SqlAlchemy: proper way to do a bare exists query

Question

I've the following construct which seems to produce the desired SQL:

>>> print session.query(exists('1').where(MyTable.name=='x'))
SELECT EXISTS (SELECT 1 
FROM my_table
WHERE my_table.name = :name_1) AS anon_1

However, when I try to execute it with .scalar() or .all() it returns the error:

*** UnboundExecutionError: Could not locate a bind configured on SQL expression or this Session

How can I bind it for this simple query? I don't want to do bool(MyTable.query.filter(MyTable.name=='x').first()) as that wastefully pulls back the entire row from the table.

Update:

I've also tried:

>>> session.connection(mapper=MyTable).execute(
        exists('1').where(MyTable.name=='x'))
StatementError: Not an executable clause 'EXISTS \
  (SELECT 1 \nFROM my_table \nWHERE my_table.name = %(name_1)s)' []

You should have a line that reads session.configure(bind=engine) — sayap
– sayap, Commented Mar 6, 2013 at 1:50
session.configure() throws error AttributeError: 'function' object has no attribute 'configure' session.__class__ is sqlalchemy.orm.scoping.ScopedSession — EoghanM
– EoghanM, Commented Mar 6, 2013 at 10:54
That doesn't sound right. Please add the relevant code where session is constructed and where session.configure() is run. — sayap
– sayap, Commented Mar 6, 2013 at 19:54

EoghanM · Accepted Answer · 2013-03-09 19:32:19Z

3

Got it I think:

>>> session.connection(mapper=MyTable).execute(
      select([exists('1').where(MyTable.name=='x')]))

answered Mar 9, 2013 at 19:32

EoghanM

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