rails_questions.md

Ruby on Rails Questions

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These questions go with a Sample App that you can find here

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1. Find all the posts commented by the first user

Tips

You can start with a joins.

Answer

Post.joins(:comments).where(:comments => {user: User.first})

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2. Find out how many comments they are for each post, be careful for N+1 queries !

Tips

One includes a day keep the n + 1 query away.

Answer

Post.includes(:comments).map { |post| post.comments.size }

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3. We want to display the first five posts on our main page, correct the method in the model Post

Tips

It's about setting boundaries.

Answer

  def self.first_five
    limit(5)
  end

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4. Find out which users are born before 2010

Tips

StackOverFlow is strong with this one.

Answer

  User.where('extract(year from birthdate) < ?', 2010)

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5. Write a method to dynamically find all users born before a given year

Tips

You need to write a scope, here is a tutorial.

Answer

    scope :born_before, ->(year) { where('extract(year from birthdate) < ?', year) }

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6. Make sure that an user can only comment an article once

Tips

You need to write a validation.

Answer

     validates :user, uniqueness: { scope: :post,
    message: "can only comment once" }

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7. Finds out all the posts without a comment

Tips

I don't really have a hint for that because it's kind of magic.

Answer

     Post.includes(:comments).where(comments: { post_id: nil })

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8. Get in a array the content of the comments for the last post

Tips

Spoiler alert: it's a one-liner thanks to this.

Answer

     Post.last.comments.pluck(:content)

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9. We want to make sure that every time a comment is made on a post, the post timestamps are updated

Tips

You'll need a callback and, I'm not kidding, this is an hint.

Answer

You'll have a after_save callback like this in comment:

    def update_post
      post.touch
    end

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