Set up Alembic for database migrations

Signed-off-by: Lukas Fleischer <lfleischer@archlinux.org>

migrations/README

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+This directory contains Alembic's environment for managing database migrations.
+
+From Alembic's documentation: Alembic is a lightweight database migration tool
+for usage with the SQLAlchemy Database Toolkit for Python.
+https://alembic.sqlalchemy.org/en/latest/index.html
+
+
+Upgrading to the latest version
+-------------------------------
+
+Simply run `alembic upgrade head` from aurweb's root.
+
+
+Creating new migrations
+-----------------------
+
+When working with Alembic and SQLAlchemy, you should never edit the database
+schema manually. Please proceed like this instead:
+
+1. Edit `aurweb/schema.py` to your liking.
+2. Run `alembic revision --autogenerate -m "your message"`
+3. Proofread the generated migration.
+4. Run `alembic upgrade head` to apply the changes to the database.
+5. Commit the new migration.
+
+To revert a migration, you may run `alembic downgrade -1` and then manually
+delete the migration file. Note that SQLite is limited and that it's sometimes
+easier to recreate the database.
+
+For anything more complicated, please read Alembic's documentation.
+
+
+Troubleshooting
+---------------
+
+- `ModuleNotFoundError: No module named 'aurweb'`
+
+  You may either install the aurweb module with pip, or set PYTHONPATH to your
+  aurweb repository. Since alembic must be run from the aurweb root, you may
+  simply use: `PYTHONPATH=. alembic [...]`.
+
+- `FAILED: No config file 'alembic.ini' found, or file has no '[alembic]' section`
+
+  You need to run Alembic from the project's root, and not from `migrations/`.
+
+- `configparser.NoSectionError: No section: 'database'`
+
+  You need to set AUR_CONFIG, as explained in `TESTING`.

migrations/env.py

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+import aurweb.db
+import aurweb.schema
+
+from alembic import context
+import logging.config
+import sqlalchemy
+
+
+# this is the Alembic Config object, which provides
+# access to the values within the .ini file in use.
+config = context.config
+
+# Interpret the config file for Python logging.
+# This line sets up loggers basically.
+logging.config.fileConfig(config.config_file_name)
+
+# model MetaData for autogenerating migrations
+target_metadata = aurweb.schema.metadata
+
+# other values from the config, defined by the needs of env.py,
+# can be acquired:
+# my_important_option = config.get_main_option("my_important_option")
+# ... etc.
+
+
+def run_migrations_offline():
+    """Run migrations in 'offline' mode.
+
+    This configures the context with just a URL
+    and not an Engine, though an Engine is acceptable
+    here as well.  By skipping the Engine creation
+    we don't even need a DBAPI to be available.
+
+    Calls to context.execute() here emit the given string to the
+    script output.
+
+    """
+    context.configure(
+        url=aurweb.db.get_sqlalchemy_url(),
+        target_metadata=target_metadata,
+        literal_binds=True,
+        dialect_opts={"paramstyle": "named"},
+    )
+
+    with context.begin_transaction():
+        context.run_migrations()
+
+
+def run_migrations_online():
+    """Run migrations in 'online' mode.
+
+    In this scenario we need to create an Engine
+    and associate a connection with the context.
+
+    """
+    connectable = sqlalchemy.create_engine(
+        aurweb.db.get_sqlalchemy_url(),
+        poolclass=sqlalchemy.pool.NullPool,
+    )
+
+    with connectable.connect() as connection:
+        context.configure(
+            connection=connection, target_metadata=target_metadata
+        )
+
+        with context.begin_transaction():
+            context.run_migrations()
+
+
+if context.is_offline_mode():
+    run_migrations_offline()
+else:
+    run_migrations_online()

migrations/script.py.mako

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+"""${message}
+
+Revision ID: ${up_revision}
+Revises: ${down_revision | comma,n}
+Create Date: ${create_date}
+
+"""
+from alembic import op
+import sqlalchemy as sa
+${imports if imports else ""}
+
+# revision identifiers, used by Alembic.
+revision = ${repr(up_revision)}
+down_revision = ${repr(down_revision)}
+branch_labels = ${repr(branch_labels)}
+depends_on = ${repr(depends_on)}
+
+
+def upgrade():
+    ${upgrades if upgrades else "pass"}
+
+
+def downgrade():
+    ${downgrades if downgrades else "pass"}