Rails - Sep 2007 - how do i check if an object has a particular field?

an easy but hard-to-google question - how can i check whether an object
has a field called "complete"?
-- 
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Is your context a Model: meaning it has the field at all, as in it
exists in the database, or whether the field has been filled in a
form?

Or some other context?

On Sep 6, 5:12 pm, Max Williams
<rails-mailing-l...-ARtvInVfO7ksV2N9l4h3zg@public.gmane.org>
wrote:
> an easy but hard-to-google question - how can i check whether an object
> has a field called "complete"?
> --
> Posted viahttp://www.ruby-forum.com/.

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Mindtonic wrote:
> Is your context a Model: meaning it has the field at all, as in it
> exists in the database, or whether the field has been filled in a
> form?
> 
> Or some other context?
> 
> On Sep 6, 5:12 pm, Max Williams
<rails-mailing-l...-ARtvInVfO7ksV2N9l4h3zg@public.gmane.org>
A model:  i have a method that takes an object that can be either a 
story or an article.  Story has a boolean field called
''complete'',
article doesn''t.  I want to check that the object has the field before
i
query the contents of the field.

thanks
max
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This is a good one.  I thought I might be able to help.  I tried lots
of things in the console and searched through the api, but everything
yields an error when the object does not have the requested property.
I tried things like blank? and empty? but got the same results.  I
looked for something like object.properties to match object.methods
thinking you could test against the result but that does not seem to
exist either.  That one would be the most handy, it does exist in the
php reflections.

Are you using this particular property to determine which type of
model the method is receiving?  If so, you could use either
object.class == Article or object.is_a?(Article) ... something like
that.  Sorry I couldn''t be more help than that!

On Sep 6, 5:27 pm, Max Williams
<rails-mailing-l...-ARtvInVfO7ksV2N9l4h3zg@public.gmane.org>
wrote:
> Mindtonic wrote:
> > Is your context a Model: meaning it has the field at all, as in it
> > exists in the database, or whether the field has been filled in a
> > form?
>
> > Or some other context?
>
> > On Sep 6, 5:12 pm, Max Williams
<rails-mailing-l...-ARtvInVfO7ksV2N9l4h3zg@public.gmane.org>
>
> A model:  i have a method that takes an object that can be either a
> story or an article.  Story has a boolean field called
''complete'',
> article doesn''t.  I want to check that the object has the field
before i
> query the contents of the field.
>
> thanks
> max
> --
> Posted viahttp://www.ruby-forum.com/.

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On Sep 6, 2007, at 1:27 PM, Mindtonic wrote:
>
> On Sep 6, 5:27 pm, Max Williams
<rails-mailing-l...-ARtvInVfO7ksV2N9l4h3zg@public.gmane.org>
> wrote:
>> Mindtonic wrote:
>>> Is your context a Model: meaning it has the field at all, as in it
>>> exists in the database, or whether the field has been filled in a
>>> form?
>>
>>> Or some other context?
>>
>>> On Sep 6, 5:12 pm, Max Williams
<rails-mailing-l...-ARtvInVfO7ksV2N9l4h3zg@public.gmane.org>
>>
>> A model:  i have a method that takes an object that can be either a
>> story or an article.  Story has a boolean field called
''complete'',
>> article doesn''t.  I want to check that the object has the
field
>> before i
>> query the contents of the field.
>>
>> thanks
>> max
>> --
>> Posted viahttp://www.ruby-forum.com/.
>
> This is a good one.  I thought I might be able to help.  I tried lots
> of things in the console and searched through the api, but everything
> yields an error when the object does not have the requested property.
> I tried things like blank? and empty? but got the same results.  I
> looked for something like object.properties to match object.methods
> thinking you could test against the result but that does not seem to
> exist either.  That one would be the most handy, it does exist in the
> php reflections.
>
> Are you using this particular property to determine which type of
> model the method is receiving?  If so, you could use either
> object.class == Article or object.is_a?(Article) ... something like
> that.  Sorry I couldn''t be more help than that!
Shouldn''t you be able to say:
	object.respond_to?(:complete)

You can also duck-type it:
	class Article
	  def complete
	    nil    # or true depending on your semantics
	  end
	end

Then you don''t need to check first.

-Rob

Rob Biedenharn		http://agileconsultingllc.com
Rob-xa9cJyRlE0mWcWVYNo9pwxS2lgjeYSpx@public.gmane.org



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Howdy,

Why can''t #instance_variables work here?  It''s not what I
would do but looks
like it can be used to return an array of an object''s instance
variables.

Alternatives:
- already mentioned #is_a?
- base class for both objects with appropriate behavior
- method_missing magic on Article to respond appropriately (could get
sticky)

Cheers,
Mel

On 9/6/07, Mindtonic <mindtonic-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>
wrote:
>
>
> This is a good one.  I thought I might be able to help.  I tried lots
> of things in the console and searched through the api, but everything
> yields an error when the object does not have the requested property.
> I tried things like blank? and empty? but got the same results.  I
> looked for something like object.properties to match object.methods
> thinking you could test against the result but that does not seem to
> exist either.  That one would be the most handy, it does exist in the
> php reflections.
>
> Are you using this particular property to determine which type of
> model the method is receiving?  If so, you could use either
> object.class == Article or object.is_a?(Article) ... something like
> that.  Sorry I couldn''t be more help than that!
>
> On Sep 6, 5:27 pm, Max Williams
<rails-mailing-l...-ARtvInVfO7ksV2N9l4h3zg@public.gmane.org>
> wrote:
> > Mindtonic wrote:
> > > Is your context a Model: meaning it has the field at all, as in
it
> > > exists in the database, or whether the field has been filled in a
> > > form?
> >
> > > Or some other context?
> >
> > > On Sep 6, 5:12 pm, Max Williams
<rails-mailing-l...-ARtvInVfO7ksV2N9l4h3zg@public.gmane.org>
> >
> > A model:  i have a method that takes an object that can be either a
> > story or an article.  Story has a boolean field called
''complete'',
> > article doesn''t.  I want to check that the object has the
field before i
> > query the contents of the field.
> >
> > thanks
> > max
> > --
> > Posted viahttp://www.ruby-forum.com/.
>
>
> >
>
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On Sep 6, 2007, at 10:12 AM, Max Williams wrote:
> an easy but hard-to-google question - how can i check whether an  
> object
> has a field called "complete"?


the_object.respond_to?("complete")

Should work.


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George Bailey wrote:
> On Sep 6, 2007, at 10:12 AM, Max Williams wrote:
> 
>> an easy but hard-to-google question - how can i check whether an  
>> object
>> has a field called "complete"?
> 
> 
> 
> the_object.respond_to?("complete")
> 
> Should work.
yep, that''s what i''m after!  thanks everyone :)
-- 
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Max Williams wrote:
> an easy but hard-to-google question - how can i check whether an object
> has a field called "complete"?

Model.column_names
>> User.column_names
=> ["id", "username", "password",
"name", "email", "showemail",
"notifications", "age", "sex",
"location_id", "interests", "music",
"comments", "image_id", "status",
"lastlogin", "lastip", "session_id",
"loggedin"]
-- 
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On Sep 6, 2007, at 4:14 PM, Matthew Rudy wrote:
> Max Williams wrote:
>> an easy but hard-to-google question - how can i check whether an  
>> object
>> has a field called "complete"?
>
> Model.column_names
>
>>> User.column_names
> => ["id", "username", "password",
"name", "email", "showemail",
> "notifications", "age", "sex",
"location_id", "interests", "music",
> "comments", "image_id", "status",
"lastlogin", "lastip", "session_id",
> "loggedin"]
Which answers the slightly different question:  Does the database  
sitting under this Model contain a column called "complete"?

The OP asked about an object (and in particular and ActiveRecord  
object), so respond_to? is probably a more apropos answer.   
ActiveRecord overrides respond_to? so that it is true for attributes  
which may actually be handled by method_missing when actually called.

Consider if the OP had:
	class Article
	  def complete
	    read_attribute(:finished)
	  end
	end

In that case, Article.column_names.include?("complete") == false
even though Article.new.respond_to?("complete") == true

-Rob

Rob Biedenharn		http://agileconsultingllc.com
Rob-xa9cJyRlE0mWcWVYNo9pwxS2lgjeYSpx@public.gmane.org



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On Thu, 6 Sep 2007 16:34:02 -0400, Rob Biedenharn wrote:
> The OP asked about an object (and in particular and ActiveRecord  
> object), so respond_to? is probably a more apropos answer.   
> ActiveRecord overrides respond_to? so that it is true for attributes  
> which may actually be handled by method_missing when actually called.
Wasn''t there a recent change to core that may be important here?  I
remember reading something about cases where, in 1.2.3, respond_to? would
be false because AR hadn''t looked at the table yet, while Edge would
correctly respond true.  Or something like that.

Jay Levitt


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Rob Biedenharn wrote:
> On Sep 6, 2007, at 4:14 PM, Matthew Rudy wrote:
>> "comments", "image_id", "status",
"lastlogin", "lastip", "session_id",
>> "loggedin"]
> 
> Which answers the slightly different question:  Does the database
> sitting under this Model contain a column called "complete"?
> 
> The OP asked about an object (and in particular and ActiveRecord
> object), so respond_to? is probably a more apropos answer.
> ActiveRecord overrides respond_to? so that it is true for attributes
> which may actually be handled by method_missing when actually called.
Max originally asked "how can i check whether an object has a field 
called ''complete''?" -- namely he is talking about the
database field.

but equally,
I like the respond_to solutions,
just thought, if we strictly want to look at "fields", that it was 
useful to point  this out.
-- better than calling "attributes.include?(field)" I feel
-- 
Posted via http://www.ruby-forum.com/.

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Matthew Rudy wrote:
> Rob Biedenharn wrote:
>> On Sep 6, 2007, at 4:14 PM, Matthew Rudy wrote:
>>> "comments", "image_id", "status",
"lastlogin", "lastip", "session_id",
>>> "loggedin"]
>> 
>> Which answers the slightly different question:  Does the database
>> sitting under this Model contain a column called "complete"?
>> 
>> The OP asked about an object (and in particular and ActiveRecord
>> object), so respond_to? is probably a more apropos answer.
>> ActiveRecord overrides respond_to? so that it is true for attributes
>> which may actually be handled by method_missing when actually called.
> 
> Max originally asked "how can i check whether an object has a field 
> called ''complete''?" -- namely he is talking about
the database field.
> 
> but equally,
> I like the respond_to solutions,
> just thought, if we strictly want to look at "fields", that it
was
> useful to point  this out.
> -- better than calling "attributes.include?(field)" I feel
Yeah, this is a useful distinction to know about, thanks Rob (and 
everyone else).  In this particular case responds_to is fine, but it''s 
good to know about column_names.include? as well, since it''s actually a
bit more ''accurate'' like you say.
-- 
Posted via http://www.ruby-forum.com/.

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