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from django.db.model.sql import Query  # Query()는 개발자가 정의한 QuerySet을 읽어서 실제 SQL을 생성해주는 구현체다


class QuerySet:
  query: Query = Query()  # 이것을 나는 메인쿼리 라고 명명한다.
  _result_cache: List[Dict[Any,Any]] = dict()  # SQL의 수행결과를 여기 저장해놓고 재사용한다 (QuerySet Cache)
                                               # QuerySet 재호출시 이 프로퍼티에 저장된 데이터가 없으면 SQL을 호출해서 데이터를 가져온다.
      
 _prefetch_related_lookups: Tuple[str] = ()    # prefetch_related(여기선언된 값들) 을 이 프로퍼티에 저장함  
                                               # 이것을 나는 추가쿼리(셋)이라고 명명한다.

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UPDATE (March 2020, thanks @ic): I don't know the exact AMI version but yum install docker now works on the latest Amazon Linux 2. The instructions below may still be relevant depending on the vintage AMI you are using.

Amazon changed the install in Linux 2. One no-longer using 'yum' See: https://aws.amazon.com/amazon-linux-2/release-notes/

Docker CE Install

sudo amazon-linux-extras install docker
sudo service docker start

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Redirects in React Router v6

An important part of "routing" is handling redirects. Redirects usually happen when you want to preserve an old link and send all the traffic bound for that destination to some new URL so you don't end up with broken links.

The way we recommend handling redirects has changed in React Router v6. This document explains why.

Background

In React Router v4/5 (they have the same API, you can read about why we had to bump the major version here) we had a <Redirect> component that you could use to tell the router when to automatically redirect to another URL. You might have used it like this:

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#! /bin/bash

# ECHO COMMAND
# echo Hello World!

# VARIABLES
# Uppercase by convention
# Letters, numbers, underscores
NAME="Bob"
# echo "My name is $NAME"

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#! /bin/bash

# ECHO COMMAND
# echo Hello World!

# VARIABLES
# Uppercase by convention
# Letters, numbers, underscores
NAME="Bob"
# echo "My name is $NAME"

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Anchors in Markdown

To create an anchor to a heading in github flavored markdown. Add - characters between each word in the heading and wrap the value in parens (#some-markdown-heading) so your link should look like so:

[create an anchor](#anchors-in-markdown)

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1. Connect to your EC2 instance
2. Install zsh : sudo apt-get update && sudo apt-get install zsh
3. Edit your passwd configuration file to tell which shell to use for user ubuntu : sudo vim /etc/passwd
4. Look for ubuntu user, and replace bin/bash by bin/zsh
5. Install OhMyZsh : sh -c "$(wget https://raw.githubusercontent.com/robbyrussell/oh-my-zsh/master/tools/install.sh -O -)"
6. Disconnect from your instance and reconnect it.