Update database docs for plugin authors for new alembic branching world

Things have gotten both easier and more complex in the branching world
view.  Plugins branch off of main with their own migrations.
Generating these has become a bit easier, but the steps for doing this
are a bit wordy to explain.

* docs/source/pluginwriter/database.rst: Update both the
  "Creating new Tables" and "Changing the Database Schema Later"
  sections accordingly.

docs/source/pluginwriter/database.rst

@@ -61,7 +61,39 @@ Here's a simple one:
 
     MODELS = [MediaSecurity]
 
-That's it.
+Next, you need to make an initial migration.  MediaGoblin uses
+`Alembic's branching model <http://alembic.readthedocs.org/en/latest/branches.html>`_
+to handle plugins adding their own content.  As such, when you are
+adding a new plugin, you need to add an initial migration adding
+the existing models, and migrate from there.
+
+You'll need to make a `migrations` subdirectory for migrations and put
+your migrations there.  If you want to look at some example
+migrations, look at `mediagoblin/media_types/image/migrations/`,
+especially the "initial" migration.  (Plugin authors with plugins that
+existed prior to the alembic switchover: you might notice how it
+checks for the table and skips the migration if it already exists.
+Plugin authors writing brand new plugins, post-Alembic migration
+switchover, do not need to do this.)
+
+Unfortunately, these migrations are a bit tedious to write.
+Fortunately, Alembic can do much of the work for us!  After adding the
+models.py file, run this command (switching out YOUR_PLUGIN_NAME of
+course)::
+
+  ./bin/gmg alembic --with-plugins revision \
+       --splice --autogenerate \
+       --branch-label YOUR_PLUGIN_NAME_plugin \
+       -m "YOUR_PLUGIN_NAME plugin initial migration"
+
+(Note that `--with-plugins` *must* come before any alembic subcommand...
+this is a quirk related to the way we handle alembic command dispatching
+with the gmg subcommand!)
+
+This will dump your migration into "the wrong place" (it'll dump it
+into the MediaGoblin core migrations directory), so you should move it
+to your plugin's migrations directory.  Open the file and make adjustments
+accordingly!
 
 Some notes:
 
@@ -78,37 +110,36 @@ Some notes:
 Changing the Database Schema Later
 ==================================
 
-If your plugin is in use and instances use it to store some
-data, changing the database design is a tricky thing.
+If your plugin is in use and instances use it to store some data,
+changing the database design is tricky and must be done with care,
+but is not impossible.
 
-1. Make up your mind how the new schema should look like.
-2. Change ``models.py`` to contain the new schema. Keep a
-   copy of the old version around for your personal
-   reference later.
-3. Now make up your mind (possibly using your old and new
-   ``models.py``) what steps in SQL are needed to convert
-   the old schema to the new one.
-   This is called a "migration".
-4. Create a file ``migrations.py`` that will contain all
-   your migrations and add your new migration.
+Luckily, Alembic can once again help with autogenerating what is
+probably very close to the migration you want.  First you will need to
+find out what the revision id of your plugin's most recent migrations
+is.  There are two ways to do this: look in your plugin's migrations/
+directory and figure it out with the hope that it's "obvious" in some
+way.  The second path: let Alembic give that info for you.
 
-Take a look at the core's ``db/migrations.py`` for some
-good examples on what you might be able to do. Here's a
-simple one to add one column:
+Assuming you've already done the latest dbupdate with your plugin
+enabled, do the following:
 
-.. code-block:: python
+  ./bin/gmg alembic --with-plugins heads
 
-    from mediagoblin.db.migration_tools import RegisterMigration, inspect_table
-    from sqlalchemy import MetaData, Column, Integer
+You should see the latest migration id for your plugin's label.
 
-    MIGRATIONS = {}
+Make changes to your
+plugin's models.py and then run::
 
-    @RegisterMigration(1, MIGRATIONS)
-    def add_license_preference(db):
-        metadata = MetaData(bind=db.bind)
+  ./bin/gmg alembic --with-plugins revision \
+       --head REVISION_HERE \
+       --autogenerate \
+       -m "YOUR_PLUGIN_NAME: Change explaination here."
 
-        security_table = inspect_table(metadata, 'yourplugin__media_security')
+Once again, this will dump the migration into the wrong place, so move
+to your plugin's migrations directory.  Open the file, adjust
+accordingly, and read carefully!  Now you should also test your
+migration with some real data.  Be sure to test it both on sqlite
+*AND* on postgresql!
 
-        col = Column('security_level', Integer)
-        col.create(security_table)
-        db.commit()
+Whew, you made it!  Get yourself a cookie to celebrate!