Having two serial keys in postgresql via sqlalchemy

Question

I have an unusual challenge. I'm modifying a table to be able to join with two other legacy groups of PostgreSQL tables.

One group pretty much requires that each record in the table have a unique integer. So, the following field definition would work:

numeric_id = sql.Column(sql.Integer, primary_key=True)

The other group of tables all use UUID fields for the expected JOIN requests. So the following field definition would work:

uu_account_id = sql.Column(UUID(as_uuid=True), primary_key=True)

But, clearly, I can't have two primary keys. So one of them needs to not be a primary key. It would be nice to simply have both still be automatically assigned when a new record is made.

Any suggestions?

I'm sure I can do a quick hack, but I'm curious if there is a nice clean answer.

(And no: changing the other tables is NOT an option. Way too much legacy code.)

Answer 1

Make the uuid column the primary key, like usual.

Define the other column as having serial type and unique . In SQL I'd write

create table mytable (
    mytable_id uuid primary key default uuid_generate_v4(),
    mytable_legacy_id serial unique not null,
    ... other cols ...
);

so you just need to do the SQLAlchemy equivalent, whatever that is, of a not null , unique field.

Note that "serial" is just shorthand for

create sequence tablename_colname_seq;
create table tablename (
    colname integer default nextval('tablename_colname_seq'),
    ... cols ...
);
alter sequence tablename_colname_seq owned by tablename.colname;

so if you can't make sqlalchemy recognise that you can have a serial field that isn't a primary key, you can do it this way instead.

Answer 2

Between the SQLAlchemy, alembic (which I also use), and PostgreSQL, this turned out to be tricky.

If creating a brand new table from scratch, the following works for my numeric_id column:

numeric_id = sql.Column(sql.Integer, sql.Sequence('mytable_numeric_id_seq'), unique=True, nullable=False)

(It is possible that the unique=True and nullable=False are overkill.)

However, if modifying an existing table, the sequence itself fails to get created. Or, at least, I couldn't get it to work.

The sequence can be created by hand, of course. Or, if using 'alembic' to make distributed migrations easier, add:

from sqlalchemy.schema import Sequence, CreateSequence

def upgrade():
    op.execute(CreateSequence(Sequence("mytable_numeric_id_seq")))

To the version script created by alembic.

Special thanks to Craig for his help.

(Note: most of the SQLAlchemy examples on the net use db. as the module alias rather than sql. . Same thing really. I used sql. simply because I'm using db. already for MongoDB.)