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Refactoring and Data Migration in Django

Often we need to change the way a piece of data is modelled on our backend. Recently I was faced with refactoring a large model into component pieces and needed a cheat sheet for the best practices to use when migrating live data on our production server to a new schema.

This is an explanation of how to perform an extractClass refactoring from within the context of a Django models.Model. For this tutorial, I'll assume you are familiar with schemamigration using South.

The basic purpose of this type of refactoring is usually one of the following:

• to DRY up your data model, such that a similar schema can be reused in multiple places
• to implement a One to Many relationship on data that is currently One to One, and tightly coupled to an existing Model

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###
#    Backbone views
###
#
# Handling subviews:
# http://stackoverflow.com/questions/9337927/how-to-handle-initializing-and-rendering-subviews-in-backbone-js
# Basic render strategy:
# http://ianstormtaylor.com/rendering-views-in-backbonejs-isnt-always-simple/

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XHR

model = {
  property_a: 'hello',
  property_b: 'world'
};

xhr=jQuery.ajax({type:'post', dataType:'json', url:'http://localhost:8000/api/...', data:JSON.stringify(model),'contentType':'application/json'})

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class MyModel extends Backbone.Model
  sync: (method, model, options) =>
    console.log 'MyModel : info : trying to coerce property_X to be property_Y when pushed to server'
    if method in ['create', 'update', 'patch']
      options.attrs = @toJSON(options)
      options.attrs.property_Y = @get('property_X')
      delete options.attrs.property_X
    super

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[contenteditable] {
    border: 2px solid rgba(0,0,0,0);                                                                          
}
[contenteditable]:hover {
    border: 2px dashed #eeeeee;                                                                               
}                                                                                                             
[contenteditable]:empty {                                                                                                                                                                              
    border: 2px dashed #ddeeee;                                                                               
    font-style: italic;
}                                                                                                             

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import pdb
from subprocess import call
breakpoint = pdb.set_trace

def set_trace():
    call(["say", "break point"])
    breakpoint()
pdb.set_trace = set_trace

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South Migration Rebasing

Framing the problem

(Note: this procedure will work pretty well for simple additions of properties on models, and the like. More complex changes will require more thought.)

Let's assume you have a branch off of master that has been creating lots of migrations. It forked when master was at 0017_migration_on_master.py. Now, on the branch, let's call it data-crazy, we're up to 0028_migration_on_data_crazy.py. Sadly, someone had to jump onto master and make a schema change to the database. This bumped master up to 0018_migration_on_master.py. Now, we need to merge those changes into data-crazy in order to eventually be able to merge back into master once we're stable and our feature is complete.

The Manual Rebase Process

• Validate the new base of your rebase.

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Auction lot spacing

Devise an algorithm to order lots in an auction based on the following guidlines:

1. Sellers of individual items should have their own lots spaced as evenly apart as possible
2. Items sold should generally go from cheapest to most expensive

---

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Push-o-matic

Want to work locally on your web site but have your changes periodically (like every minute) updated on Heroku?

#!/bin/bash

while [ true ]
do
	echo "Pushing recent changes to Heroku..."

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<script src="http://jashkenas.github.com/coffee-script/extras/coffee-script.js"
        type="text/javascript"
        charset="utf-8"></script>
<script type="text/coffeescript">
  # Some CoffeeScript
</script>