StevenJL

# The error_log directive sets the location of the nginx error 
# log as well as the security level.
error_log /var/log/nginx/error.log warn;

# Specifies the number of worker processes spawned by the master 
# process.  It's these worker processes that listen to sockets and
# accepts the incoming connections.  Should be set equal to the
# number of cores on the machine.
worker_processes 8;

StevenJL

error_log /var/log/nginx/error.log warn;

events {
  use epoll;
  worker_connections 1024;
}

http {
  include mime.types;
  default_type application/octet-stream;

StevenJL

# Suppose we want to select from these n emails those that
# are not already in the `users` table.
emails = [
  "[email protected]",
  "[email protected]",
  "[email protected]",
  ...
  "[email protected]"
]

StevenJL

# When creating a lot of records from an array of hashes
new_users = [
  {name: "Teemo", email: "[email protected]"},
  {name: "Heim", email: "[email protected]"},
  {name: "Annie", email: "[email protected]" },
  ...
  {name: "Volli", email: "[email protected]" },
]

# Do not create each one individually as it results in one query, per record.

StevenJL

# If we care to know a record exists, but actually don't
# need to do anything with the record...
if User.where(email: "[email protected]").present?
  puts "There is a user with email address [email protected]"
else
  puts "There is no user with email address [email protected]"
end
# then using `present?` is wasteful, as it selects all the column
# and loads the object into memory.
# SELECT * FROM users WHERE email` = '[email protected]';

StevenJL

# Deleting a large amount of records by invoking `destroy` on
# each record results in a single query per record.
deadbeat_users = User.where(payment_status: "deadbeat")
deadbeat_users.each do |deadbeat|
  deadbeat.destroy
end
# DELETE FROM users WHERE id = 13;
# DELETE FROM users WHERE id = 42;
# DELETE FROM users WHERE id = 49;
# DELETE FROM users WHERE id = 420;

StevenJL

# Suppose we load all user records, instantiate them as `User` objects
# only to select one attribute. This is wasteful since we select all
# columns from the database and load them all into memory.
user_emails = User.where(status: "active").map(&:email)
# SELECT * FROM users;

# Instead, we can use `pluck` to modify the query to select only the 
# column(s) we need. It also returns the attributes as an array of strings 
# instead of `User` objects, reducing memory use.
user_emails = User.where(status: "active").pluck(:email)

StevenJL

# Selecting all users
User.all.each do |user|
  notify(user)
end

# Generates this query:
# SELECT * FROM users;

# Selecting all users with `find_each` will query by batches of size
# determined by the :batch_size option (defaults to 1000)

StevenJL

# Eager-load all users' posts with `includes`
users = User.includes(:posts)

users.each do |user|
  user.posts
end

# This will only run 2 queries.
# SELECT * FROM users;
# SELECT * FROM posts WHERE posts.user_ids IN (1,2,3,4,....,n);

StevenJL

# Assuming User has the following association
class User
  has_many :posts
end

# Running this activerecord code
users = User.all

users.each do |user|
  user.posts