Create multilevel constant in ruby

defined?(Foo) # => nil module Foo; end defined?(Foo) # => "constant" defined?(Foo::Bar) # => nil module Foo::Bar; end defined?(Foo::Bar) # => "constant" defined?(Foo::Bar::Baz) # => nil module Foo::Bar; Baz = :baz end defined?(Foo::Bar::Baz) # => "constant" 

Other people talk about the defined? operator (yes, it's a built-in unary operator, not a method!!!), but there are other ways. I personally favor this one:

constants #=> a big array of constant names constants.include? :Foo #=> false module Foo; end constants.include? :Foo #=> true Foo.constants.include? :Bar #=> false module Foo module Bar; end end Foo.constants.include? :Bar #=> true # etc. 

One thing I have to admit about defined? operator is its reliability. It is not a metho and thus can never be redefined, and thus always does what you expect of it. On the other hand, it is possible to use more roundabout (and less reliable) ways, such as:

begin Foo::Bar::Baz puts "Foo::Bar::Baz exists!" rescue NameError puts "Foo::Bar::Baz does not exist!" end 

Here's what I ended up doing in case anyone else runs into this:

unless Object.const_defined? const_name const_name.split('::').inject(Object) do |obj, const| unless obj.const_defined? const # create the missing object here... end obj.const_get(const) end end 

It sounds like you want to use the defined? operator. Check if a constant is already defined has more on this.