How do I prevent sql alchemy from inserting the None value to field? - postgresql

Question

The Alembic migration script :
def upgrade():
uuid_gen = saexp.text("UUID GENERATE V1MC()")
op.create_table(
'foo',
sa.Column('uuid', UUID, primary_key=True, server_default=uuid_gen),
sa.Column(
'inserted',
sa.DateTime(timezone=True),
server_default=sa.text("not null now()"))
sa.Column('data', sa.Text)
)
This is my Base class for SQL Alchemy:
Class Foo(Base):
__tablename__ = 'foo'
inserted = Column(TIMESTAMP)
uuid = Column(UUID, primary_key=True)
data = Column(TEXT)
It has a static mehtod for insert :
#staticmethod
def insert(session, jsondata):
foo = Foo()
foo.data = jsondata['data']
if 'inserted' in jsondata:
foo.inserted = jsondata['inserted']
if 'uuid' in jsondata:
foo.uuid = jsondata['uuid']
session.add(foo)
return foo
the purpose of the 2 if's are to simplify testing. this way i can "inject" a uuid and inserted date, to get predictible data for my tests
When trying to insert data
foo = Foo()
foo.insert(session, {"data": "foo bar baz"})
session.commit()
I get an IntegrityError :
[SQL: 'INSERT INTO foo (inserted, data) VALUES (%(inserted)s, %(data)s) RETURNING foo.uuid'] [parameters: {'data': 'foo bar baz', 'inserted': None}]
wich seem normal to me because the insert violates the "not-null" constraint in the postgres database.
How do I prevent sql alchemy from inserting the None value to the inserted field ?
While playing and testing around, I found that if the "inserted" column is defined as primary key , sql alchemy does not include the field in the insert statement.
def upgrade():
uuid_gen = saexp.text("UUID GENERATE V1MC()")
op.create_table(
'foo',
sa.Column('uuid', UUID, primary_key=True, server_default=uuid_gen),
sa.Column(
'inserted',
primary_key=True,
sa.DateTime(timezone=True),
server_default=sa.text("not null now()"))
sa.Column('data', sa.Text)
)
But this is not what I want.

Answer 1

The primary problem is the server_default which is missing in the inserted member in class Foo. It's only present in the alembic script. Note that the alembic definitions are only used when running the migrations. They do not affect the application. For this reason, it's a good idea to copy the exact same definitions from the alembic script to your application (or vice-versa).
Because no value is defined in the model definition, sqlalchemy seems to set this to None when the class is instantiated. This will then be sent to the DB which will complain. To fix this, either set default or server_default on the model definition (the class inheriting from Base).
Some additional notes/questions:
Where does UUID GENERATE V1MC() come from? The official docs look different. I replaced it with func.uuid_generate_v1mc().
The server_default value in your case contains not null which is incorrect. You should set nullable=False on you column attribute (see below).
alembic script
# revision identifiers, used by Alembic.
revision = THIS_IS_DIFFERENT_ON_EACH_INSTANCE! # '1b7e145f2138'
down_revision = None
branch_labels = None
depends_on = None
from alembic import op
import sqlalchemy as sa
from sqlalchemy.dialects.postgresql import UUID
def upgrade():
op.create_table(
'foo',
sa.Column('uuid', UUID, primary_key=True,
server_default=sa.func.uuid_generate_v1mc()),
sa.Column(
'inserted',
sa.DateTime(timezone=True),
nullable=False,
server_default=sa.text("now()")),
sa.Column('data', sa.Text)
)
def downgrade():
op.drop_table('foo')
tester.py
from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy import Column, create_engine, func
from sqlalchemy.orm import scoped_session, sessionmaker
from sqlalchemy.dialects.postgresql import (
TEXT,
TIMESTAMP,
UUID,
)
engine = create_engine('postgresql://michel#/michel')
Session = scoped_session(sessionmaker(autocommit=False,
autoflush=False,
bind=engine))
Base = declarative_base()
class Foo(Base):
__tablename__ = 'foo'
inserted = Column(TIMESTAMP, nullable=False,
server_default=func.now())
uuid = Column(UUID, primary_key=True,
server_default=func.uuid_generate_v1mc()),
data = Column(TEXT)
#staticmethod
def insert(session, jsondata):
foo = Foo()
foo.data = jsondata['data']
if 'inserted' in jsondata:
foo.inserted = jsondata['inserted']
if 'uuid' in jsondata:
foo.uuid = jsondata['uuid']
session.add(foo)
return foo
if __name__ == '__main__':
session = Session()
Foo.insert(session, {"data": "foo bar baz"})
session.commit()
session.close()
output after execution
[9:43:54] michel#BBS-nexus [1 background job(s)]
/home/users/michel/tmp› psql -c "select * from foo"
uuid | inserted | data
--------------------------------------+-------------------------------+-------------
71f5fd32-0602-11e6-aebb-27be4bbac26e | 2016-04-19 09:43:45.297191+02 | foo bar baz
(1 row)