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#respond_to? not working for dynamically generated methods


It seems Object#respond_to doesn't work for dynamically generated
methods:

SomeRailsModel.respond_to? 'find_by_name' #=> false

1. Is there some way to *really* check if an object will respond to a
message, even if it will respond by a dynamically generated method?

Right now, Object#respond_to? just doesn't do what it name promises: it
doesn't check if an object responds to a message :foo, only if the
object has a :foo method defined.

Thanks,
Maurice B. Gladwell

--
Posted via http://www.ruby-forum.com/.

Alle mercoled 30 maggio 2007, Maurice Gladwell ha scritto:

> It seems Object#respond_to doesn't work for dynamically generated
> methods:

> SomeRailsModel.respond_to? 'find_by_name' #=> false

> 1. Is there some way to *really* check if an object will respond to a
> message, even if it will respond by a dynamically generated method?

> Right now, Object#respond_to? just doesn't do what it name promises: it
> doesn't check if an object responds to a message :foo, only if the
> object has a :foo method defined.

> Thanks,
> Maurice B. Gladwell

Are you sure find_by_name is a class method and not an instance method (i.e,
do you call SomeRailsModel.find_by_name or something like
SomeRailsModel.new.find_by_name)? I don't know Rails, so I may be wrong, but
to me it seems that find_by_name is an instance method. If it is so, then you
can't expect SomeRailsModel.respond_to? to return true. This has nothing to
do with the method being dynamically generated. For instance,

Array.respond_to?(:select)

gives false, because select is an instance method.

Array.new.respond_to?(:select)

gives true, instead.

If you want to know whether a class has an *instance* method without first
creating an instance of the class, you can call instance_methods on the
class. It will return an array with the names of the methods instances of the
class will have. For example:

Array.instance_methods
=>["select", "[]=", "inspect", "<<", ... ]

In your case, if find_by_name is an instance method, you can do this:

SomeRailsModel.instance_methods.include? 'find_by_name'

I hope this helps

Stefano

Stefano Crocco wrote:
> Are you sure find_by_name is a class method and not an instance method

Yes, I'm positive:

>> SomeRailsModel.respond_to? 'find_by_name'

=> false

>> SomeRailsModel.find_by_name 'foo'

=> #<SomeRailsModel:@name='foo'>

M.

--
Posted via http://www.ruby-forum.com/.

Hi,

At Wed, 30 May 2007 18:01:59 +0900,
Stefano Crocco wrote in [ruby-talk:253530]:

> In your case, if find_by_name is an instance method, you can do this:

> SomeRailsModel.instance_methods.include? 'find_by_name'

method_defined? would be better in almost cases.

--
Nobu Nakada

Maurice Gladwell wrote:
> It seems Object#respond_to doesn't work for dynamically generated
> methods:

> SomeRailsModel.respond_to? 'find_by_name' #=> false

> 1. Is there some way to *really* check if an object will respond to a
> message, even if it will respond by a dynamically generated method?

> Right now, Object#respond_to? just doesn't do what it name promises: it
> doesn't check if an object responds to a message :foo, only if the
> object has a :foo method defined.

> Thanks,
> Maurice B. Gladwell

I think 'Object#respond_to?' just looks at a list of previously
*defined* methods, as does 'method_defined?', it doesn't matter if
they're defined dynamically at runtime or not. 'find_by_name' is not
really a method in Rails, it's just the magic of method_missing that you
can call it on SomeRailsModel.

In order to find out which methods SomeRailsModel would respond to you'd
have to write a method that looks at 'method_missing' and works out all
the possibilities there. I'm by no means an expert (neither for Ruby nor
Rails), but SomeRailsModel will respond to every 'find_by_foo' method,
where foo is column of the underlying database, so your task seems way
too big for such like me..

Sascha

--
Posted via http://www.ruby-forum.com/.

Hi --

On Wed, 30 May 2007, Maurice Gladwell wrote:
> It seems Object#respond_to doesn't work for dynamically generated
> methods:

> SomeRailsModel.respond_to? 'find_by_name' #=> false

> 1. Is there some way to *really* check if an object will respond to a
> message, even if it will respond by a dynamically generated method?

> Right now, Object#respond_to? just doesn't do what it name promises: it
> doesn't check if an object responds to a message :foo, only if the
> object has a :foo method defined.

That's what responding to a method means.  Otherwise, it would be
meaningless, since you can send any message to any object.

Things like find_by_name are made possible by method_missing, which
intercepts uninterpretable messages.  It's possible for method_missing
to generate a new method dynamically, but it doesn't have to (and in
fact "find_by_name" does not get generated as a method; it just gets
examined as a string).  Even if it does, there's no way for respond_to
to figure it out.

Basically, to know what's going to happen dynamically, you have to run
the program and send the messages.

David

--
Q. What is THE Ruby book for Rails developers?
A. RUBY FOR RAILS by David A. Black (http://www.manning.com/black)
    (See what readers are saying!  http://www.rubypal.com/r4rrevs.pdf)
Q. Where can I get Ruby/Rails on-site training, consulting, coaching?
A. Ruby Power and Light, LLC (http://www.rubypal.com)

dbl@wobblini.net wrote:
> Basically, to know what's going to happen dynamically, you have to run
> the program and send the messages.

Or you have to patch rails so that respond_to? will take this case into
account. It's open source, what are you waiting for? ;-)

Daniel

On 5/30/07, dbl@wobblini.net <dbl@wobblini.net> wrote:
> Hi --

> On Wed, 30 May 2007, Maurice Gladwell wrote:

> > It seems Object#respond_to doesn't work for dynamically generated
> > methods:

> > SomeRailsModel.respond_to? 'find_by_name' #=> false

> > 1. Is there some way to *really* check if an object will respond to a
> > message, even if it will respond by a dynamically generated method?

> > Right now, Object#respond_to? just doesn't do what it name promises: it
> > doesn't check if an object responds to a message :foo, only if the
> > object has a :foo method defined.

Does not look right though

# vim: sts=2 sw=2 expandtab nu tw=0:

module M
  def m; "m" end
end
class A
  def a; "a" end
end

class B < A
  include M
end

b = B.new
def b.b; "b" end

p B.new.respond_to?(:a)
p B.new.respond_to?(:m)
p B.method_defined?(:a)
p B.method_defined?(:m)
p b.respond_to?(:b)
true
true
true
true
true

looks pretty good to me.
Is Rails fooling around with my beloved Ruby ;) ?
For what concerns intercepted messages for which no methods exist
David has said it all below:...

Cheers
Robert
--
You see things; and you say Why?
But I dream things that never were; and I say Why not?
-- George Bernard Shaw
Hi --

No, it's just using method_missing, which intercepts the
"find_by_name" message.

> For what concerns intercepted messages for which no methods exist
> David has said it all below:...

I'll go along with that :-)

David

--
Q. What is THE Ruby book for Rails developers?
A. RUBY FOR RAILS by David A. Black (http://www.manning.com/black)
    (See what readers are saying!  http://www.rubypal.com/r4rrevs.pdf)
Q. Where can I get Ruby/Rails on-site training, consulting, coaching?
A. Ruby Power and Light, LLC (http://www.rubypal.com)

David wrote:
> [...]
> Basically, to know what's going to happen dynamically, you have to run
> the program and send the messages.

It would be possible to define at writing time the entire range of
messages that an object will respond to. Here's how:

There are two kinds of messages to which an object responds by something
that is not a NoMethodError:

1) Those corresponding to defined instance methods.

2) Those that are handled in some way by method_missing.

#1 is already well defined and thus handled by #respond_to?. So if we
could just define the range of messages recognized by method_missing, we
could have an object return the entire range of messages to which it
will respond.

Of course, we don't have standard support for that currently, but we
could (e.g. with some DSL) declare inside the class definition (e.g.
above the `def method_missing`) patterns to which method_missing would
respond.

So for example, in ActiveRecord::Base we would have something like:

responds_to /find_by_.+/

Thus `SomeRailsModel.respond_to? :find_by_name` would have the knowledge
it needs to return the correct answer: true.

--
M.

--
Posted via http://www.ruby-forum.com/.

On 5/30/07, dbl@wobblini.net <dbl@wobblini.net> wrote:

> > Is Rails fooling around with my beloved Ruby ;) ?

> No, it's just using method_missing, which intercepts the
> "find_by_name" message.

Ok that is acceptable ;) thx for the clarification.

> > For what concerns intercepted messages for which no methods exist
> > David has said it all below:...

> I'll go along with that :-)

We are risking recursive mutual agreement causing stack overflow ;)
Cheers
Robert

--
You see things; and you say Why?
But I dream things that never were; and I say Why not?
-- George Bernard Shaw

Hi --

On Wed, 30 May 2007, Maurice Gladwell wrote:
> David wrote:
>> [...]
>> Basically, to know what's going to happen dynamically, you have to run
>> the program and send the messages.

> It would be possible to define at writing time the entire range of
> messages that an object will respond to. Here's how:

> There are two kinds of messages to which an object responds by something
> that is not a NoMethodError:

> 1) Those corresponding to defined instance methods.

> 2) Those that are handled in some way by method_missing.

That's every possible string you can sent to an object.

That's a bit of a conversation-stopper :-)  But I'll just observe that
"responding to a message" is really a kind of high-level, abstract way
of saying that the object has a method it can run whose name
corresponds to the message.  That's not true of "find_by_name" in the
case of an ActiveRecord::Base subclass; there is no such method, and
the only way an object can handle the message is with method_missing,
which is not the same as the message itself being understood by the
object.

You could of course shoehorn find_by_* into respond_to? for AR
objects, if you don't mind, essentially, writing method_missing twice
(once for real, once as a kind of pseudo-static twin).  But in the
general case, method_missing really means it's missing, and it's
handled at the time in ways that cannot be predicted or registered
with the system in advance.  For example:

   class C
     def method_missing(m)
       super unless rand(2).zero?
       puts "I'm handing your message!"
     end
   end

   c = C.new

Does c respond to "blah"?

In sum, the way it's engineered, with objects responding to certain
messages (in the sense of having corresponding methods), and
method_missing available to handle the missing ones, gives you more
latitude and flexibility than you'd be able to encapsulate in a
missing-method registry.

David

--
Q. What is THE Ruby book for Rails developers?
A. RUBY FOR RAILS by David A. Black (http://www.manning.com/black)
    (See what readers are saying!  http://www.rubypal.com/r4rrevs.pdf)
Q. Where can I get Ruby/Rails on-site training, consulting, coaching?
A. Ruby Power and Light, LLC (http://www.rubypal.com)

On May 30, 2007, at 7:53 AM, dbl@wobblini.net wrote:

> You could of course shoehorn find_by_* into respond_to? for AR
> objects, if you don't mind, essentially, writing method_missing twice
> (once for real, once as a kind of pseudo-static twin).

I don't really understand this stance.  My opinion is that providing  
a method_missing() implementation is a convenient way for a  
programmer to define a lot of dynamic methods.  This increases the  
messages an object responds to.

Following from that logic, I believe you should also override  
respond_to?() to reflect those new messages.  It's my opinion  
therefore that this thread has exposed a bug in Rails that should be  
patched.

As for the double implementation, I would think we would handle that  
in the usual way.  You use an Extract Method refactoring to pull out  
the matching logic into a separate private method and use that in  
both implementations.

Those are just my opinions though.

James Edward Gray II

On 5/30/07, James Edward Gray II <j@grayproductions.net> wrote:

> On May 30, 2007, at 7:53 AM, dbl@wobblini.net wrote:

> > You could of course shoehorn find_by_* into respond_to? for AR
> > objects, if you don't mind, essentially, writing method_missing twice
> > (once for real, once as a kind of pseudo-static twin).

> I don't really understand this stance.  My opinion is that providing
> a method_missing() implementation is a convenient way for a
> programmer to define a lot of dynamic methods.  This increases the
> messages an object responds to.

I have always felt that dynamic method creation and dynamic message
interception are not quite the same beast. But I guess you have some
thoughts behind your claim of which I fail to grasp the concept. Would
you mind to elaborate?

To be honest, with my lack of imagination, right now I really do not
like the idea to tweak #respond_to? .

Am I right that in the following case

class A
   def method_missing name, *args, &blk
      return whatever(name, *args, &blk)
   end
end

A.new.respond_to(x) would be true for whatever x - if it were for your idea.

Is this really a good idea?

Cheers
Robert

<snip>

> Those are just my opinions though.
Same here :)

> James Edward Gray II

Robert

--
You see things; and you say Why?
But I dream things that never were; and I say Why not?
-- George Bernard Shaw

On Wed, May 30, 2007 at 08:56:08PM +0900, Maurice Gladwell wrote:
> Of course, we don't have standard support for that currently, but we
> could (e.g. with some DSL) declare inside the class definition (e.g.
> above the `def method_missing`) patterns to which method_missing would
> respond.

> So for example, in ActiveRecord::Base we would have something like:

> responds_to /find_by_.+/

Hmm, everybody forgets to anchor their regular expressions :-)

Anyway, this doesn't seem right. find_by_foo will fail if there is no column
called 'foo'. So really you should really check that the column exists.

But that doesn't work either, when you remember that AR also supports
find_by_foo_and_bar(x,y), find_by_foo_and_bar_and_baz(x,y,z) etc.

It would be too expensive to enumerate all these possibilities up-front.
Rather, I think you need to check whether a particular method name is valid
or not. In which case, what you really want is to define a respond_to?
method in ActiveRecord::Base which has the same logic as 'find'.

class ActiveRecord::Base
  alias :respond_to :real_respond_to
  def respond_to?(m)
    return true if real_respond_to?(m)

    # now also return true if m is of the form find_xxxxxxxx and
    # xxxxxx is a valid finder pattern
  end
end

Regards,

Brian.

On May 30, 2007, at 8:34 AM, Robert Dober wrote:

I'm not really sure what else to say.  I read the documentation for  
respond_to?():

$ ri -T Object#respond_to?
----------------------------------------------------- Object#respond_to?
      obj.respond_to?(symbol, include_private=false) => true or false
------------------------------------------------------------------------
      Returns +true+> if _obj_ responds to the given method. Private
      methods are included in the search only if the optional second
      parameter evaluates to +true+.

I know that ActiveRecord model classes will except find_by_*()  
methods, so the fact that respond_to?() returns false for them makes  
the above documentation lie.  That does not seem good to me and it's  
certainly possible to fix it.  Thus, I can only reason that it is a  
bug in ActiveRecord.

> Is this really a good idea?

Is it a good idea not to?  What exactly is the use case for  
respond_to?()?  I have only ever used it to find out if something I  
want is available.  If I can't do that reliably, it doesn't really  
help me much.

James Edward Gray II

James Gray wrote:
> On May 30, 2007, at 7:53 AM, dbl@wobblini.net wrote:

>> You could of course shoehorn find_by_* into respond_to? for AR
>> objects, if you don't mind, essentially, writing method_missing twice
>> (once for real, once as a kind of pseudo-static twin).

> I don't really understand this stance.  My opinion is that providing
> a method_missing() implementation is a convenient way for a
> programmer to define a lot of dynamic methods.  This increases the
> messages an object responds to.

I must agree with James Edward Gray here.

David's contrived example is of course possible, but highly unlikely in
practice; a programmer will generally not code his program into a
situation in which he can't be sure whether an object will respond to a
particular message sent to it. That's probably a major reason for having
Object#respond_to?.

In practice, and certainly in the Rails' find_by_* case, you have a
well-defined  range of messages to which an object will respond, known
already at development time. I don't know of a single real-life example
in which a programmer doesn't know at development time the entire range
of messages his objects might respond to. And Rails - with all its magic
- notwithstanding.

--
M.

(Of course, even if there would be an exception to that rule, in which
we really cannot delimit - for reasons unimaginable - the range of
messages to which the object will respond to at runtime - that rare and
probably bizarre case would not obviate the usefulness of the other
99.99% more common cases.)

--
Posted via http://www.ruby-forum.com/.

As an edge case, consider a DRb client proxy object. Should respond_to? be
handled locally, or should it be proxied to the server? This involves an
expensive network round trip.

In many cases it may be cheaper to attempt the call and to rescue a
NoMethodError, than to have 'respond_to?' perform almost as much work as the
call you're testing.

Regards,

Brian.

On May 30, 2007, at 9:08 AM, Brian Candler wrote:

I agree with everything you just said and it still doesn't convince  
me respond_to?() should lie.  ;)  Whether or not we use respond_to?()  
in a given situation seems like an entirely different issue to me.

James Edward Gray II

Hi --

But it can happen; hence NoMethodError.

> In practice, and certainly in the Rails' find_by_* case, you have a
> well-defined  range of messages to which an object will respond, known
> already at development time. I don't know of a single real-life example
> in which a programmer doesn't know at development time the entire range
> of messages his objects might respond to. And Rails - with all its magic
> - notwithstanding.

If it's absolutely known what messages every object will respond to,
then we don't need #respond_to? :-)  I don't know... it's obviously
just a slightly different way of looking at it.  I'm uneasy with
having to rewrite respond_to? every time I implement method_missing;
it seems like a lot of work and an awfully tight coupling.

David

--
Q. What is THE Ruby book for Rails developers?
A. RUBY FOR RAILS by David A. Black (http://www.manning.com/black)
    (See what readers are saying!  http://www.rubypal.com/r4rrevs.pdf)
Q. Where can I get Ruby/Rails on-site training, consulting, coaching?
A. Ruby Power and Light, LLC (http://www.rubypal.com)

Brian Candler wrote:
> In many cases it may be cheaper to attempt the call and to rescue a
> NoMethodError, than to have 'respond_to?' perform almost as much work as
> the
> call you're testing.

That's certinly true, but the rescue technique you describe is not a
substitute for a working #respond_to?. You're actually sending the
message, not asking the object whether it will respond to it.

These are two completely different operations. The only relevant
solution here would be if you aspired to emulate Object#respond_to? by
checking if the call raises NoMethodError, but that's infeasible because
method call commonly have side effects and/or are destructive.

--
M.

--
Posted via http://www.ruby-forum.com/.

On Wed, May 30, 2007 at 11:40:57PM +0900, Maurice Gladwell wrote:
> Brian Candler wrote:
> > In many cases it may be cheaper to attempt the call and to rescue a
> > NoMethodError, than to have 'respond_to?' perform almost as much work as
> > the
> > call you're testing.

> That's certinly true, but the rescue technique you describe is not a
> substitute for a working #respond_to?. You're actually sending the
> message, not asking the object whether it will respond to it.

> These are two completely different operations. The only relevant
> solution here would be if you aspired to emulate Object#respond_to? by
> checking if the call raises NoMethodError, but that's infeasible because
> method call commonly have side effects and/or are destructive.

I'm saying that if you're doing

    object.foo if object.respond_to? :foo

then you may as well do

    object.foo rescue nil

(or a more specific rescue for NoMethodError)

I guess there are other uses for respond_to?, but I very rarely use it -
especially with ActiveRecord, which was the example given. Why do you want
to check that an ActiveRecord class responds usefully to
find_by_foo_and_bar, but not actually call find_by_foo_and_bar?

Regards,

Brian.

James Gray wrote:

> I don't really understand this stance.  My opinion is that providing
> a method_missing() implementation is a convenient way for a
> programmer to define a lot of dynamic methods.  This increases the
> messages an object responds to.

> Following from that logic, I believe you should also override
> respond_to?() to reflect those new messages.  It's my opinion
> therefore that this thread has exposed a bug in Rails that should be
> patched.

FWIW, there's an entry about this in the Ruby Style Guide on
rubygarden.org:
http://wiki.rubygarden.org/Ruby/page/show/RubyStyleGuide/RespondToGoe...

--
Posted via http://www.ruby-forum.com/.

David wrote:
> If it's absolutely known what messages every object will respond to,
> then we don't need #respond_to? :-)

#respond_to? is usually used when we don't know exactly which object is
referenced by a particular name, and especially what it can do ("duck
typing"). So we check if `foo.respond_to? :to_s` to see if we can tell
it to convert itself to a String.

But if we do know that foo is, say, an Array, we should - as the system
programmers - know which messages it will respond_to and how. It's
generally never the case that we know which object we have, yet are not
sure which messages it will respond to.

> I don't know... it's obviously
> just a slightly different way of looking at it.  I'm uneasy with
> having to rewrite respond_to? every time I implement method_missing;
> it seems like a lot of work and an awfully tight coupling.

Yes, that is true. Everyone would be happy (some much happier, others a
tiny bit happier :-) if we had a really working #respond_to? that would
cover all cases. But implementing such a thing elegantly seems
non-trivial.

It would be interesting to see an implementation of James Edward Gray's
suggestion above. ("You use an Extract Method refactoring to pull out
the matching logic into a separate private method and use that in
both implementations.")

--
M.

--
Posted via http://www.ruby-forum.com/.

Hi --

Hmmm... perhaps there needs to be a second entry explaining why it's
better not to put oneself in the position of having to maintain two
almost identical methods in parallel instead of one :-)

David

--
Q. What is THE Ruby book for Rails developers?
A. RUBY FOR RAILS by David A. Black (http://www.manning.com/black)
    (See what readers are saying!  http://www.rubypal.com/r4rrevs.pdf)
Q. Where can I get Ruby/Rails on-site training, consulting, coaching?
A. Ruby Power and Light, LLC (http://www.rubypal.com)

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