Hi there!
I'm looking to create a helper or decorator function that will enable us to
"mark" a column or table as “unused”, and raise an error if that column or
table is queried for in SQL.
Context
The motivation behind this is to help us catch accidental references to
deleted columns in our code. We currently follow this process for database
migrations that involve dropping columns or tables:
1. Remove all usages of the column in the code, except the column
definition. (We keep the column definition around since we have tests to
ensure our column definitions match up with our migrated database schemas.)
2. Make the column deferred
3. Wait a few days, then remove the column definition and run the
database migration that actually drops the column
The main issue with this process is that if there’s any code left in that
loads in the deleted column, we run into a number of issues upon running
the migration. We’d like to introduce a helper or decorator for dropped
columns/tables that will trigger an error when the column/table is
referenced / loaded in any of the following ways:
1. session.query(MyTable.my_column).all()
2. session.query(MyTable).options(undefer('*')).all()
3. my_table_instance.my_column
4. session.query(MyDroppedTable).all()
Ideally, this would be a lightweight wrapper or decorator that could be
applied to any column or table — something like my_column =
unused(Column(…)) or an @unused decorator on a class.
Prior Work
We’ve looked at using hybrid_property to achieve this:
class MyTable():
...
_my_column = deferred(Column('my_column', String(255), nullable=True))
@hybrid_property
def my_column(self):
raise Exception('Unused column')
@my_column.setter
def my_column(self, value):
raise Exception('Unused column')
What this gets us:
- Exception on my_table_instance.my_column
- Exception on session.query(MyTable.my_column).all()
- Exception on session.query(MyTable).options(undefer('my_column')).all()
Issues with this approach:
1. No exception on session.query(MyTable).options(undefer('*')).all()
2. Requires a fair amount of implementation overhead for each column
3. Not applicable to unused tables
Any tips on how we can best use SQLAlchemy to accomplish all of the above?
Thank you!
Neena
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