# Dynamic method access from an Object

**URL:** <https://www.ruby-forum.com/t/dynamic-method-access-from-an-object/131015>\
**Category:** Rails\
**Created:** [March 5, 2008, 7:23am UTC](https://www.ruby-forum.com/t/dynamic-method-access-from-an-object/131015 "2008-03-05T07:23:08Z")\
**Posts on this page:** 6\
**Page:** 1

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**Author:** ![vasan\_cec](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/v/258eb7/32.png) [@vasan\_cec](https://www.ruby-forum.com/u/vasan_cec)\
**Post date:** [March 5, 2008, 7:23am UTC](https://www.ruby-forum.com/t/dynamic-method-access-from-an-object/131015/1 "2008-03-05T07:23:08Z")

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Hi Everyone,  
I have a requirement where the method name of an Object will be  
generated and accessed dynamically in code. When I tried to access it  
showed me “undefined method”. Am attaching a sample code of what i want  
to accomplish:

@user\_information = UserInformation.new  
@table\_fields = UserInformation.find\_by\_sql(“DESC user\_informations”)  
for table\_field in @table\_fields  
temp = table\_field.Field # considering Field is `user_name`  
@user\_information.temp = “vasanth”  
# referring to @user\_information.user\_name = “vasanth”  
end

In the above example i will get an error like "undefined method `temp`"

Can anyone tell me how to do this in Ruby on Rails? (I need something  
equivalent to “eval()” function present in Java Script)

Thanks in Advance.  
Regards,  
VASANTH

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**Author:** ![vasan\_cec](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/v/258eb7/32.png) [@vasan\_cec](https://www.ruby-forum.com/u/vasan_cec)\
**Post date:** [March 5, 2008, 8:15am UTC](https://www.ruby-forum.com/t/dynamic-method-access-from-an-object/131015/2 "2008-03-05T08:15:40Z")

</div>

Guo Y.-2 wrote:

> for table\_field in @table\_fields  
> temp = table\_field.Field # considering Field is `user_name`  
> @user\_information.temp = “vasanth”  
> # referring to @user\_information.user\_name = “vasanth”  
> end
> 
> In the above example i will get an error like “undefined method `temp`”
> 
> Can anyone tell me how to do this in Ruby on Rails? (I need something  
> equivalent to “eval()” function present in Java Script)

instance\_variable\_set is one method you should investigate for doing  
dynamic  
setters. You can find it sort of undocumented here:  
[http://www.ruby-doc.org/core/classes/Object.src/M000381.html](http://www.ruby-doc.org/core/classes/Object.src/M000381.html). But  
seriously,  
Google turns up lots on this topic.

Your code would then read:

for table\_field in @table\_fields  
@user\_information.instance\_variable\_set(“@#{table\_field}”,  
“vasanth”)  
# referring to @user\_information.send(“@#{table\_field}”) # =\>  
“vasanth”  
end

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Sent from the RubyOnRails Users mailing list archive at [Nabble.com](http://Nabble.com).

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**Author:** ![vasan\_cec](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/v/258eb7/32.png) [@vasan\_cec](https://www.ruby-forum.com/u/vasan_cec)\
**Post date:** [March 5, 2008, 10:05am UTC](https://www.ruby-forum.com/t/dynamic-method-access-from-an-object/131015/3 "2008-03-05T10:05:19Z")

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Vasanthakumar C. wrote:

> I have a requirement where the method name of an Object will be  
> generated and accessed dynamically in code. When I tried to access it  
> showed me “undefined method”. Am attaching a sample code of what i want  
> to accomplish:

As well as Steve’s suggestion for this case, there is a general method  
available in Ruby for sending messages to invoke methods where the  
method name is in a string. The method #send does this:

[http://www.ruby-doc.org/core/classes/Object.html#M000334](http://www.ruby-doc.org/core/classes/Object.html#M000334)

However:

> @user\_information = UserInformation.new  
> @table\_fields = UserInformation.find\_by\_sql(“DESC user\_informations”)

Try to avoid embedding SQL in your code if possible. You’ll make it  
more readable and more portable (especially if you decide to change your  
database at some point). The above will return an array of column  
properties which is not what you’re after anyway. To get the column  
names for a model, use:

@table\_fields = UserInformation.column\_names

> for table\_field in @table\_fields  
> temp = table\_field.Field # considering Field is `user_name`  
> @user\_information.temp = “vasanth”  
> # referring to @user\_information.user\_name = “vasanth”  
> end

If you are using a standard Rails model, then one of your columns will  
be called “id” and the above will try to overwrite this (numerical)  
value with a string. Not a good idea.

This code is also very model oriented but gives the impression you are  
wanting to put this into a controller. Whenever you find yourself doing  
model logic like this, re-think your strategy. This sort of stuff  
should be in the model. So, if you really want the model to be able to  
set all attributes (except “id”!) to a particular string, then in your  
model:

def set\_to\_string new\_string  
attributes.keys.each do | attr |  
self.send “#{attr}=”, new\_string  
end  
end

This form means that any setters you’ve overridden for any reason will  
be used and using #attributes to get the names automatically excludes  
“id”.

This is, of course, dangerous. It assumes that all your attributes are  
string values (does the model have “created\_at” and “updated\_at”?) and  
that if you ever add columns, you’ll want them to get the same  
treatment.

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**Author:** ![vasan\_cec](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/v/258eb7/32.png) [@vasan\_cec](https://www.ruby-forum.com/u/vasan_cec)\
**Post date:** [March 5, 2008, 10:43am UTC](https://www.ruby-forum.com/t/dynamic-method-access-from-an-object/131015/4 "2008-03-05T10:43:55Z")

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Thanks a lot Steve and Bush. All the informations that you gave was new  
and very useful to me. Here is a bit more explanation of what i am  
trying to do:

> This code is also very model oriented but gives the impression you are  
> wanting to put this into a controller. Whenever you find yourself doing  
> model logic like this, re-think your strategy. This sort of stuff  
> should be in the model. So, if you really want the model to be able to  
> set all attributes (except “id”!) to a particular string, then in your  
> model:  
> I understand that my code is model oriented, so its in model only.  
> Anyway thanks again for your suggestion.

> This is, of course, dangerous. It assumes that all your attributes are  
> string values (does the model have “created\_at” and “updated\_at”?) and  
> that if you ever add columns, you’ll want them to get the same  
> treatment.  
> Sorry that i couldn’t explain you the real scenario or give the actual  
> code for you to analyse. But the case is like this,  
> I have a “table A” with columns a1,a2,a3,a4 & a5.  
> Now, there will be a procedure (user action) which will change only  
> one/two column(s) in that “table A” eg: a3. The need of this is to  
> generate a report based on changed value. Also i should not over-write  
> the original value of a3 in “table A”.

Instead, i want to modify that object dynamically at run-time for  
further calculations. However, i dont know which column that user is  
going to change (till run-time).

So i have to write IF condition for all 5 columns of “table A”. But this  
will get tedious because i have more than 20 tables with 40 columns  
each. Hope you understood my problem. As you said, the user wont/cant  
modify “id” or “created\_at” or “updated\_at” etc.

Hope you can help me in minimizing the code further, if any methods  
available.

Thanks in Advance.  
Regards,  
VASANTH

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**Author:** ![vasan\_cec](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/v/258eb7/32.png) [@vasan\_cec](https://www.ruby-forum.com/u/vasan_cec)\
**Post date:** [March 5, 2008, 1:01pm UTC](https://www.ruby-forum.com/t/dynamic-method-access-from-an-object/131015/5 "2008-03-05T13:01:55Z")

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Hi Mark,  
Thanks a lot for your help. Its working fine. Learnt few new things in  
ROR because of you, thanks once again for that.

Regards,  
Vasanth

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**Author:** ![vasan\_cec](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/v/258eb7/32.png) [@vasan\_cec](https://www.ruby-forum.com/u/vasan_cec)\
**Post date:** [March 5, 2008, 12:11pm UTC](https://www.ruby-forum.com/t/dynamic-method-access-from-an-object/131015/6 "2008-03-05T12:11:46Z")

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Vasanthakumar C. wrote:

> Instead, i want to modify that object dynamically at run-time for  
> further calculations. However, i dont know which column that user is  
> going to change (till run-time).

In your model:

def set\_values columns, values  
(0…columns.length).each do |i|  
self.send “#{columns[i]}=”, values[i]  
end  
end

Then call this on the object you want to change with two arrays, one the  
set of columns the user wants to change and the other with the values.  
If the values will always be the same, then this reduces to:

def set\_values columns, new\_value  
columns.each do | attr |  
seld.send “#{attr}=”, new\_value  
end  
end

You should be able to fit any of these to your particular situation.