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+This plan has several problems and has been modified; new plan is discussed in wiki:RepositoryFormat2
+
+----
+
+One problem with [http://www.rubyonrails.org/ Ruby on Rails'] (very good) schema migration system is the behavior of scripts that depend on outside sources; ie. the application. If those change, there's no guarantee that such scripts will behave as they did before, and you'll get strange results. 
+
+For example, suppose one defines a SQLAlchemy table:
+{{{
+users = Table('users', metadata, 
+    Column('user_id', Integer, primary_key = True),
+    Column('user_name', String(16), nullable = False),
+    Column('password', String(20), nullable = False)
+)
+}}}
+and creates it in a change script:
+{{{
+from project import table
+
+def upgrade():
+  table.users.create()
+}}}
+
+Suppose we later add a column to this table. We write an appropriate change script:
+{{{
+from project import table
+
+def upgrade():
+  # This syntax isn't set in stone yet
+  table.users.add_column('email_address', String(60), key='email')
+}}}
+...and change our application's table definition:
+{{{
+users = Table('users', metadata, 
+    Column('user_id', Integer, primary_key = True),
+    Column('user_name', String(16), nullable = False),
+    Column('password', String(20), nullable = False),
+    Column('email_address', String(60), key='email')  #new column
+)
+}}}
+
+Modifying the table definition changes how our first script behaves - it will create the table with the new column. This might work if we only apply change scripts to a few database which are always kept up to date (or very close), but we'll run into errors eventually if our migration scripts' behavior isn't consistent.
+
+----
+
+One solution is to generate .sql files from a Python change script at the time it's added to a repository. The sql generated by the script for each database is set in stone at this point; changes to outside files won't affect it.
+
+This limits what change scripts are capable of - we can't write dynamic SQL; ie., we can't do something like this:
+{{{
+for row in db.execute("select id from table1"):
+  db.execute("insert into table2 (table1_id, value) values (:id,42)",**row)
+}}}
+But SQL is usually powerful enough to where the above is rarely necessary in a migration script:
+{{{
+db.execute("insert into table2 select id,42 from table1")
+}}}
+This is a reasonable solution. The limitations aren't serious (everything possible in a traditional .sql script is still possible), and change scripts are much less prone to error.