After several months of working with Ruby on Rails, I want to gain a deeper understanding of Active Record in efforts to optimize performance and understand more of the good ol’ Rails magic. I drew information from many resources, which I will include as links at the bottom. This is a general overview of common concepts of Active Record.

**Active Record Classes**

ActiveRecord::FinderRelations
Returns model instance, and run queries immediatelyraises error if not found
#find
returns nil if not found
#find_by
#first
#last
#exists?

**ActiveRecord::QueryMethods**
Returns relation, only runs query when calledreturns an array
#where
#group
#distinct
#includes
#joins
#select

Aggregations

Useful when performing mathematical operations, such as #count or #max.

Post.joins(:tags).group("tags.name").count
  # => {"tag1" => 4, "tag2" => 2, "tag3" => 5}

Scopes

Chains of ActiveRecord methods that are used with a relation. Scopes return relations, so you can chain them.

class User < ActiveRecord::Base
  scope :millennials, -> { where('age < 35') }
end

Retrieve large batches

Works for models and relations, and can chain methods

#find_eachUser.find_each(batch_size: 5000) do |user|
  NewsMailer.weekly(user).deliver_now
end
options: batch_size, start, finish, error_on_ignore#find_in_batches

Conditions can be used with ActiveRelations

Client.where("orders_count = ?", params[:orders])

Joins (lazy loading)

Lazy loading: queries are not fired until called upon

> user = User.where('age > 18')
> user.all #queries fired

# Returns all Users that have a blog post

users_with_posts = User.joins(:post)
users_with_posts.all

# Returns all User objects with posts in which have comments

User.joins(:post, :comment)

# join nested associations

User.joins(post: :something)

Includes (eager loading, minimal queries possible)

ActiveRecored lets you specify 1+ associations beforehand. It eagerly loads the table so there are not extra queries

User.includes(:posts, :followers)

N + 1 problem

We want to avoid unnecessary queries through associations.

#includes uses eager loading

An example of eager loading is #pluck. It pulls up a bunch of records and stores in memory, grabs the column and returns an array.

Joins vs Includes

Use joins when you don’t want to access data from the associated table

Use includes when you want to access data from the associated table

More Resources api.rubyonrails.org guides.rubyonrails.org (rubyinrails.com)[rubyinrails.com]