I've written about Open Structs before:
To review, an Open Struct is a hash like data structure that responds to dot methods (i.e. .id or .name). I use these in a lot of different places and I just finished using them for a form collection_select operation.
The Rails collection_select form helper is really designed to work with database objects that you need to make pick lists from. The canonical usage is like this:
<%= form.label :project_id %> <%= form.collection_select :project_id, current_user.projects.all, :id, :name %>
The expression current_user.projects.all is a list of all projects as an iterable ActiveRecord collection and what collection_select does under the hood is loop over the collection and call the .id and .name methods (.id is the value in the collection and .name is what's displayed). And this works great – until what you have to select isn't a database collection. And that, dear reader, is where open structs come out to play!
Here's what I just did:
@contexts =  @contexts << OpenStruct.new(id: "web_browser_on_computer", name: "Web Browser (Desktop Computer)") @contexts << OpenStruct.new(id: "mobile_app", name: "Mobile App")
This defines an array of open struct objects. And an array is an iterable collection just like the database collection so this implementation, despite being an entirely different underlying data structure, works just like the ActiveRecord collection based version.
And here's the view side:
<%= form.label :context %> <%= form.collection_select :context, @contexts, :id, :name %>
Here's how this looks by default and then when dropped down: