Database table has different schema than model description in Flask application - postgresql

Question

I have an old application that I'm redeploying. The following model class:
class TableA(db.Model):
id = db.Column(db.Integer, primary_key=True)
name_key = db.Column(db.String(20), index=True, unique=True, nullable=False)
acronym_key = db.Column(db.String(6), index=True, unique=True, nullable=False)
has the following table in postgres:
Table "public.tablea"
Column | Type | Collation | Nullable | Default
-------------+------------------------+-----------+----------+----------------------------------
id | integer | | not null | nextval('tablea_id_seq'::regclass)
name_key | character varying(10) | | not null |
acronym_key | character varying(6) | | not null |
Notice the length of the column name_key, it does not match.
As I worked in this when I still didn't know what I was doing with the migrations, I double checked if I had left changes that were not saved as migrations, with
flask db migrate and flask db upgrade. I got some changes to the db, but not this one. Do column lengths do not generate migration changes? What am I missing? Any help is appreciated.

Answer 1

Adding to #PeterBlahos' link, alembic needs to be configured to notice differences in column lengths.
For that you need to modify {project-root}/migrations/env.py, modify the run_migrations_* methods context.configure segments by adding the compare_type=True as in the snippet bellow:
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.
"""
url = config.get_main_option("sqlalchemy.url")
context.configure(
url=url, target_metadata=target_metadata, literal_binds=True, compare_type=True)
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 = engine_from_config(
config.get_section(config.config_ini_section),
prefix='sqlalchemy.',
poolclass=pool.NullPool)
with connectable.connect() as connection:
context.configure(
connection=connection,
target_metadata=target_metadata,
compare_type=True
)
with context.begin_transaction():
context.run_migrations()
After that just run on your terminal flask db migrate -m "some explanation for the changes" to create the migration file and flask db upgrade for the changes to actually affect the db.
Big thanks to #PeterBlahos who actually answered 85% of the question :).