Add Get An Empty ActiveRecord Relation as a rails til

rails/get-an-empty-activerecord-relation.md

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+# Get An Empty ActiveRecord Relation
+
+When you query for something (with `#where`) and there are no results matching
+that query, you get something that looks like an empty array (`[]`), but it's
+not quite.
+
+```ruby
+> result
+[]
+> result.class
+Book::ActiveRecord_Relation
+```
+
+It's an empty [`ActiveRecord`
+relation](https://api.rubyonrails.org/classes/ActiveRecord/Relation.html).
+
+You can get an instance of an empty `ActiveRecord` relation without
+constructing a _no result_ query.
+
+```ruby
+> Book.none
+[]
+> Book.none.class
+Book::ActiveRecord_Relation
+```
+
+I can think of a couple scenarios where this would be useful:
+
+- as a default value for a method parameter
+- as a test value for a method that expects to have an `ActiveRecord` relation
+  passed in
+
+[source](https://stackoverflow.com/questions/4877931/how-to-return-an-empty-activerecord-relation)