'Return a count of rows this Query would return.
This generates the SQL for this Query as follows::
SELECT count(1) AS count_1 FROM (
SELECT <rest of query follows...>
) AS anon_1
.. versionchanged:: 0.7
The above scheme is newly refined as of 0.7b3.
For fine grained control over specific columns
to count, to skip the usage of a subquery or
otherwise control of the FROM clause,
or to use other aggregate functions,
use :attr:`~sqlalchemy.sql.expression.func`
expressions in conjunction
with :meth:`~.Session.query`, i.e.::
from sqlalchemy import func
# count User records, without
# using a subquery.
session.query(func.count(User.id))
# return count of user "id" grouped
# by "name"
session.query(func.count(User.id)).\
group_by(User.name)
from sqlalchemy import distinct
# count distinct "name" values
session.query(func.count(distinct(User.name)))'

def count(self):

    col = sql.func.count(sql.literal_column('*'))
    return self.from_self(col).scalar()
