On Wed, Sep 23, 2020, at 4:17 PM, agrot...@gmail.com wrote: > I actually don't really care that much to have the attribute remain dynamic. > In fact there is only one *specific* filtering that I want to apply to it, > but that filtering will vary from (web) request to (web) request. This is > what made me think of using contains_eager. > > Right now this is the best solution I have come up with, which is to define a > temporary class that extends A and add to that class a new relationship with > the custom filter applied. I then specify to selectinload that property. Is > there a better way to do this?
I would still use a separate relationship on the same class, you can always make a @hybrid_property that switches between the two relationships depending on what you want to do. In version 1.4, which will be in betas as soon as I can get a huge amount of new docs written, you will have a potentially better option for this which is the PropComparator.and_() operator. you can play with this now from git master if you were interested: https://docs.sqlalchemy.org/en/14/orm/loading_relationships.html#adding-criteria-to-loader-options that way you'd say: session.query(A).options(selectinload(A.bs).and_(B.some_field == value)) that might be what you're waiting for here > > # Run this code for each web request, reading some_field_value from the value > the client specifies in the request: > some_field_value = ... > > class ATmp(A): > bs_temp = relationship( > lambda: models.B, > primaryjoin=( > (models.A.id == models.B.a_id) > & (models.B.some_field == some_field_value) > ), > ) > > q = return db.session.query(ATmp).options(selectinload(cls.bs_temp)) > # iterate over the q (which in iterable of "A"s) and for each A, iterate over > the bs_temp, which is a filtered collection of Bs. > > This also leads to a warning: > SAWarning: This declarative base already contains a class with the same class > name and module name as my_app.graphql.queries.ATmp, and will be replaced in > the string-lookup table > > though it does seem to work (I am able to avoid n+1 and do the filtering in > the DB). > On Wednesday, September 23, 2020 at 8:10:44 AM UTC-4 Mike Bayer wrote: >> >> >> On Wed, Sep 23, 2020, at 5:43 AM, agrot...@gmail.com wrote: >>> Let's say I have a model with a one to many relationship as such: >>> class A(Base): >>> id = ... >>> >>> class B(Base): >>> id = ... >>> some_field = .... >>> a_id = Column(ForeignKey(A.id)... >>> a = relationship(A, backref=backref('bs', lazy='dynamic')) >>> >>> I can define a method on A: >>> class A(Base): >>> ... >>> def get_b_with_some_field(self, some_field): >>> return self.bs.filter(B.some_field==some_field) >>> >>> to get all b's that have a certain value of `some_field`. >>> >>> Is there any way to accomplish this with eager fetching to avoid the n+1 >>> select problem that will occur if I want to query for a lot of `A`s and >>> then iterate over the collection and for each call `get_b_with_some_field` >>> with the same value of `some_field`?' >> >> if you want to have that attribute remain on "dynamic" then you'd need to >> define a second relationship where you can use normal eager fetching >> strategies, and then use that for those cases. >> >> IMO "dynamic" is not really worth it, you can get the same queries more >> programmatically by using query(B).filter(with_parent(some_a, A.bs)). >> >> >> >> >> >>> >>> >>> One option is to to change relationship to from `lazy='dynamic'` to >>> `lazy='subquery'` or ``lazy='selectin'`` and then implement the filtering >>> in `get_b_with_some_field` in Python. This will address the n+1 select >>> problem, but will cause pulling extra data from the database (and extra >>> work in Python). >>> >>> I thought `contains_eager` >>> <https://docs.sqlalchemy.org/en/13/orm/loading_relationships.html?highlight=contains_eager#using-contains-eager-to-load-a-custom-filtered-collection-result> >>> might be relevant; however, I only see it being mentioned in the case of >>> joined loads. >>> >>> The reason I am looking for this functionality is I am defining a graphql >>> API that looks like the following: >>> type query { >>> as: [A!! >>> } >>> >>> type A { >>> ... >>> bs(some_field: String): [B!]! >>> } >>> >>> type B { >>> ... >>> some_field: String! >>> } >>> where I would like to be able to specify a filter on the `bs` relationship >>> from `A`. I would ideally like to 1. avoid the n+1 select issue and 2. >>> perform the some_field filtering at the database level, and 3. leverage as >>> much of the ORM as possible ;-) >>> >>> Is it possible to do this within SQLA? >>> >>> -- >>> 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. >>> --- >>> You received this message because you are subscribed to the Google Groups >>> "sqlalchemy" group. >>> To unsubscribe from this group and stop receiving emails from it, send an >>> email to sqlalchemy+...@googlegroups.com. >>> To view this discussion on the web visit >>> https://groups.google.com/d/msgid/sqlalchemy/55c7b3a6-bfee-45b6-83bc-25185bf7af87n%40googlegroups.com >>> >>> <https://groups.google.com/d/msgid/sqlalchemy/55c7b3a6-bfee-45b6-83bc-25185bf7af87n%40googlegroups.com?utm_medium=email&utm_source=footer>. >> > > -- > 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. > --- > You received this message because you are subscribed to the Google Groups > "sqlalchemy" group. > To unsubscribe from this group and stop receiving emails from it, send an > email to sqlalchemy+unsubscr...@googlegroups.com. > To view this discussion on the web visit > https://groups.google.com/d/msgid/sqlalchemy/e20cd72d-796f-4e99-a91d-55f4252f5fd6n%40googlegroups.com > > <https://groups.google.com/d/msgid/sqlalchemy/e20cd72d-796f-4e99-a91d-55f4252f5fd6n%40googlegroups.com?utm_medium=email&utm_source=footer>. -- 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. --- You received this message because you are subscribed to the Google Groups "sqlalchemy" group. 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