How do I implement a null coalescing operator in SQLAlchemy?

Question

Or how do I make this thing work?

I have an Interval object:

class Interval(Base):
    __tablename__ = 'intervals'
    id = Column(Integer, primary_key=True)
    start = Column(DateTime)
    end = Column(DateTime, nullable=True)
    task_id = Column(Integer, ForeignKey('tasks.id'))

@hybrid_property #used to just be @property
def hours_spent(self):
    end = self.end or datetime.datetime.now()
    return (end-start).total_seconds()/60/60

And a Task:

class Task(Base):
    __tablename__ = 'tasks'
    id = Column(Integer, primary_key=True)
    title = Column(String)
    intervals = relationship("Interval", backref="task")

@hybrid_property  # Also used to be just @property
def hours_spent(self):
    return sum(i.hours_spent for i in self.intervals)

Add all the typical setup code, of course.

Now when I try to do session.query(Task).filter(Task.hours_spent > 3).all()

I get NotImplementedError: <built-in function getitem> from the sum(i.hours_spent... line.

So I was looking at this part of the documentation and theorized that there might be some way that I can write something that will do what I want. This part also looks like it may be of use, and I’ll be looking at it while waiting for an answer here 😉

Asked By: Wayne Werner

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Answer 1

SQLAlchemy is not smart enough to build SQL expression tree from these operands, you have to use explicit propname.expression decorator to provide it. But then comes another problem: there is no portable way to convert interval to hours in-database. You’d use TIMEDIFF in MySQL, EXTRACT(EPOCH FROM ... ) / 3600 in PostgreSQL etc. I suggest changing properties to return timedelta instead, and comparing apples to apples.

from sqlalchemy import select, func


class Interval(Base):
    ...

    @hybrid_property
    def time_spent(self):
        return (self.end or datetime.now()) - self.start

    @time_spent.expression
    def time_spent(cls):
        return func.coalesce(cls.end, func.current_timestamp()) - cls.start


class Task(Base):
    ...

    @hybrid_property
    def time_spent(self):
        return sum((i.time_spent for i in self.intervals), timedelta(0))

    @time_spent.expression
    def hours_spent(cls):
        return (select([func.sum(Interval.time_spent)])
            .where(cls.id==Interval.task_id)
            .label('time_spent'))

The final query is:

session.query(Task).filter(Task.time_spent > timedelta(hours=3)).all()

which translates to (on PostgreSQL backend):

SELECT task.id AS task_id, task.title AS task_title 
FROM task 
WHERE (SELECT sum(coalesce(interval."end", CURRENT_TIMESTAMP) - interval.start) AS sum_1 
FROM interval 
WHERE task.id = interval.task_id) > %(param_1)s

Answered By: rkhayrov

Answer 2

For a simple example of SQLAlchemy’s coalesce function, this may help: Handling null values in a SQLAlchemy query – equivalent of isnull, nullif or coalesce.

Here are a couple of key lines of code from that post:

from sqlalchemy.sql.functions import coalesce
my_config = session.query(Config).order_by(coalesce(Config.last_processed_at, datetime.date.min)).first()

Answered By: Steve Saporta

Answer 3

There is a complete example of making a func action similar to coalesc or nvl.

Note how it takes in arguements, and renders an expression… in this case NVL(a, b) when used with Oracle.

http://docs.sqlalchemy.org/en/latest/core/compiler.html#subclassing-guidelines

from sqlalchemy.ext.compiler import compiles
from sqlalchemy.sql.expression import FunctionElement

class coalesce(FunctionElement):
     name = 'coalesce'

@compiles(coalesce)
def compile(element, compiler, **kw):
     return "coalesce(%s)" % compiler.process(element.clauses)

@compiles(coalesce, 'oracle')
def compile(element, compiler, **kw):
     if len(element.clauses) > 2:
          raise TypeError("coalesce only supports two arguments on Oracle")
     return "nvl(%s)" % compiler.process(element.clauses)

Then when you want to use it…

from my_oracle_functions_sqla import coalesce
select([coalesce(A.value, '---')])  # etc

Hope that helps.

Answered By: phyatt

Answer 4

I needed to use the text function and could not use 0 as an integer.

import sqlalchemy as sa

session.query(sa.func.coalesce(table1.col1, sa.text("0"))).all()

Answered By: NameVergessen

How do I implement a null coalescing operator in SQLAlchemy?

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