docs/source/pluginwriter/database.rst

   1 .. MediaGoblin Documentation
   2
   3    Written in 2013 by MediaGoblin contributors
   4
   5    To the extent possible under law, the author(s) have dedicated all
   6    copyright and related and neighboring rights to this software to
   7    the public domain worldwide. This software is distributed without
   8    any warranty.
   9
  10    You should have received a copy of the CC0 Public Domain
  11    Dedication along with this software. If not, see
  12    <http://creativecommons.org/publicdomain/zero/1.0/>.
  13
  14
  15 ========
  16 Database
  17 ========
  18
  19
  20 Accessing Existing Data
  21 =======================
  22
  23 If your plugin wants to access existing data, this is quite
  24 straight forward. Just import the appropiate models and use
  25 the full power of SQLAlchemy. Take a look at the (upcoming)
  26 database section in the Developer's Chapter.
  27
  28
  29 Creating new Tables
  30 ===================
  31
  32 If your plugin needs some new space to store data, you
  33 should create a new table.  Please do not modify core
  34 tables.  Not doing so might seem inefficient and possibly
  35 is.  It will help keep things sane and easier to upgrade
  36 versions later.
  37
  38 So if you create a new plugin and need new tables, create a
  39 file named ``models.py`` in your plugin directory. You
  40 might take a look at the core's db.models for some ideas.
  41 Here's a simple one:
  42
  43 .. code-block:: python
  44
  45     from mediagoblin.db.base import Base
  46     from sqlalchemy import Column, Integer, Unicode, ForeignKey
  47
  48     class MediaSecurity(Base):
  49         __tablename__ = "yourplugin__media_security"
  50
  51         # The primary key *and* reference to the main media_entry
  52         media_entry = Column(Integer, ForeignKey('core__media_entries.id'),
  53             primary_key=True)
  54         get_media_entry = relationship("MediaEntry",
  55             backref=backref("security_rating", cascade="all, delete-orphan"))
  56
  57         rating = Column(Unicode)
  58
  59     MODELS = [MediaSecurity]
  60
  61 That's it.
  62
  63 Some notes:
  64
  65 * Make sure all your ``__tablename__`` start with your
  66   plugin's name so the tables of various plugins can't
  67   conflict in the database. (Conflicts in python naming are
  68   much easier to fix later).
  69 * Try to get your database design as good as possible in
  70   the first attempt.  Changing the database design later,
  71   when people already have data using the old design, is
  72   possible (see next chapter), but it's not easy.
  73
  74
  75 Changing the Database Schema Later
  76 ==================================
  77
  78 If your plugin is in use and instances use it to store some
  79 data, changing the database design is a tricky thing.
  80
  81 1. Make up your mind how the new schema should look like.
  82 2. Change ``models.py`` to contain the new schema. Keep a
  83    copy of the old version around for your personal
  84    reference later.
  85 3. Now make up your mind (possibly using your old and new
  86    ``models.py``) what steps in SQL are needed to convert
  87    the old schema to the new one.
  88    This is called a "migration".
  89 4. Create a file ``migrations.py`` that will contain all
  90    your migrations and add your new migration.
  91
  92 Take a look at the core's ``db/migrations.py`` for some
  93 good examples on what you might be able to do. Here's a
  94 simple one to add one column:
  95
  96 .. code-block:: python
  97
  98     from mediagoblin.db.migration_tools import RegisterMigration, inspect_table
  99     from sqlalchemy import MetaData, Column, Integer
 100
 101     MIGRATIONS = {}
 102
 103     @RegisterMigration(1, MIGRATIONS)
 104     def add_license_preference(db):
 105         metadata = MetaData(bind=db.bind)
 106
 107         security_table = inspect_table(metadata, 'yourplugin__media_security')
 108
 109         col = Column('security_level', Integer)
 110         col.create(security_table)
 111         db.commit()