Add Where Am I In The Partial Iteration as a rails til

rails/where-am-i-in-the-partial-iteration.md

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+# Where Am I In The Partial Iteration?
+
+Let's say I am going to render a collection of posts with a post partial.
+
+```erb
+<%= render collection: @posts, partial: "post" %>
+```
+
+The
+[`ActionView::PartialIteration`](http://api.rubyonrails.org/classes/ActionView/PartialIteration.html)
+module provides a couple handy methods when rendering collections.
+I'll have access in the partial template to `#{template_name}_iteration`
+(e.g. `post_iteration`) which will, in turn, give me access to `#index`,
+`#first?`, and `#last?`.
+
+This is great if I need to do something special with the first or last item
+in the collection or if I'd like to do some sort of numbering based on the
+index of each item.
+
+[source](http://stackoverflow.com/questions/13397848/rails-render-collection-partial-getting-size-of-collection-inside-partial)
+
+h/t Josh Davey