Alembic migration to convert serial field to integer

Question

I initially defined one of my SQLAlchemy models as:

class StreetSegment(db.Model):
    id = db.Column(db.Integer, autoincrement=True)    # surrogate ID
    seg_id = db.Column(db.Integer, primary_key=True)  # assigned in another system; don't increment

not realizing that seg_id would become a SERIAL field in my Postgres database. What I really wanted was an INTEGER field with a PK constraint (no autoincrement). I turned off autoincrement like so:

class StreetSegment(db.Model):
    id = db.Column(db.Integer, autoincrement=True)
    seg_id = db.Column(db.Integer, primary_key=True, autoincrement=False)  # <--- see here

but the change isn’t reflected when I run migrate in Alembic. I’ve also tried writing a custom migration with the following operations:

def upgrade():
    op.alter_column('street_segment', 'seg_id', autoincrement=False)

def downgrade():
    op.alter_column('street_segment', 'seg_id', autoincrement=True)

but that gives the warning autoincrement and existing_autoincrement only make sense for MySQL. So my question is: is there any way of using Alembic to convert a SERIAL to an INTEGER in Postgres?

Asked By: serverpunk

||

Source

Answer 1

Just set the type explicitly to the one you want. This should work:

op.alter_column('street_segment', 'seg_id', _type=Integer)

Answered By: Oin

Answer 2

I ended up using the following code, basically using raw sql queries, which drops the default value to make it an integer(since serial is integer with a sequence value for default). And for downgrading, the sequence is created and the column’s default is set.

table = "users"
id_column = "id"

def upgrade():
    op.execute(f"ALTER TABLE {table} ALTER COLUMN {id_column} DROP DEFAULT")
    op.execute(f"DROP SEQUENCE {table}_{id_column}_seq")

    # ALTER TABLE yourtable ALTER COLUMN yourcolumn DROP DEFAULT;

def downgrade():
    op.execute(f"CREATE SEQUENCE {table}_{id_column}_seq")
    op.execute(f"SELECT SETVAL('{table}_{id_column}_seq',  (SELECT MAX({id_column})+1 FROM {table}));")
    op.execute(f"ALTER TABLE {table} ALTER {id_column} SET DEFAULT nextval('{table}_{id_column}_seq')")
    op.execute(f"ALTER SEQUENCE {table}_{id_column}_seq OWNED BY {table}.{id_column}")

Answered By: lordvcs

Alembic migration to convert serial field to integer

Question:

Answers: