tests accomplish?

Similarly, calling should_not on a string works like this:

  string.should_not(be_blank)

  def be_blank
    Blank.new
  end

  class Blank
    def matches?(string)
       @actual = string

        string.blank?
     end

     ...
     ...
  end

And as David Chelimsky pointed out it is change() and be_blank() that 
care about what type of the object calling should and should_not--the 
object is either the correct type, or an error occurs.

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7stud --

2011-Jul-26 08:50 UTC

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Re: What does using a lambda in rspec tests accomplish?

Whoops.  This:

==where the Change class is defined something like this:

  class Change
    def initialize(a_class, message) #(User, :count)
      @receiver = a_class
      @msg = message
    end

    def matches?(callable)  #(a_function)
      before = @receiver.send(msg)  #same as calling User.count()
      callable.call        #execute a_function()
      after = @receiver.send(msg) #same as calling User.count()

      before == after
    end

    ...
    ...
  end
==
should look more like this:


where the Change class is defined something like this:

  class Change
    def initialize(a_class, message) #(User, :count)
      @receiver = a_class
      @msg = message
    end

    def matches?(callable)  #(a_function)
      before = @receiver.send(@msg)  #same as calling User.count()
      callable.call        #execute a_function()
      after = @receiver.send(@msg) #same as calling User.count()

      before == after
    end

    ...
    ...
  end

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