w3d3 rails

Question 1

How do you run migrations

Answer 1

bundle exec rails db:migrate

Question 2

How do you create a database with rails

Answer 2

set up config/database.yml

bundle exec rails db:create

Question 3

What are 5 gems to add to rails for development

Answer 3

annotate
better_errors
binding_of_caller
byebug
pry-rails

Question 4

How do you run migrations?

Answer 4

rails db:migrate

Question 5

Where can you see a copy of your database’s current schema as reported by rails?

Answer 5

db/schema.rb

Question 6

What’s the best/fastest way to initialize a new database? What does this rely on?

Answer 6

rails db:schema:load

db/schema.rb

Question 7

How do you enforce that a column must have a value?

Answer 7

within create_table:

t.string :name, null: false

Question 8

What’s the difference between belongs_to and has_many ?

What do they have in common?

Answer 8

belongs_to exists on the model that contains the primary key

has_many exists on the other model

They allow the use of helper instance methods

Question 9

What arguments are passed to a belongs_to or has_many association?

Answer 9

(trick) answer: two

better answer: we use ruby’s ability to use a hash as a final argument without braces to pass several options to the association method like so:

belongs_to :table_name, primary_key: :id,
foreign_key: :foreign_key_id,
class_name: ‘SomeClass’

example:

belongs_to :house,
brimary_key: :id,
foreign_key: :house_id,
class_name: ‘House’

Question 10

What arguments are passed to a belongs_to or has_many association?

Answer 10

(trick) answer: two

better answer: we use ruby’s ability to use a hash as a final argument without braces to pass several options to the association method like so:

belongs_to :table_name, primary_key: :id,
foreign_key: :foreign_key_id,
class_name: ‘SomeClass’

example:

belongs_to :house,
brimary_key: :id,
foreign_key: :house_id,
class_name: ‘House’

Question 11

What does this do:

validates :attribute, presence: true

Answer 11

It validates that a particular attribute exists on a method when we call #save

Question 12

How do we validate the uniqueness of an attribute for an object?

Answer 12

Add this to a validates macro:

uniqueness: true

Question 13

How do we create a custom validator for a model?

Answer 13

1. create a method that conditionally shovels an error message into a new key on the self.errors hash
2. add this line:

validate :my_validator_method_name

Question 14

How do we see the errors for an object?

Answer 14

errors.full_messages

Question 15

What’s the difference between ApplicationModel#save and #save!

Answer 15

save will return false if there are errors.

Question 16

What’s the difference between ApplicationModel#save and #save!

Answer 16

save will return false if there are errors.

Question 17

How would you use a symbol as a value in a SQL string?

Answer 17

awesome_symbol: 55

<

Question 18

What are two levels of data integrity-protection in rails?

Answer 18

DB-level constraints

Model-level validations

Question 19

Why shouldn’t you roll back a migration?

Answer 19

Can make things out-of-sync with other people working on the same database.

Impossible to keep consistent with a production database.

Question 20

How do you get to a sql console in rails?

Answer 20

rails db

Question 21

What is the superclass for all rails models?

Answer 21

ApplicationRecord

Question 22

What is ApplicationRecord

Answer 22

The superclass that all rails models inherit from.

Question 23

When does code directly inside a class definition run?

Answer 23

When the class is first parsed.

Question 24

How do you recreate your db from scratch? What steps does this take?

Answer 24

rails db:reset

1. drops entire database
2. runs db:migrate
3. runs db:seed

Question 25

What arguments are passed to has_many? How many of these are optional?

Answer 25

has_many :relation_name,
class_name: __,
foreign_key: __,
primary_key: __

All but the relation_name are optional

Question 26

What’s the difference between has_one and has_many?

Answer 26

has_one is just a has_many with a call to Array#first on the result

Question 27

How does a “has_many through” association know which association to go through?

What does this do in SQL?

Answer 27

through: :which_association

joins the model’s table to the through: table

Question 28

How do you search for files in Atom?

Answer 28

cmd + T

Question 29

How do you center a div?

Answer 29

margin 0 auto

Question 30

Why do inline-block elements have spacing between them?

How do you fix this?

Answer 30

They’re treated like words. The “\n” after a tag is interpreted as whitespace.

To fix: float one element to align to the other, or set text-size: 0

Question 31

SQL: How do you specify an INTEGER to auto-increment?

Answer 31

SERIAL

Question 32

How do you list all dbs via psql?

Answer 32

\l (lowercase L)

Question 33

How do you escape quotes in SQL

Answer 33

double the quote

Question 34

How do you do emojis in Atom

Answer 34

cmd + ctrl + space

Question 35

What’s the default join type?

Answer 35

INNER JOIN

Question 36

How do you validate that a model have a unique name?

Answer 36

validate :name, uniqueness: true

Question 37

What will this do:

validate :name, uniqueness: true

Answer 37

Validate that a model has a unique name

Question 38

How do you validate that a model contain a particular attribute?

Answer 38

validate :attribute_name, presence: true

Question 39

What will this do:

validate :attribute_name, presence: true

Answer 39

validate that a model contains a particular attribute

Question 40

How do you create an optional belongs_to association?

Answer 40

pass in this parameter to belongs_to

optional: true

Question 41

How do we skip a validation if an object is nil? How about if it is empty?

Answer 41

pass in this parameter to validates :

allow_nil: true

allow_blank: true

Question 42

How do you optionally specify a validation?

Answer 42

pass in this parameter to validates :

if: :my_boolean_method?

Question 43

What’s the major difference between an ActiveRecord::Relation and an array?

Answer 43

The former is lazy. The contents are not fetched until needed.

Question 44

What are two ways of forcing the evaluation of a Relation?

Answer 44

to_a

Relation#load

Question 45

In relation to queries within ActiveRecord::Base, what is a scope?

Why is it called a scope?

Answer 45

An ActiveRecord::Base class method that returns all or part of a query and then returns the resulting Relation object

It’s called a scope because we can tack more query method onto the end of it. It effectively “scopes” the space in which we’re querying data from.

Question 46

Can we use a scope from an instance? Why or why isn’t this surprising?

Answer 46

Yes, we can.

It’s surprising because we define it in a class method but it can be used as an instance method.

Question 47

What’s the difference between ::find and ::find_by ?

Answer 47

::find accepts a single argument, the id of the record in question, and raises an exception if the record is not found.

::find_by accepts an options hash, which allows for more detailed search criteria, and returns nil if the record is not found.

Question 48

What query method can you use to retrieve records in a specific order?

Answer 48

::order

Question 49

How do you use your own SQL to locate a record?

Answer 49

::find_by_sql

Question 50

How does rails deal with the SQL ternary problem? What is the SQL ternary problem?

Answer 50

Comparing something with a NULL object always returns NULL.

rails uses the ::where method to correctly specify one of these two comparisons:

obj = ?
obj IS NULL

Question 51

How would you run a rails query for a User that is not NULL?

Answer 51

User.where.not(name: nil)

Question 52

How do you set a column to be required within a migration?

Answer 52

add this to the end of the t.coltype line:

null: false

ex:

t.text “body”, null: false

Question 53

How do you prefetch associations for a model?

Answer 53

includes(:association_name)

Question 54

includes(:association_name)

What does this code do?

Answer 54

Prefetches objects associated with an association

Question 55

What’s an N+1 query? Why is it bad? How can you get around it?

Answer 55

It means you run N additional queries when fetching multiple objects from the same relation.

This is expensive in DB overhead.

You can get around it by prefetching data from the association using #includes

Question 56

CSS: What is a clearfix?

Answer 56

An element that has the following properties for the purpose of containing a float (using the dimensions of the float object to dictate the dimensions of its parent):

.clearfix:after {
content: "";
display: block;
clear: both;
}

Question 57

How do you debug queries generated by rails?

Answer 57

add .to_sql on the end of a query