Problems downgrading discrete geometry migration

[jameshiebert]

The discrete sampling geometry upgrade cannot be downgraded if data has already been added.

The following error message results:

alembic -x db=docker_postgres downgrade -1
### Creating modelmeta ORM
/home/lzeman/Code/modelmeta-newdb/modelmeta/venv/lib/python3.5/site-packages/psycopg2/__init__.py:144: UserWarning: The psycopg2 wheel package will be renamed from release 2.8; in order to keep installing from binary please use "pip install psycopg2-binary" instead. For details see: <http://initd.org/psycopg/docs/install.html#binary-install-from-pypi>.
  """)
INFO  [alembic.runtime.migration] Context impl PostgresqlImpl.
INFO  [alembic.runtime.migration] Will assume transactional DDL.
INFO  [alembic.runtime.migration] Running downgrade 12f290b63791 -> 614911daf883, handle variant sampling geometries
Traceback (most recent call last):
  File "/home/lzeman/Code/modelmeta-newdb/modelmeta/venv/lib/python3.5/site-packages/sqlalchemy/engine/base.py", line 1193, in _execute_context
    context)
  File "/home/lzeman/Code/modelmeta-newdb/modelmeta/venv/lib/python3.5/site-packages/sqlalchemy/engine/default.py", line 509, in do_execute
    cursor.execute(statement, parameters)
psycopg2.IntegrityError: insert or update on table "data_file_variables" violates foreign key constraint "data_file_variables_grid_id_fkey"
DETAIL:  Key (grid_id)=(0) is not present in table "grids".


The above exception was the direct cause of the following exception:

Traceback (most recent call last):
  File "/home/lzeman/Code/modelmeta-newdb/modelmeta/venv/bin/alembic", line 11, in <module>
    sys.exit(main())
  File "/home/lzeman/Code/modelmeta-newdb/modelmeta/venv/lib/python3.5/site-packages/alembic/config.py", line 486, in main
    CommandLine(prog=prog).main(argv=argv)
  File "/home/lzeman/Code/modelmeta-newdb/modelmeta/venv/lib/python3.5/site-packages/alembic/config.py", line 480, in main
    self.run_cmd(cfg, options)
  File "/home/lzeman/Code/modelmeta-newdb/modelmeta/venv/lib/python3.5/site-packages/alembic/config.py", line 463, in run_cmd
    **dict((k, getattr(options, k, None)) for k in kwarg)
  File "/home/lzeman/Code/modelmeta-newdb/modelmeta/venv/lib/python3.5/site-packages/alembic/command.py", line 294, in downgrade
    script.run_env()
  File "/home/lzeman/Code/modelmeta-newdb/modelmeta/venv/lib/python3.5/site-packages/alembic/script/base.py", line 427, in run_env
    util.load_python_file(self.dir, 'env.py')
  File "/home/lzeman/Code/modelmeta-newdb/modelmeta/venv/lib/python3.5/site-packages/alembic/util/pyfiles.py", line 81, in load_python_file
    module = load_module_py(module_id, path)
  File "/home/lzeman/Code/modelmeta-newdb/modelmeta/venv/lib/python3.5/site-packages/alembic/util/compat.py", line 82, in load_module_py
    spec.loader.exec_module(module)
  File "<frozen importlib._bootstrap_external>", line 665, in exec_module
  File "<frozen importlib._bootstrap>", line 222, in _call_with_frames_removed
  File "alembic/env.py", line 118, in <module>
    run_migrations_online()
  File "alembic/env.py", line 112, in run_migrations_online
    context.run_migrations()
  File "<string>", line 8, in run_migrations
  File "/home/lzeman/Code/modelmeta-newdb/modelmeta/venv/lib/python3.5/site-packages/alembic/runtime/environment.py", line 836, in run_migrations
    self.get_context().run_migrations(**kw)
  File "/home/lzeman/Code/modelmeta-newdb/modelmeta/venv/lib/python3.5/site-packages/alembic/runtime/migration.py", line 330, in run_migrations
    step.migration_fn(**kw)
  File "/home/lzeman/Code/modelmeta-newdb/modelmeta/alembic/versions/12f290b63791_handle_variant_sampling_geometries.py", line 197, in downgrade
    ['grid_id'], ['grid_id'])
  File "/usr/lib/python3.5/contextlib.py", line 66, in __exit__
    next(self.gen)
  File "/home/lzeman/Code/modelmeta-newdb/modelmeta/venv/lib/python3.5/site-packages/alembic/operations/base.py", line 300, in batch_alter_table
    impl.flush()
  File "/home/lzeman/Code/modelmeta-newdb/modelmeta/venv/lib/python3.5/site-packages/alembic/operations/batch.py", line 53, in flush
    fn(*arg, **kw)
  File "/home/lzeman/Code/modelmeta-newdb/modelmeta/venv/lib/python3.5/site-packages/alembic/ddl/impl.py", line 177, in add_constraint
    self._exec(schema.AddConstraint(const))
  File "/home/lzeman/Code/modelmeta-newdb/modelmeta/venv/lib/python3.5/site-packages/alembic/ddl/impl.py", line 115, in _exec
    return conn.execute(construct, *multiparams, **params)
  File "/home/lzeman/Code/modelmeta-newdb/modelmeta/venv/lib/python3.5/site-packages/sqlalchemy/engine/base.py", line 948, in execute
    return meth(self, multiparams, params)
  File "/home/lzeman/Code/modelmeta-newdb/modelmeta/venv/lib/python3.5/site-packages/sqlalchemy/sql/ddl.py", line 68, in _execute_on_connection
    return connection._execute_ddl(self, multiparams, params)
  File "/home/lzeman/Code/modelmeta-newdb/modelmeta/venv/lib/python3.5/site-packages/sqlalchemy/engine/base.py", line 1009, in _execute_ddl
    compiled
  File "/home/lzeman/Code/modelmeta-newdb/modelmeta/venv/lib/python3.5/site-packages/sqlalchemy/engine/base.py", line 1200, in _execute_context
    context)
  File "/home/lzeman/Code/modelmeta-newdb/modelmeta/venv/lib/python3.5/site-packages/sqlalchemy/engine/base.py", line 1413, in _handle_dbapi_exception
    exc_info
  File "/home/lzeman/Code/modelmeta-newdb/modelmeta/venv/lib/python3.5/site-packages/sqlalchemy/util/compat.py", line 265, in raise_from_cause
    reraise(type(exception), exception, tb=exc_tb, cause=cause)
  File "/home/lzeman/Code/modelmeta-newdb/modelmeta/venv/lib/python3.5/site-packages/sqlalchemy/util/compat.py", line 248, in reraise
    raise value.with_traceback(tb)
  File "/home/lzeman/Code/modelmeta-newdb/modelmeta/venv/lib/python3.5/site-packages/sqlalchemy/engine/base.py", line 1193, in _execute_context
    context)
  File "/home/lzeman/Code/modelmeta-newdb/modelmeta/venv/lib/python3.5/site-packages/sqlalchemy/engine/default.py", line 509, in do_execute
    cursor.execute(statement, parameters)
sqlalchemy.exc.IntegrityError: (psycopg2.IntegrityError) insert or update on table "data_file_variables" violates foreign key constraint "data_file_variables_grid_id_fkey"
DETAIL:  Key (grid_id)=(0) is not present in table "grids".
 [SQL: 'ALTER TABLE data_file_variables ADD CONSTRAINT data_file_variables_grid_id_fkey FOREIGN KEY(grid_id) REFERENCES grids (grid_id)'] (Background on this error at: http://sqlalche.me/e/gkpj)

There is no grid_id 0 in this database; the grid_ids start at 1. No entries in any other table reference grid_id 0, either, I'm not sure why it's trying to access grid_id 0 at all.

Since we don't expect to downgrade the live database, this is unlikely to be a problem, but it does come up in testing.