Essential Books

.group and .count

suggest change

We have a Product model and we want to group them by their category. 

Product.select(:category).group(:category)

This will query the database as follows: 

SELECT "product"."category" FROM "product" GROUP BY "product"."category"

Make sure that the grouped field is also selected. Grouping is especially useful for counting the occurrence - in this case - of categories. 

Product.select(:category).group(:category).count

As the query shows, it will use the database for counting, which is much more efficient, than retrieving all record first and do the counting in the code: 

SELECT COUNT("products"."category") AS count_categories, "products"."category" AS products_category FROM "products" GROUP BY "products"."category"
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ActiveRecord Query Interface:
* ActiveRecord Query Interface
* .where
* .where with an array
* Scopes
* Get first and last record
* where.not
* Ordering
* Includes
* .delete all
* ActiveRecord case insensitive search
* .find by
* .group and .count
* .distinct or .uniq
* Joins
* Limit and Offset
* ActiveRecord Bang methods
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