Stupid ActiveRecord Tricks

Short presentation given at the August 2016 indy.rb meetup covering some lesser-knowns bits of Rails' ActiveRecord (and just a touch of Arel)

Transcript

1. Stupid ActiveRecord Tricks Little used goodness in Rails' database API
2. replace Replaces a collection with an array of new items and diffs just the new stuff. @post = Post.find(params[:id]) new_tags = params[:tags].split(",").map(&:strip).map do |tag_name| Tag.find_or_initialize_by(name: tag_name) end @post.tags.replace(new_tags) @post.tags.replace(["doo", "ggie", "gaga"]) # => ActiveRecord::AssociationTypeMismatch: Tag expected, got String
3. many? & any? Do we have one of these? Do we have more than one of these? @post.tags.any? # Boolean @post.tags.many? # Boolean
4. More than first and last @post.tags.second @post.tags.second_to_last @post.tags.third @post.tags.third_to_last @post.tags.fourth @post.tags.fifth @post.tags.forty_two
5. Not the whole thing Several methods to limit the returned attributes: Post.all.select(:id) # SELECT "posts"."id" FROM "posts" # => #<ActiveRecord::Relation [#<Post id: 1>]> Post.all.pluck(:id) # SELECT "posts"."id" FROM "posts" # => [1] Post.all.ids # SELECT "posts"."id" FROM "posts" # => [1]
6. Maths AR methods that do math in the database Book.average(:price) # SELECT AVG("books"."price") FROM "books" Book.minimum(:price) # SELECT MIN("books"."price") FROM "books" Book.maximum(:price) # SELECT MAX("books"."price") FROM "books" Book.sum(:price) # SELECT SUM("books"."price") FROM "books"
7. Group and having Adding conditions to your groupings. Post.group(:word_count).having("created_at > ?", 1.years.ago) # Posts grouped by their word_count if they were created in the last year
8. New in Rails 5: or Generate an OR SQL query. Works great with scope class Post < ApplicationRecord scope :featured, -> { where(featured: true) } scope :recent, -> { where("created_at > ?", 2.days.ago) } end Post.featured.or(Post.recent) # SELECT "posts".* FROM "posts" # WHERE ("posts"."featured" = ? # OR (created_at > '2016-08-08 17:07:58.750856')) [["featured", true]]
9. New in Rails 5: left_outer_joins # Rails 4 authors = Author.join('LEFT OUTER JOIN "posts" ON "posts"."author_id" = "authors"."id"') .uniq .select("authors.*, COUNT(posts.*) as posts_count") .group("authors.id") # Rails 5 authors = Author.left_outer_joins(:posts) .uniq .select("authors.*, COUNT(posts.*) as posts_count") .group("authors.id") # Also Author.left_joins :posts, :comments # SELECT "authors".* FROM "authors" # LEFT OUTER JOIN "posts" ON "posts"."author_id" = "authors"."id" # LEFT OUTER JOIN "comments" ON "comments"."author_id" = "authors"."id"
10. Arel predicates Who wants to write SQL? Not this guy. Arel is the code that powers ActiveRecord's queries, and we can use it to write Ruby instead of SQL fragments.
11. class Post < ApplicationRecord # Helper method to make things a bit easier def self.[](name) self.arel_table[name] end end Post.where(Post[:created_at].gt 2.days.ago) # Same as : Post.where(Post.arel_table[:created_at].gt 2.days.ago) # Same as: Post.where('created_at > ?', 2.days.ago) # SELECT "posts".* FROM "posts" WHERE ("posts"."created_at" > '2016-08-08 17:38:46.068891')
12. Gem recommendation
13. Groupdate Post.group_by_day(:created_at).count # { # Sat, 28 May 2016 => 50, # Sun, 29 May 2016 => 100, # Mon, 30 May 2016 => 34 # } https://github.com/ankane/groupdate