Postgres function as column in SQLAlchemy

Question

I have two functions in Postgesql database:

• id_generator , which generates id from timestamp
• timestamp_from_id(id) , which reverses id to timestamp

Let's assume this is my model:

class DatModel(object):
    id = Column(
        BigInteger,
        primary_key=True,
        server_default=text('id_generator()')
    )
    name = Column(String(50), index=True)

What I want to do is query by timestamp generated by timestamp_from_id(id) function, for example:

dbsession.query(DatModel).filter(DatModel.timestamp > datetime.now()).all()

or

obj = dbsession.query(DatModel).first()
created = obj.timestamp

My question is:

How to create virtual column based on postgres function?

Thanks in advance

---

Edit:

Unsatisfying method #1

As Roman Dryndik suggested possible solution is to use func . It's working method, but little problematic way while retrieving timestamp:

timestamp = session.execute(func.timestamp_from_id(obj.id)).fetchone()

I'd prefer to do it something like:

timestamp = obj.timestamp

Unsatisfying method #2

Second possible solution is to use Compilation Extension

from datetime import datetime

from sqlalchemy import DateTime
from sqlalchemy.ext.compiler import compiles
from sqlalchemy.sql.functions import FunctionElement

class Timestamp(FunctionElement):
    type = DateTime()
    name = 'timestamp'

@compiles(Timestamp)
def default_timestamp(element, compiler, **kw):
    return compiler.visit_function(element)

@compiles(Timestamp, 'postgresql')
def pg_created(element, compiler, **kw):
    arg1 = list(element.clauses)[0]
    return 'timestamp_from_id({})'.format(compiler.process(arg1))


# filter example
obj = session.query(DatModel).filter(Timestamp < datetime.now()).first()

# get timestamp attribute example
timestamp = session.execute(Timestamp(obj.id)).fetchone()

Answer 1

Please read Using column_property section of SQL Expressions as Mapped Attributes documentation.

Using it you should be able to do:

class DatModel(Base):
    id = Column(
        Integer,
        primary_key=True,
        server_default=text('id_generator()')
    )
    name = Column(String(50), index=True)

    timestamp = column_property(func.timestamp_from_id(id))

timestamp will also be included in your query, and you can use it in the filtering and/or ordering expressions:

q = (session.query(DatModel)
     .filter(DatModel.timestamp > datetime.now())
     .order_by(DatModel.timestamp.desc())
    )

Answer 2

If I correctly understand your problem you can call the stored procedure timestamp_from_id using func .

So you will get something like this:

from sqlalchemy import func

# some code here

dbsession.query(DatModel).filter(func.timestamp_from_id(DataModel.id) > datetime.now()).all()

# some code here

Didn't try the code. Probably you should do func.timestamp_from_id(DataModel.id).scalar() as well.

More info you can find here and here .

Answer 3

The solution is to use hybrid_property and expression ( docs )

class DatModel(Base)::
    id = Column(
        BigInteger,
        primary_key=True,
        server_default=text('id_generator()')
    )

    @hybrid_property
    def timestamp(self):
        return object_session(self). \
            scalar(select([func.timestamp_from_id(self.id)]))

    @created.expression
    def timestamp(self):
        return func.timestamp_from_id(self.id)

Example usage:

obj = DatModel()
with transaction.manager:
    session.add(obj)
obj = session.query(DatModel).one()

assert type(obj.timestamp) == datetime.datetime
assert obj.timestamp.date() == datetime.date.today()


objs = session.query(DatModel) \
    .filter(DatModel.timestamp < datetime.datetime.now())
assert objs.count() == 1

objs = session.query(Gateway) \
    .filter(DatModel.timestamp > datetime.datetime.now())
assert objs.count() == 0

Answer 4

You could achieve this using a column_property with a literal_column . For example:

class DatModel(object):
    id = Column(
        BigInteger,
        primary_key=True,
        server_default=text('id_generator()')
    )
    name = Column(String(50), index=True)
    timestamp = column_property(literal_column("timestamp_from_id(datmodel.id)", type_=DateTime).label('timestamp'))