optional local variables in rails partial templates: how do I get out of the (defined? foo) mess?

Question

I've been a bad kid and used the following syntax in my partial templates to set default values for local variables if a value wasn't explicitly defined in the :locals hash when rendering the partial --

<% foo = default_value unless (defined? foo) %>

This seemed to work fine until recently, when (for no reason I could discern) non-passed variables started behaving as if they had been defined to nil (rather than undefined).

As has been pointed by various helpful people on SO, http://api.rubyonrails.org/classes/ActionView/Base.html says not to use

defined? foo

and instead to use

local_assigns.has_key? :foo

I'm trying to amend my ways, but that means changing a lot of templates.

Can/should I just charge ahead and make this change in all the templates? Is there any trickiness I need to watch for? How diligently do I need to test each one?

Answer 1

How about

<% foo ||= default_value %>

This says "use foo if it is not nil or true. Otherwise assign default_value to foo"

Answer 2

I do this:

<% some_local = default_value if local_assigns[:some_local].nil? %>

I hope this helps...

Answer 3

A helper can be created to look like this:

somearg = opt(:somearg) { :defaultvalue }

Implemented like:

module OptHelper
  def opt(name, &block)
    was_assigned, value = eval(
      "[ local_assigns.has_key?(:#{name}), local_assigns[:#{name}] ]", 
      block.binding)
    if was_assigned
      value
    else
      yield
    end
  end
end

See my blog for details on how and why.

Note that this solution does allow you to pass nil or false as the value without it being overridden.