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

Author: jbranchaud

README.md

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-_1078 TILs and counting..._
+_1079 TILs and counting..._
 
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 - [Comparing DateTimes Down To Second Precision](rails/comparing-datetimes-down-to-second-precision.md)
 - [Conditional Class Selectors in Haml](rails/conditional-class-selectors-in-haml.md)
 - [Convert A Symbol To A Constant](rails/convert-a-symbol-to-a-constant.md)
+- [Count The Number Of Records By Attribute](rails/count-the-number-of-records-by-attribute.md)
 - [Create A Custom Named References Column](rails/create-a-custom-named-references-column.md)
 - [Creating Records of Has_One Associations](rails/creating-records-of-has-one-associations.md)
 - [Custom Validation Message](rails/custom-validation-message.md)

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.