Designing Django's Migrations by Andrew Godwin

Transcript

1. Andrew Godwin
   @andrewgodwin
   Designing
   migrations
   's
   

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2. Andrew Godwin
   Author of South migrations library
   Hi, I'm
   Author of 1.7 Django migrations
   Author of "Please, stop using MySQL"
   

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3. Migrations
   They're pretty good.
   

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4. South
   ...it was good for its time
   

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5. The Initial Plan
   Django
   Schema backend
   ORM Hooks
   South 2
   Migration handling
   User interface
   

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6. The Revised Plan
   Django
   Schema backend
   ORM Hooks
   South 2
   Migration handling
   User interface
   Backport for 1.4 - 1.6
   

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7. Logically Separate
   Schema backend
   ORM Hooks
   Migration handling
   User interface
   SchemaEditor Migrations
   

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8. Moving South into Django
   instead, "Adding migrations to Django"
   

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9. SchemaEditor
   Abstracts schema operations across DBs
   Works in terms of Django fields/models
   Contains per-database workarounds
   

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10. django.db.migrations
    Migration file reader/writer
    Dependency resolver
    Autodetector
    Applied/unapplied tracking
    

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11. A New Format
    More concise
    Declarative
    Introspectable
    

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12. Migration actions
    Frozen ORM
    

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14. In-memory running
    Creates models from migration sets
    Autodetector diffs created from on-disk
    Used to feed SchemaEditor / ORM
    

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15. Database Peculiarities
    

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16. PostgreSQL
    None. Let's move on.
    

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17. MySQL
    No transactional DDL
    No CHECK constraints
    Conflates UNIQUE and INDEX
    

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18. Oracle
    Different SQL syntax
    Picky about names
    Can't convert to/from TextField (LOB)
    

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19. SQLite
    AAAAAAAAAAAAAHHHHHHHHHHHHHH
    Altering tables? Schema introspection? What?
    

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20. Backwards Compatability
    Django generally very good at this
    Auto-applies first migration if tables exist
    Ignores South-style migrations
    

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21. Lessons Learnt
    

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22. Explicit is better than implicit
    I'm sure I've heard this somewhere before...
    

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23. Abstracting DBs is hard
    Would I do it from scratch? Possibly not.
    

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24. Composability rocks
    It's simplified the code so much.
    

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25. Feedback is vital
    I'm just not mad enough to do nasty things to my code.
    

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26. Thanks, to everyone. @andrewgodwin
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