Add Count The Number Of Records By Attribute as a Rails til

rails/count-the-number-of-records-by-attribute.md

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+# Count The Number Of Records By Attribute
+
+In [Count How Many Records There Are Of Each
+Type](postgres/count-how-many-records-there-are-of-each-type.md), I walked
+through how to use SQL (in PostgreSQL) to get a count of how many records there
+are with each unique value in a given column. This is something I tend to do
+with a `type` or `status` column.
+
+We can ask the same question with Rails, with very little code. It produces a
+nearly identical query and the same results.
+
+```ruby
+> Book.group(:status).count
+#=> { nil => 123, "published" => 611, "draft" => 364, "review" => 239 }
+```
+
+We've picked the `Book` model and we want it to group books by their `status`.
+Tacking on the `#count` at the end tells it to apply the `count` aggregate. The
+result is a hash of each unique value of the specified attribute (`status`)
+paired with the count.