Active Record Query Interface (2), Season 1

The 10th Round of ROR Lab.

Active Record
Query Interface(2)

April 7th, 2012

Hyoseong Choi
ROR Lab.

Short Review

ROR Lab.

Query Interface(1)
- How to retrieve data from DB -

Bi-directional Associations
:inverse_of
class Customer < ActiveRecord::Base
has_many :orders, :inverse_of => :customer
end

class Order < ActiveRecord::Base
belongs_to :customer, :inverse_of => :orders
end
---------------------------------------------------
c = Customer.first
o = c.orders.first
c.first_name == o.customer.first_name # => true
c.first_name = 'Manny'
c.first_name == o.customer.first_name # => true

ROR Lab.

Overriding
Conditions
- reorder -
class Post < ActiveRecord::Base
  ..
  ..
  has_many :comments, :order => 'posted_at DESC'
end

SELECT * FROM posts WHERE id = 10 ORDER BY name

SELECT * FROM posts WHERE id = 10 ORDER BY posted_at DESC

ROR Lab.

Locking Records
for Update
• To prevent “race conditions”
• To ensure “atomic updates”
• Two locking mechanisms
‣ Optimistic Locking : version control
‣ Pessimistic Locking : DB lock

ROR Lab.

Optimistic Locking
• “lock_version” in DB table (default to 0)
• set_locking_column to change column name
c1 = Client.find(1)
c2 = Client.find(1)

c1.first_name = "Michael"
c1.save # increments the lock_version column

c2.name = "should fail"

ROR Lab.

Joining Tables
- Using Array/Hash of Named Associations -

only with INNER JOIN
class Category < ActiveRecord::Base
  has_many :posts
end

class Post < ActiveRecord::Base • Multiple Associations
  belongs_to :category
  has_many :comments
  has_many :tags
end Post.joins(:category, :comments)

class Comment < ActiveRecord::Base
  belongs_to :post
  has_one :guest SELECT posts.* FROM posts
end

  INNER JOIN categories
class Guest < ActiveRecord::Base ON posts.category_id = categories.id
  belongs_to :comment   INNER JOIN comments
end ON comments.post_id = posts.id

class Tag < ActiveRecord::Base “return all posts that have a category and at least one comment”
  belongs_to :post
end

ROR Lab.

Joining Tables
- Using Array/Hash of Named Associations -

only with INNER JOIN
class Category < ActiveRecord::Base
  has_many :posts
end

class Post < ActiveRecord::Base • Nested Associations(Single Level)
  belongs_to :category
  has_many :comments
  has_many :tags
end Post.joins(:comments => :guest)

class Comment < ActiveRecord::Base
  belongs_to :post
  has_one :guest SELECT posts.* FROM posts
end

  INNER JOIN comments
class Guest < ActiveRecord::Base ON comments.post_id = posts.id
  belongs_to :comment   INNER JOIN guests
end ON guests.comment_id = comments.id

class Tag < ActiveRecord::Base
  belongs_to :post
“return all posts that have a comment made by a guest”
end

ROR Lab.

Query Interface(2)
- How to retrieve data from DB -

Eager Loading
Associations
: as few queries as possible

N + 1 queries problem

clients = Client.limit(10)

clients.each do |client|
puts client.address.postcode

ROR Lab.

Eager Loading
Associations
: as few queries as possible

Solution to N + 1 queries problem Only 2 queries

clients = Client.includes(:address).limit(10)

clients.each do |client|
puts client.address.postcode

SELECT * FROM clients LIMIT 10
SELECT addresses.* FROM addresses
WHERE (addresses.client_id IN (1,2,3,4,5,6,7,8,9,10))

ROR Lab.

Eager Loading
Associations
- Eager Loading Multiple Associations -
Array of Multiple Associations

Nested Associations Hash

Category.includes(
:posts => [{:comments => :guest}, :tags]).ﬁnd(1)

ROR Lab.

Eager Loading
Associations
- Conditions on Eager Loaded Associations -

conditional “joins” > conditional “includes”

Post.includes(:comments)
.where("comments.visible", true)

SELECT "posts"."id" AS t0_r0, ... "comments"."updated_at" AS t1_r5
FROM "posts"
LEFT OUTER JOIN "comments"
ON "comments"."post_id" = "posts"."id"
WHERE (comments.visible = 1)

ROR Lab.

Eager Loading
Associations
- Conditions on Eager Loaded Associations -
INNER JOIN LEFT OUTER JOIN
conditional “joins” > conditional “includes”

Post.includes(:comments)
.where("comments.visible", true)

SELECT "posts"."id" AS t0_r0, ... "comments"."updated_at" AS t1_r5
FROM "posts"
LEFT OUTER JOIN "comments"
ON "comments"."post_id" = "posts"."id"
WHERE (comments.visible = 1)

ROR Lab.

Scopes

• To specify ARel queries
• Referenced as method calls on the
association objects or model
• ﬁnder methods in a scope
• return an ActiveRecord::Relation object
ROR Lab.

Scopes

scope :published, where(:published => true)
end
ARel query

ROR Lab.

Scopes
chainable
  scope :published,
where(:published => true).joins(:category)
end

chainable within scopes
  scope :published, where(:published => true)
  scope :published_and_commented,
published.and(self.arel_table[:comments_count].gt(0))

ROR Lab.

Scopes
To call the scope

Post.published # => [published posts]

category = Category.ﬁrst
category.posts.published
# => [published posts belonging to this category]

ROR Lab.

Scopes
Working with times

scope :last_week,
lambda { where("created_at < ?", Time.zone.now ) }
end a lambda so that the scope is evaluated every time

ROR Lab.

Scopes
Passing in arguments
  scope :1_week_before,
lambda { |time| where("created_at < ?", time) }
end

  def self.1_week_before(time)
    where("created_at < ?", time)
  end
***What about “as a class method” ?

ROR Lab.

Scopes
  scope :1_week_before,
lambda { |time| where("created_at < ?", time) }
end

le
eab
erclass Post < ActiveRecord::Base
ef
pr   def self.1_week_before(time)
    where("created_at < ?", time)
  end
***What about “as a class method” ?

ROR Lab.

Scopes

ROR Lab.

Scopes
Working with Scopes

client = Client.ﬁnd_by_ﬁrst_name("Ryan")

ROR Lab.

Scopes
Applying a default scope

class Client < ActiveRecord::Base
default_scope where("removed_at IS NULL")

SELECT * FROM clients WHERE removed_at IS NULL

ROR Lab.

Scopes
Removing all scopes

Especially useful,
when escaping the default_scope

ROR Lab.

Dynamic Finders
• Model.find_by_attribute_name
• Model.find_all_by_attribute_name
• Model.find_last_by_attribute_name
• Model.find_by_attribute_name!
• Model.find_by_attr1_and_attr2
Rails 3.1 < Argument Error on missing fields

ROR Lab.

Find or build
a new object

• first_or_create
• first_or_create! Exception on invalid

• first_or_initialize

ROR Lab.

Find or build
a new object
• first_or_create
Client.where(:first_name => 'Andy').first_or_create(:locked =>
false)
# => #<Client id: 1, first_name: "Andy", orders_count: 0, locked: false, created_at:
"2011-08-30 06:09:27", updated_at: "2011-08-30 06:09:27">

Client.find_or_create_by_first_name(:first_name =>
"Andy", :locked => false)

SELECT * FROM clients WHERE (clients.first_name = 'Andy') LIMIT 1
BEGIN
INSERT INTO clients (created_at, first_name, locked, orders_count, updated_at) VALUES
('2011-08-30 05:22:57', 'Andy', 0, NULL, '2011-08-30 05:22:57')
COMMIT

ROR Lab.

Find or build
a new object
• first_or_create!
class Client < ActiveRecord::Base
validates :orders_count, :presence => true
end

Client.where(:first_name => 'Andy').first_or_create!(:locked =>
false)
# => ActiveRecord::RecordInvalid: Validation failed: Orders count can't be blank

ROR Lab.

Find or build
a new object
• first_or_initialize
nick = Client.where(:first_name =>
'Nick').first_or_initialize(:locked => false)
# => <Client id: nil, first_name: "Nick", orders_count: 0, locked: false,
created_at: "2011-08-30 06:09:27", updated_at: "2011-08-30 06:09:27">

SELECT * FROM clients WHERE (clients.first_name = 'Nick') LIMIT 1
nick.persisted?
# => false
nick.new_record?
# => true
nick.save
# => true

persisted? vs new_record? ROR Lab.

Finding by SQL
: return an array of objects
where each object indicates a record

Client.ﬁnd_by_sql("SELECT * FROM clients
INNER JOIN orders ON clients.id = orders.client_id

cf. connection.select_all
an array of hash

ROR Lab.

select_all
: return an array of hash
where each hash indicates a record

Client.connection.select_all("SELECT * FROM clients WHERE id =

cf. ﬁnd_by_sql -> instantiates objects

ROR Lab.

pluck
To query a single column from the underlying table of a model
To return an array of values

Client.where(:active => true).pluck(:id)
# SELECT id FROM clients WHERE active = 1

Client.uniq.pluck(:role)

Client.select(:id).map { |c| c.id }

Client.pluck(:id)

ROR Lab.

Existence of
Objects
Client.exists?(1)

Client.exists?(1,2,3)
# or
Client.exists?([1,2,3])

Client.where(:ﬁrst_name => 'Ryan').exists?

cf. ﬁnd method
ROR Lab.

Existence of
Objects
# via a model
Post.any?
Post.many?

# via a named scope
Post.recent.any?
Post.recent.many?

# via a relation
Post.where(:published => true).any?
Post.where(:published => true).many?

# via an association
Post.ﬁrst.categories.any?

ROR Lab.

Calculations
Client.count
# SELECT count(*) AS count_all FROM clients

Client.where(:first_name => 'Ryan').count
# SELECT count(*) AS count_all FROM clients WHERE (first_name =
'Ryan')

Client.includes("orders").where(:first_name => 'Ryan', :orders

SELECT count(DISTINCT clients.id) AS count_all FROM clients
LEFT OUTER JOIN orders ON orders.client_id = client.id WHERE
(clients.first_name = 'Ryan' AND orders.status = 'received')

ROR Lab.

Calculations
Client.count(:age)

Client.average("orders_count")

Client.minimum("age")

Client.maximum("age")

ROR Lab.

Running EXPLAIN
a pretty printing that emulates the one of the database shells

User.where(:id => 1).joins(:posts).explain

EXPLAIN for: SELECT "users".* FROM "users" INNER JOIN "posts" ON "posts"."user_id" = "users"."id" WHERE "users"."id" = 1
                                  QUERY PLAN
------------------------------------------------------------------------------
Nested Loop Left Join (cost=0.00..37.24 rows=8 width=0)
   Join Filter: (posts.user_id = users.id)
   -> Index Scan using users_pkey on users (cost=0.00..8.27 rows=1 width=4)
         Index Cond: (id = 1)
   -> Seq Scan on posts (cost=0.00..28.88 rows=8 width=4)
         Filter: (posts.user_id = 1)
(6 rows)

under PostgreSQL
ROR Lab.

Running EXPLAIN
Automatic EXPLAIN

To set the threshold in seconds
conﬁg.active_record.auto_explain_threshold_in_seconds

• disables automatic EXPLAIN => nil or no AR logger
• Development mode => default, 0.5 sec
• Test mode => nil
• Production mode => nil

ROR Lab.

Running EXPLAIN
Disabling Automatic EXPLAIN

To be selectively silenced with
ActiveRecord::Base.silence_auto_explain do
# no automatic EXPLAIN is triggered here

• useful for queries are slow but ﬁne
: a heavyweight report of a admin interface

ROR Lab.

Active Record Query Interface (2), Season 1

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Editor's Notes