Chapter 7 - Active Record Associations

Question 1

Add index

Answer 1

add_index :timesheets, :user_id

add_index :expense_reports, :user_id

Question 2

pluck

Answer 2

Use pluck as a shortcut to select one or more attributes without loading a bunch of records just to grab the attributes you want.
Pluck returns an array.

Question 3

has_many

Answer 3

the has_many association allows you to define a relationship in which one model has many other models that belong to it.

has_many :timesheets

has_many :expense_reports

Question 4

One-To-Many

Answer 4

class User

Question 5

where(*conditions)

Answer 5

has_many :pending_comments, -> { where( approved: true) }, class_name: ‘Comment’

Question 6

has_and_belongs_to_many

Answer 6

I must clear my conscience by stating that

has_and_belongs_to_many is practically obsolete.

Question 7

create_join_table

Answer 7

create_join_table :billing_codes, :timesheets

class Timesheet

Question 8

has_many :through

Answer 8

The has_many :through association allows you to specify a one-to-many relationship indirectly via an intermediate join table. In fact, you can specify more than one such relationship via the same table, which effectively makes it a replacement for has_and_belongs_to_many. The biggest advantage is that the join table contains full-fledged model objects complete with primary keys and ancillary data.

Question 9

has_many :through implementation

Answer 9

class Client

Question 10

has_many :through as aggregating

Answer 10

class Grandparent

Question 11

Join models and validations

Answer 11

You can add validates_uniqueness_of constraints on the join model to keep duplicate joins from happening.

validates_uniqueness_of :client_id, scope: :timesheet_id

Question 12

One-to-One Relationship

Answer 12

One of the most basic relationship types is a one-to-one object relationship. In Active Record we declare a one-to-one relationship using the has_one and belongs_to methods together.

class User