creating a temporary table from a query using sqlalchemy orm

I can create a temporary table this way:

session.execute("CREATE TABLE temptable SELECT existingtable.id, "
    "existingtable.column2 FROM existingtable WHERE existingtable.id<100000")

but the new table is unreadable because it says it has no primary key. existingtable.id is the primary key of exisitingtable, so I expected it to get the same treatment in the temp table.

However, I would rather find some ORM way of doing this anyway. Given:

temp_table = Table('temptable', metadata, 
    Column('id', Integer, primary_key=True),
    Column('column2', Integer),
    useexisting=True )
class TempTable(object):
    pass
mapper(TempTable, temp_table)
temp_table.create(bind=session.bind, checkfirst=True)
if session.query(TempTable).delete(): #make sure it's empty
    session.commit()

How can I populate temp_table with some selected contents of existingtable without doing 100000 session.query.add(TempTable(...)) commands? Or is there a way of creating the table from a query similar to the plain SQL version above?

Answer

It’s not exactly ORM, but to create the table initially, I’d clone the table structure (see cloneTable in the example below). For copying the data, I then would use the InsertFromSelect example.

Edit: Since version 0.8.3, SqlAlchemy supports Insert.from_select() out of the box. Hence the InsertFromSelect class and the respective visitor in the example below can be directly replaced and are no longer needed. I leave the original example unchanged for historic reasons.

Here is a working example

from sqlalchemy import Table
from sqlalchemy.ext.compiler import compiles
from sqlalchemy.sql.expression import UpdateBase

class InsertFromSelect(UpdateBase):
    def __init__(self, table, select):
        self.table = table
        self.select = select

@compiles(InsertFromSelect)
def visit_insert_from_select(element, compiler, **kw):
    return "INSERT INTO %s %s" % (
        compiler.process(element.table, asfrom=True),
        compiler.process(element.select)
    )

def cloneTable(name, table, metadata):
    cols = [c.copy() for c in table.columns]
    constraints = [c.copy() for c in table.constraints]
    return Table(name, metadata, *(cols + constraints))

# test data
from sqlalchemy import MetaData, Column, Integer
from sqlalchemy.engine import create_engine
e = create_engine('sqlite://')
m = MetaData(e)
t = Table('t', m, Column('id', Integer, primary_key=True),
          Column('number', Integer))
t.create()
e.execute(t.insert().values(id=1, number=3))
e.execute(t.insert().values(id=9, number=-3))

# create temp table
temp = cloneTable('temp', t, m)
temp.create()

# copy data
ins = InsertFromSelect(temp, t.select().where(t.c.id>5))
e.execute(ins)

# print result
for r in e.execute(temp.select()):
    print(r)