Ruby: Dynamically calling available methods raising undefined method (metaprogramming)

Question

I have an Activerecord object called Foo:

Foo.attribute_names.each do |attribute|
  puts Foo.find(:all)[0].method(attribute.to_sym).call
end

Here I'm calling all attributes on this model (ie, querying for each column value). However, sometimes, I'll get an undefined method error.

How can ActiveRecord::Base#attribute_names return an attribute name that when converted into its own method call, raises an undefined method error?

Keep in mind this only happens on certain objects for only certain methods. I can't identify a pattern.

Thank you.

I can't tell from the code you posted, but you might want to look at what the failing attribute.to_sym is. Also, you probably know this already, but Foo.find(:all)[0] is horribly inefficient- use Foo.first instead. — igul222
– igul222, Commented Apr 21, 2010 at 23:33
attribute.to_sym converts an attribute name that is a String (eg, 'author' to a symbol ':author'. Symbols are the data-type accepted by the Object#method function. This works in principle as only some of the methods are undefined. Also, I know the db query is terrible, it's not what I'm actually using, it's just easier for everyone else to understand. — user94154
– user94154, Commented Apr 21, 2010 at 23:40

Marc-André Lafortune · Accepted Answer · 2010-04-22 01:09:06Z

2

The NoMethodError should be telling you which method does not exist for what object. Is it possible that your find returns no record? In that case, [][0] is nil and you will get a NoMethodError for sure.

I would use .fetch(0) instead of [0], and you will get a KeyError if ever there is no element with index 0.

Note: no need for to_sym; all builtin methods accept name methods as strings or symbols (both in 1.8 and 1.9)

answered Apr 22, 2010 at 1:09

Marc-André Lafortune

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1 Comment

user94154 Over a year ago

Don't worry about the query, it's not returning a nil result. And thanks for the .to_sym tip.

JRL · Accepted Answer · 2010-04-22 00:06:08Z

0

Maybe something to do with access? Like if a class has an attr_protected attribute, or something along that line. Or for attributes that are not database columns, which have no accessors defined?

answered Apr 22, 2010 at 0:06

JRL

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1 Comment

user94154 Over a year ago

All of the attributes that tripped the error were database columns.

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