Ruby, Rails and Sql Specific Questions

Interview_questions.md

Rails

1. Fix the following snippet. Given a collection of 1000 draft Articles, update all of them to be published

   Post.where(status: :draft).each { |p| p.update_attributes(status: :published}

   #Ans. Post.where(status: :draft).update_all(status: :published)

2. Given the following model tables

   users

   id	username	email
   2	Leslie	leslie.dunn@mail.com
   3	Amy	amy.cook@mail.com
   5	Gavin	gavin.olson@mail.com
   7	Lucas	lucas.carr@mail.com
   11	Jennie	jennie.cruz@mail.com

   posts

   id	user_id	title
   10	2	Trump, in Poland, Asks if West Has the 'Will to Survive'
   11	3	Merkel Has to Deal With Trump, the Question Is How
   12	2	New Jersey Transit Train Derails at Pennsylvania Station
   13	5	Faces of Intermarriage, 50 Years After the Loving Case
   14	11	In Milan, Beer, History and, of Course, Fashion
   15	3	Review: Spider-Man (Again) and All That Sticky Kid Stuff
   16	7	Editorial: Showdown in Hamburg

   comments

   id	post_id	content
   10	11	There are various options that can be discussed
   11	12	This is crazy
   12	11	You could almost see her analyze Trump, run through the various scenarios and approaches for dealing with him
   13	15	Wow!!! Another somewhat positive review of a super hero movie.
   14	16	This is a very dangerous man who is making the U.S. into a backwater.
   15	12	They had been aware of the problem but had underestimated just how urgently it needed to be addressed

   Use ActiveRecord to present the following serialized response. Optimize your code for performance.

   RESPONSE

   {
     "users": [
       {
         id: 2,
         "username": "Leslie",
         "email": "leslie.dunn@mail.com",
         "posts": [
           {
             "id": 10,
             "title": "Trump, in Poland, Asks if West Has the 'Will to Survive'",
             "comments": []
           },
           {
             "id": 12,
             "title": "New Jersey Transit Train Derails at Pennsylvania Station",
             "comments": [
               {
                 "id": 12,
                 "content": "This is crazy"
               },
               ...
             ]
           },
           ...
         ]
       },
       ...
     ]
   
   }

   #Ans. 
   
   Gem : active_model_serializers
   
   class UserSerializer < ActiveModel::Serializer
       attributes :id, :username, :email
       has_many :posts
   end
   
   class PostSerializer < ActiveModel::Serializer
       attributes :id, :title
       has_many :comments
   end
   
   class CommentSerializer < ActiveModel::Serializer
       attributes :id, :content
   end
   
   class UsersController < ApplicationController
       def index
         users = User.all.includes(posts: [:comments])
         render json: users, each_serializer: UserSerializer
       end
   end
   
   Api -> /users

Ruby

1. What is self.included? How would you use it in mixins?

    #Ans.  It is a class method that will be called automatically when another class or module try to include as their extension. Included module methods will be accesed with instance variable.
    eg: 
    module B
     def say
        "hello"
     end
    end
    class A 
      include B
    end
    A.new.say

2. Why is it bad to use rescue in the following manner? What do you understand about the ruby exception hierarchy?

   begin
     do_something()
   rescue Exception => e
     puts e
   end

   #Ans.  Exception is class of root error in error hirarchy.  It can raise any error which not proper with the method or running process. Hence its better to use specific class or use standard class
   
   Eg: begin; 1+1=2; rescue Exception => e; puts e;end
   
   If there is issue with database, the Exception will be called which is not related with the process 1+1=2

SQL

1. A cooperation booth sells gloves in singles and pairs. Each transaction may be a purchase of a single side of glove, or a combination of left/right gloves. It is invalid, however, when a combination of right-right or left-left gloves to be sold.

   Given the following data, write a SQL query to list the duplicate gloves (for both left-left and right-right) sold.

   Transaction

   id	sold_at
   1	'2017-05-07 15:29:36 +0800'
   2	'2017-05-09 17:01:59 +0800'
   3	'2017-06-10 09:32:01 +0800'
   4	'2017-06-12 09:34:47 +0800'

   Gloves

   id	transaction_id	glove_type
   10	1	"right"
   11	1	"left"
   12	2	"left"
   13	3	"left"
   14	3	"left"
   15	4	"right"
   16	5	"left"
   17	4	"left"
   18	5	"left"
   19	6	"right"

   Your query result should be:

   id	transaction_id	glove_type
   14	3	"left"
   18	5	"left"

   #Ans.  SELECT transaction_id, glove_type, count(*) FROM gloves group by transaction_id, glove_type HAVING ( COUNT(*) > 1 )