Michael Bayer mike...@zzzcomputing.com writes:
http://www.sqlalchemy.org/docs/05/mappers.html?highlight=contains_eager#routing-explicit-joins-statements-into-eagerly-loaded-collections
Ok, that's embarrassing... I even recall skimming that section, but
now looking at it can't for the life of me figure out why I skipped
past it. Even if I didn't fully grok it the first paragraph obviously
shows it is intended for my use case. Sigh.
If I may ask for one clarification though - contains_eager is like
eagerload and doesn't support working like eagerload_all, right? In
other words, I need to add contain_eager options for each step through
a multi-stage join, correct?
I think that matches the behavior I'm seeing, but the example in the
documentation that showed Users-orders-Order-items-Item seemed to
imply it wouldn't be necessary.
For example, with these mappings/relationships:
class ClassPeriod(object): pass
class Class_(object): pass
class Session(object): pass
class Weekday(object): pass
mapper(ClassPeriod, 'class_periods', autoload=True,
properties = { 'class_': relation(Class_, backref='periods'),
'weekday_': relation(Weekday, lazy=False) })
mapper(Class_, 'classes', autoload=True,
properties = { 'session': relation(Session, backref='classes'),
'type': relation(ClassType, backref='classes') })
mapper(Session, 'sessions' autoload=True)
mapper(Weekday, 'weekday', autoload=True)
then a query like:
query(ClassPeriod).join(Class_, Session).\
options(contains_eager('class_','session'))
doesn't seem to perform the eager loading:
SELECT class_periods.class_id AS class_periods_class_id,
class_periods.weekday AS class_periods_weekday,
class_periods.start_time AS class_periods_start_time,
class_periods.duration AS class_periods_duration, weekdays_1.weekday
AS weekdays_1_weekday, weekdays_1.day AS weekdays_1_day
FROM class_periods JOIN classes ON classes.class_id =
class_periods.class_id JOIN sessions ON sessions.session_id =
classes.session_id LEFT OUTER JOIN weekdays AS weekdays_1 ON
weekdays_1.weekday = class_periods.weekday
but if I add the intermediate class_ relationship:
query(ClassPeriod).join(Class_, Session).\
options(contains_eager('class_'),
contains_eager('class_','session'))
then it adds both the classes.* and sessions.* columns for eager loading:
SELECT sessions.session_id AS sessions_session_id, sessions.site_id AS
sessions_site_id, sessions.name AS sessions_name, sessions.start_date
AS sessions_start_date, sessions.end_date AS sessions_end_date,
classes.class_id AS classes_class_id, classes.session_id AS
classes_session_id, classes.class_type_id AS classes_class_type_id,
classes.start_date AS classes_start_date, classes.end_date AS
classes_end_date, classes.capacity AS classes_capacity,
classes.overfill AS classes_overfill, class_periods.class_id AS
class_periods_class_id, class_periods.weekday AS
class_periods_weekday, class_periods.start_time AS
class_periods_start_time, class_periods.duration AS
class_periods_duration, weekdays_1.weekday AS weekdays_1_weekday,
weekdays_1.day AS weekdays_1_day
FROM class_periods JOIN classes ON classes.class_id =
class_periods.class_id JOIN sessions ON sessions.session_id =
classes.session_id LEFT OUTER JOIN weekdays AS weekdays_1 ON
weekdays_1.weekday = class_periods.weekday
Given the above, I'm not sure how the Users/Order/Item example, which
only had the single contains_eager('orders','items') would works?
Thanks.
-- David
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