On 04/13/2017 10:24 AM, Kent wrote:
Suppose we have the documentation's example of *Concrete Table
Inheritance, *where
session.query(Employee).all()
produces this:
SELECT pjoin.type AS pjoin_type,
pjoin.manager_data AS pjoin_manager_data,
pjoin.employee_id AS pjoin_employee_id,
pjoin.name AS pjoin_name, pjoin.engineer_info AS pjoin_engineer_info
FROM (
SELECT employees.employee_id AS employee_id,
CAST(NULL AS VARCHAR(50)) AS manager_data, employees.name AS name,
CAST(NULL AS VARCHAR(50)) AS engineer_info, 'employee' AS type
FROM employees
UNION ALL
SELECT managers.employee_id AS employee_id,
managers.manager_data AS manager_data, managers.name AS name,
CAST(NULL AS VARCHAR(50)) AS engineer_info, 'manager' AS type
FROM managers
UNION ALL
SELECT engineers.employee_id AS employee_id,
CAST(NULL AS VARCHAR(50)) AS manager_data, engineers.name AS name,
engineers.engineer_info AS engineer_info, 'engineer' AS type
FROM engineers
) AS pjoin
Suppose we want to*filter certain managers*, which we can do with:
session.query(Employee)\
.filter(or_(
Employee.manager_data == u'whatineed',
Employee.manager_data == None))\
.all()
If manager_data is indexed, many databases can no longer use this index.
What we really want is:
SELECT pjoin.type AS pjoin_type,
pjoin.manager_data AS pjoin_manager_data,
pjoin.employee_id AS pjoin_employee_id,
pjoin.name AS pjoin_name, pjoin.engineer_info AS pjoin_engineer_info
FROM (
SELECT employees.employee_id AS employee_id,
CAST(NULL AS VARCHAR(50)) AS manager_data, employees.name AS name,
CAST(NULL AS VARCHAR(50)) AS engineer_info, 'employee' AS type
FROM employees
UNION ALL
SELECT managers.employee_id AS employee_id,
managers.manager_data AS manager_data, managers.name AS name,
CAST(NULL AS VARCHAR(50)) AS engineer_info, 'manager' AS type
FROM managers
*WHERE manager_data = 'whatineed'*
UNION ALL
SELECT engineers.employee_id AS employee_id,
CAST(NULL AS VARCHAR(50)) AS manager_data, engineers.name AS name,
engineers.engineer_info AS engineer_info, 'engineer' AS type
FROM engineers
) AS pjoin
Is there a way to accomplish this?
Certainly, construct the complete UNION query that you want, most likely
using Core select() and union(), and supply it to Query using
with_polymorphic; see the "custom selectable" example in
http://docs.sqlalchemy.org/en/latest/orm/inheritance.html#basic-control-of-which-tables-are-queried.
Automating this process, perhaps you could compose some enhanced version
of the polymorhic_union() feature that accepts additional criteria.
However, I would suggest that if the "manager_data is NULL" part is what
screws up the index, you might want to query like this instead:
session.query(Employee)\
.filter(or_(Employee.manager_data == u'whatineed',
pjoin.c.type != 'manager')).all()
Thanks in advance,
Kent
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SQLAlchemy -
The Python SQL Toolkit and Object Relational Mapper
http://www.sqlalchemy.org/
To post example code, please provide an MCVE: Minimal, Complete, and Verifiable
Example. See http://stackoverflow.com/help/mcve for a full description.
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