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#method_missing is a very useful part of the ruby language. However, there’s one common mistake that developers make when using it. They forget to add a call to #super.
Let’s take a look:
| | | copy code | | ? |
| 01 | $ irb |
| 02 | >> class Foo |
| 03 | >> end |
| 04 | => nil |
| 05 | >> f=Foo.new |
| 06 | => #<Foo:0xb75122fc> |
| 07 | >> f.i_do_not_exist |
| 08 | NoMethodError: undefined method `i_do_not_exist' for #<Foo:0xb75122fc> |
| 09 | from (irb):5 |
| 10 | >> class Foo |
| 11 | >> def method_missing(symbol,*args) |
| 12 | >> if (symbol.to_s === "i_do_not_exist") |
| 13 | >> puts "Yes, actually you do" |
| 14 | >> end |
| 15 | >> end |
| 16 | >> end |
| 17 | => nil |
| 18 | >> f.i_do_not_exist |
| 19 | Yes, actually you do |
| 20 | => nil |
| 21 | >> f.i_exist |
| 22 | => nil |
Unfortunately, we no longer get the error when we try to invoke a method which does not exist. When we add an invocation to super, it’ll behave as expected:
| | | copy code | | ? |
| 01 | >> class Foo |
| 02 | def method_missing(symbol,*args) |
| 03 | if (symbol.to_s === "i_do_not_exist") |
| 04 | puts "Yes, actually you do" |
| 05 | else |
| 06 | super |
| 07 | end |
| 08 | end |
| 09 | end |
| 10 | >> >> >> >> ?> >> >> >> => nil |
| 11 | >> f.i_exist |
| 12 | NoMethodError: undefined method `i_exist' for #<Foo:0xb75122fc> |
| 13 | from (irb):31:in `method_missing' |
| 14 | from (irb):35 |
| 15 | >> f.i_do_not_exist |
| 16 | Yes, actually you do |
| 17 | => nil |
| 18 | >> |
You’re still missing something. This is better:
def method_missing(meth, *args, &block)
if (meth == :i_do_not_exist)
puts “Yes, actually you do”
else
super(meth, *args, &block)
end
end
You are most definitely correct. Oops! Thanks for catching that.