From: Mike Bayer Date: Mon, 7 Dec 2015 17:36:36 +0000 (-0500) Subject: - write a real inline documentation section for from_self(), X-Git-Tag: rel_1_1_0b1~84^2~70^2~13 X-Git-Url: http://git.ipfire.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=8fa62d652b8d1834e23ba36e56c7bd711cc37e37;p=thirdparty%2Fsqlalchemy%2Fsqlalchemy.git - write a real inline documentation section for from_self(), building on the example of query.join(). Ideally all the methods in Query would have this kind of thing. fixes #3544 --- diff --git a/lib/sqlalchemy/orm/query.py b/lib/sqlalchemy/orm/query.py index 4441975bf7..6f4373ec53 100644 --- a/lib/sqlalchemy/orm/query.py +++ b/lib/sqlalchemy/orm/query.py @@ -984,8 +984,169 @@ class Query(object): """return a Query that selects from this Query's SELECT statement. - \*entities - optional list of entities which will replace - those being selected. + :meth:`.Query.from_self` essentially turns the SELECT statement + into a SELECT of itself. Given a query such as:: + + q = session.query(User).filter(User.name.like('e%')) + + Given the :meth:`.Query.from_self` version:: + + q = session.query(User).filter(User.name.like('e%')).from_self() + + This query renders as: + + .. sourcecode:: sql + + SELECT anon_1.user_id AS anon_1_user_id, + anon_1.user_name AS anon_1_user_name + FROM (SELECT "user".id AS user_id, "user".name AS user_name + FROM "user" + WHERE "user".name LIKE :name_1) AS anon_1 + + There are lots of cases where :meth:`.Query.from_self` may be useful. + A simple one is where above, we may want to apply a row LIMIT to + the set of user objects we query against, and then apply additional + joins against that row-limited set:: + + q = session.query(User).filter(User.name.like('e%')).\\ + limit(5).from_self().\\ + join(User.addresses).filter(Address.email.like('q%')) + + The above query joins to the ``Address`` entity but only against the + first five results of the ``User`` query: + + .. sourcecode:: sql + + SELECT anon_1.user_id AS anon_1_user_id, + anon_1.user_name AS anon_1_user_name + FROM (SELECT "user".id AS user_id, "user".name AS user_name + FROM "user" + WHERE "user".name LIKE :name_1 + LIMIT :param_1) AS anon_1 + JOIN address ON anon_1.user_id = address.user_id + WHERE address.email LIKE :email_1 + + **Automatic Aliasing** + + Another key behavior of :meth:`.Query.from_self` is that it applies + **automatic aliasing** to the entities inside the subquery, when + they are referenced on the outside. Above, if we continue to + refer to the ``User`` entity without any additional aliasing applied + to it, those references wil be in terms of the subquery:: + + q = session.query(User).filter(User.name.like('e%')).\\ + limit(5).from_self().\\ + join(User.addresses).filter(Address.email.like('q%')).\\ + order_by(User.name) + + The ORDER BY against ``User.name`` is aliased to be in terms of the + inner subquery: + + .. sourcecode:: sql + + SELECT anon_1.user_id AS anon_1_user_id, + anon_1.user_name AS anon_1_user_name + FROM (SELECT "user".id AS user_id, "user".name AS user_name + FROM "user" + WHERE "user".name LIKE :name_1 + LIMIT :param_1) AS anon_1 + JOIN address ON anon_1.user_id = address.user_id + WHERE address.email LIKE :email_1 ORDER BY anon_1.user_name + + The automatic aliasing feature only works in a **limited** way, + for simple filters and orderings. More ambitious constructions + such as referring to the entity in joins should prefer to use + explicit subquery objects, typically making use of the + :meth:`.Query.subquery` method to produce an explicit subquery object. + Always test the structure of queries by viewing the SQL to ensure + a particular structure does what's expected! + + **Changing the Entities** + + :meth:`.Query.from_self` also includes the ability to modify what + columns are being queried. In our example, we want ``User.id`` + to be queried by the inner query, so that we can join to the + ``Address`` entity on the outside, but we only wanted the outer + query to return the ``Address.email`` column:: + + q = session.query(User).filter(User.name.like('e%')).\\ + limit(5).from_self(Address.email).\\ + join(User.addresses).filter(Address.email.like('q%')) + + yielding: + + .. sourcecode:: sql + + SELECT address.email AS address_email + FROM (SELECT "user".id AS user_id, "user".name AS user_name + FROM "user" + WHERE "user".name LIKE :name_1 + LIMIT :param_1) AS anon_1 + JOIN address ON anon_1.user_id = address.user_id + WHERE address.email LIKE :email_1 + + **Looking out for Inner / Outer Columns** + + Keep in mind that when referring to columns that originate from + inside the subquery, we need to ensure they are present in the + columns clause of the subquery itself; this is an ordinary aspect of + SQL. For example, if we wanted to load from a joined entity inside + the subquery using :func:`.contains_eager`, we need to add those + columns. Below illustrates a join of ``Address`` to ``User``, + then a subquery, and then we'd like :func:`.contains_eager` to access + the ``User`` columns:: + + q = session.query(Address).join(Address.user).\\ + filter(User.name.like('e%')) + + q = q.add_entity(User).from_self().\\ + options(contains_eager(Address.user)) + + We use :meth:`.Query.add_entity` above **before** we call + :meth:`.Query.from_self` so that the ``User`` columns are present + in the inner subquery, so that they are available to the + :func:`.contains_eager` modifier we are using on the outside, + producing: + + .. sourcecode:: sql + + SELECT anon_1.address_id AS anon_1_address_id, + anon_1.address_email AS anon_1_address_email, + anon_1.address_user_id AS anon_1_address_user_id, + anon_1.user_id AS anon_1_user_id, + anon_1.user_name AS anon_1_user_name + FROM ( + SELECT address.id AS address_id, + address.email AS address_email, + address.user_id AS address_user_id, + "user".id AS user_id, + "user".name AS user_name + FROM address JOIN "user" ON "user".id = address.user_id + WHERE "user".name LIKE :name_1) AS anon_1 + + If we didn't call ``add_entity(User)``, but still asked + :func:`.contains_eager` to load the ``User`` entity, it would be + forced to add the table on the outside without the correct + join criteria - note the ``anon1, "user"`` phrase at + the end: + + .. sourcecode:: sql + + -- incorrect query + SELECT anon_1.address_id AS anon_1_address_id, + anon_1.address_email AS anon_1_address_email, + anon_1.address_user_id AS anon_1_address_user_id, + "user".id AS user_id, + "user".name AS user_name + FROM ( + SELECT address.id AS address_id, + address.email AS address_email, + address.user_id AS address_user_id + FROM address JOIN "user" ON "user".id = address.user_id + WHERE "user".name LIKE :name_1) AS anon_1, "user" + + :param \*entities: optional list of entities which will replace + those being selected. """ fromclause = self.with_labels().enable_eagerloads(False).\