[python] SQLAlchemy default DateTime

This is my declarative model:

import datetime
from sqlalchemy import Column, Integer, DateTime
from sqlalchemy.ext.declarative import declarative_base

Base = declarative_base()

class Test(Base):
    __tablename__ = 'test'

    id = Column(Integer, primary_key=True)
    created_date = DateTime(default=datetime.datetime.utcnow)

However, when I try to import this module, I get this error:

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "orm/models2.py", line 37, in <module>
    class Test(Base):
  File "orm/models2.py", line 41, in Test
    created_date = sqlalchemy.DateTime(default=datetime.datetime.utcnow)
TypeError: __init__() got an unexpected keyword argument 'default'

If I use an Integer type, I can set a default value. What's going on?

As per PostgreSQL documentation, https://www.postgresql.org/docs/9.6/static/functions-datetime.html

now, CURRENT_TIMESTAMP, LOCALTIMESTAMP return the time of transaction.

This is considered a feature: the intent is to allow a single transaction to have a consistent notion of the "current" time, so that multiple modifications within the same transaction bear the same time stamp.

You might want to use statement_timestamp or clock_timestamp if you don't want transaction timestamp.

statement_timestamp()

returns the start time of the current statement (more specifically, the time of receipt of the latest command message from the client). statement_timestamp

clock_timestamp()

returns the actual current time, and therefore its value changes even within a single SQL command.

Calculate timestamps within your DB, not your client

For sanity, you probably want to have all datetimes calculated by your DB server, rather than the application server. Calculating the timestamp in the application can lead to problems because network latency is variable, clients experience slightly different clock drift, and different programming languages occasionally calculate time slightly differently.

SQLAlchemy allows you to do this by passing func.now() or func.current_timestamp() (they are aliases of each other) which tells the DB to calculate the timestamp itself.

Use SQLALchemy's server_default

Additionally, for a default where you're already telling the DB to calculate the value, it's generally better to use server_default instead of default. This tells SQLAlchemy to pass the default value as part of the CREATE TABLE statement.

For example, if you write an ad hoc script against this table, using server_default means you won't need to worry about manually adding a timestamp call to your script--the database will set it automatically.

Understanding SQLAlchemy's onupdate/server_onupdate

SQLAlchemy also supports onupdate so that anytime the row is updated it inserts a new timestamp. Again, best to tell the DB to calculate the timestamp itself:

from sqlalchemy.sql import func

time_created = Column(DateTime(timezone=True), server_default=func.now())
time_updated = Column(DateTime(timezone=True), onupdate=func.now())

There is a server_onupdate parameter, but unlike server_default, it doesn't actually set anything serverside. It just tells SQLalchemy that your database will change the column when an update happens (perhaps you created a trigger on the column ), so SQLAlchemy will ask for the return value so it can update the corresponding object.

One other potential gotcha:

You might be surprised to notice that if you make a bunch of changes within a single transaction, they all have the same timestamp. That's because the SQL standard specifies that CURRENT_TIMESTAMP returns values based on the start of the transaction.

PostgreSQL provides the non-SQL-standard statement_timestamp() and clock_timestamp() which do change within a transaction. Docs here: https://www.postgresql.org/docs/current/static/functions-datetime.html#FUNCTIONS-DATETIME-CURRENT

UTC timestamp

If you want to use UTC timestamps, a stub of implementation for func.utcnow() is provided in SQLAlchemy documentation. You need to provide appropriate driver-specific functions on your own though.

The default keyword parameter should be given to the Column object.

Example:

Column(u'timestamp', TIMESTAMP(timezone=True), primary_key=False, nullable=False, default=time_now),

The default value can be a callable, which here I defined like the following.

from pytz import timezone
from datetime import datetime

UTC = timezone('UTC')

def time_now():
    return datetime.now(UTC)

You can also use sqlalchemy builtin function for default DateTime

from sqlalchemy.sql import func

DT = Column(DateTime(timezone=True), default=func.now())

You likely want to use onupdate=datetime.now so that UPDATEs also change the last_updated field.

SQLAlchemy has two defaults for python executed functions.

• default sets the value on INSERT, only once
• onupdate sets the value to the callable result on UPDATE as well.