Re: [sqlalchemy] Limiting queries from single-table inheritance declarative models

mike bayer Wed, 28 Dec 2016 14:20:07 -0800


On 12/27/2016 04:36 PM, Tucker Beck wrote:

Mike (or whoever else might have insight):

I used the pattern above, and it worked great until I tried use it for a
really large table that I was querying and filtering by the hybrid
property. Something like this:
query(HybridModel).filter_by(hybrid_type_name='alpha')


Well I mentioned a correlated subquery won't scale here.


The resulting query does a seq-scan applying a sub-select to get the
hybrid_type_name and then comparing it to the filter value ('alpha' in
the example). Obviously, this is sub-optimal, and I want the query to do
an index-scan comparing the hybrid_type_id matching the filter_value to
the hybrid_type_id of each row. The table has an index on
hybrid_type_id, so that query is orders of magnitude faster. Before
using this revised pattern we discussed, I got the query I wanted by
adding a comparator to my class that basically inverts the select used
by the hybrid property expression. That worked great before using this
pattern.


I'm not sure what "invert the select" looks like.


However, with the new pattern, the delcared_attr __mapper_args__
attempts to use the expression from the comaprator for polymorphic_on,
and that query results in an integer (primary key) which won't work
because we defined our polymorphic identities using the name

I'm not understanding this part either, can this be demonstrated withthe simple test script I gave you ?


Do you have any ideas on how I might resolve this issue?

On Sat, Dec 3, 2016 at 4:25 PM, mike bayer <mike...@zzzcomputing.com
<mailto:mike...@zzzcomputing.com>> wrote:



    On 12/01/2016 05:29 PM, Tucker Beck wrote:

        This is what I ended up with, and it *seems* to work pretty
        well. Does
        my approach here pass the sniff test?


    everything looks idiomatic, so if it does what you need, then it's
    greatg!




        from sqlalchemy.ext.hybrid import hybrid_property
        from sqlalchemy.ext.declarative import declarative_base,
        declared_attr
        from sqlalchemy.orm import sessionmaker, relationship,
        column_property
        from sqlalchemy import (
            Column, Integer, ForeignKey, Text,
            select, create_engine,
        )

        Base = declarative_base()


        class HybridType(Base):
            __tablename__ = 'hybrid_types'
            id = Column(Integer, primary_key=True)
            name = Column(Text)


        class HybridModel(Base):
            __tablename__ = 'hybrids'
            hybrid_type_identity = 'base'

            id = Column(Integer, primary_key=True)
            name = Column(Text)
            hybrid_type_id = Column(Integer, ForeignKey('hybrid_types.id
        <http://hybrid_types.id>
        <http://hybrid_types.id>'))
            hybrid_type = relationship('HybridType')

            @declared_attr
            def __mapper_args__(cls):
                return dict(
                    polymorphic_on=cls.hybrid_type_name,
                    polymorphic_identity=cls.hybrid_type_identity,
                )

            def __repr__(self):
                return "{} ({}:{})".format(type(self).__name__,
        self.name <http://self.name>
        <http://self.name>, self.id <http://self.id> <http://self.id>)

            def __init__(self, **kwargs):
                self.hybrid_type_name = self.hybrid_type_identity
                super(HybridModel, self).__init__(**kwargs)

            @hybrid_property
            def hybrid_type_name(self):
                return self.hybrid_type.name
        <http://self.hybrid_type.name> <http://self.hybrid_type.name>

            @hybrid_type_name.expression
            def hybrid_type_name(cls):
                return (
                    select([HybridType.name]).
                    where(HybridType.id == cls.hybrid_type_id).
                    as_scalar()
                )

            @hybrid_type_name.setter
            def hybrid_type_name(self, value):
                self.hybrid_type_id = (
                    select([HybridType.id]).
                    where(HybridType.name == value)
                )


        class HybridAlpha(HybridModel):
            hybrid_type_identity = 'alpha'


        class HybridBeta(HybridModel):
            hybrid_type_identity = 'beta'


        engine = create_engine('sqlite:///:memory:', echo=False)
        Base.metadata.create_all(engine)
        Session = sessionmaker(bind=engine)
        session = Session()
        session.add(HybridType(name='base'))
        session.add(HybridType(name='alpha'))
        session.add(HybridType(name='beta'))
        session.add(HybridModel(name='base_instance'))
        session.add(HybridAlpha(name='alpha_instance'))
        session.add(HybridBeta(name='beta_instance'))
        print(session.query(HybridModel).all())
        print(session.query(HybridAlpha).all())
        print(session.query(HybridAlpha).one().hybrid_type_name)
        session.query(HybridAlpha).one().hybrid_type_name = 'beta'
        print(session.query(HybridBeta).all())


        On Wed, Nov 30, 2016 at 7:20 AM, mike bayer
        <mike...@zzzcomputing.com <mailto:mike...@zzzcomputing.com>
        <mailto:mike...@zzzcomputing.com
        <mailto:mike...@zzzcomputing.com>>> wrote:



            On 11/29/2016 07:31 PM, Tucker Beck wrote:

                Thanks for this! What allows the query to filter in the
        derived
                classes
                when using a column property instead of a hybrid
        property? Is it
                declaring the __mapper_args__ in the base class declaration
                instead of
                after it?


            Yea I don't think setting __mapper_args__ after the fact on the
            class does anything.   Those __mapper_args__ are only for the
            mapper() constructor directly, which gets called when the
        class is
            declared.

            you can set the actual polymorphic_on property right now
        only using
            the special class.__mapper__._set_polymorphic_on(<something>),
            that's one of a very few setters that's available on the mapper
            after the fact and even then it's not fully public API right
        now.



                Also, it seems like having the hybrid_type_prop column
        property
                obviates
                the need for the hybrid property hybrid_type_name...that is,
                except for
                the setter. Is there a way to give a column_property a
        setter?


            a column_property against a SQL expression is strictly a
        read-only
            thing, since "writing" to it makes no sense in terms of
        persistence.


            the column_property aspect here isn't strictly necessary,
        the mapper
            will create the property internally as a special case for the
            polymorphic_on setting if not provided.  the example can be
            re-organized w/ your hybrid in at least two ways:


            class HybridModel(Base):
                # ...

                @declared_attr
                def __mapper_args__(cls):
                    return {
                        "polymorphic_on": cls.hybrid_type_name
                    }


            or


            class HybridModel(Base):
                # ...

                _hybrid_name_expr = (
                    select([HybridType.name]).
                    where(HybridType.id == hybrid_type_id).
                    as_scalar()
                )

                __mapper_args__ = {
                    "polymorphic_on": _hybrid_name_expr
                }

                # ...

                @hybrid_type_name.expression
                def hybrid_type_name(cls):
                    return cls._hybrid_name_expr



                On Tue, Nov 29, 2016 at 2:59 PM, mike bayer
                <mike...@zzzcomputing.com
        <mailto:mike...@zzzcomputing.com>
        <mailto:mike...@zzzcomputing.com <mailto:mike...@zzzcomputing.com>>
                <mailto:mike...@zzzcomputing.com
        <mailto:mike...@zzzcomputing.com>
                <mailto:mike...@zzzcomputing.com
        <mailto:mike...@zzzcomputing.com>>>> wrote:



                    On 11/29/2016 05:01 PM, Tucker Beck wrote:

                        The issue with using the foreign_key integer
        value as the
                        discriminator
                        is that you won't know what that is at class
        declaration
                time. The
                        type_name, however, you can declare as a part of the
                class as
                        you would
                        with a normal string discriminator. I'm not sure
        how you
                would do a
                        correlated subquery for the polymorphic_on
        attribute. I
                modified the
                        query object so that I could filter results for
        the derived
                        classes. It
                        doesn't seem like that happens automatically.
        Maybe I'm
                just not
                        doing
                        it right.


                    The correlated subquery will not scale as well as a
        direct
                    identifier, but here is an adaption of your test using
                    column_property(), and you should be able to set
                polymorphic_on to
                    the select() object directly too.  There's another
        example
                of this
                    at


        
http://docs.sqlalchemy.org/en/latest/orm/mapping_api.html?highlight=polymorphic_on#sqlalchemy.orm.mapper.params.polymorphic_on
        
<http://docs.sqlalchemy.org/en/latest/orm/mapping_api.html?highlight=polymorphic_on#sqlalchemy.orm.mapper.params.polymorphic_on>

        
<http://docs.sqlalchemy.org/en/latest/orm/mapping_api.html?highlight=polymorphic_on#sqlalchemy.orm.mapper.params.polymorphic_on
        
<http://docs.sqlalchemy.org/en/latest/orm/mapping_api.html?highlight=polymorphic_on#sqlalchemy.orm.mapper.params.polymorphic_on>>


        
<http://docs.sqlalchemy.org/en/latest/orm/mapping_api.html?highlight=polymorphic_on#sqlalchemy.orm.mapper.params.polymorphic_on
        
<http://docs.sqlalchemy.org/en/latest/orm/mapping_api.html?highlight=polymorphic_on#sqlalchemy.orm.mapper.params.polymorphic_on>

        
<http://docs.sqlalchemy.org/en/latest/orm/mapping_api.html?highlight=polymorphic_on#sqlalchemy.orm.mapper.params.polymorphic_on
        
<http://docs.sqlalchemy.org/en/latest/orm/mapping_api.html?highlight=polymorphic_on#sqlalchemy.orm.mapper.params.polymorphic_on>>>.

                    For the "use the id" approach, you would need to
        query from
                    HybridType up front and populate the
        polymorphic_identity
                attributes
                    after the fact, this is a feature that is not directly
                supported yet
                    but there is a recipe to do so at


        
https://bitbucket.org/zzzeek/sqlalchemy/issues/2792/settable-polymorphic-identity
        
<https://bitbucket.org/zzzeek/sqlalchemy/issues/2792/settable-polymorphic-identity>

        
<https://bitbucket.org/zzzeek/sqlalchemy/issues/2792/settable-polymorphic-identity
        
<https://bitbucket.org/zzzeek/sqlalchemy/issues/2792/settable-polymorphic-identity>>


        
<https://bitbucket.org/zzzeek/sqlalchemy/issues/2792/settable-polymorphic-identity
        
<https://bitbucket.org/zzzeek/sqlalchemy/issues/2792/settable-polymorphic-identity>

        
<https://bitbucket.org/zzzeek/sqlalchemy/issues/2792/settable-polymorphic-identity
        
<https://bitbucket.org/zzzeek/sqlalchemy/issues/2792/settable-polymorphic-identity>>>.



                    from sqlalchemy.ext.hybrid import hybrid_property
                    from sqlalchemy.ext.declarative import declarative_base
                    from sqlalchemy.orm import sessionmaker, relationship,
                column_property
                    from sqlalchemy import (
                        Column, Integer, ForeignKey, Text,
                        select, create_engine,
                    )

                    Base = declarative_base()


                    class HybridType(Base):
                        __tablename__ = 'hybrid_types'
                        id = Column(Integer, primary_key=True)
                        name = Column(Text)


                    class HybridModel(Base):
                        __tablename__ = 'hybrids'

                        id = Column(Integer, primary_key=True)
                        name = Column(Text)
                        hybrid_type_id = Column(Integer,
                ForeignKey('hybrid_types.id <http://hybrid_types.id>
        <http://hybrid_types.id>
                    <http://hybrid_types.id>'))
                        hybrid_type = relationship('HybridType')

                        hybrid_type_prop = column_property(
                            select([HybridType.name]).
                            where(HybridType.id == hybrid_type_id).
                            as_scalar()
                        )

                        __mapper_args__ = {
                            "polymorphic_on": hybrid_type_prop
                        }

                        def __repr__(self):
                            return "{} ({}:{})".format(type(self).__name__,
                self.name <http://self.name> <http://self.name>
                    <http://self.name>, self.id <http://self.id>
        <http://self.id> <http://self.id>)

                        def __init__(self, **kwargs):
                            self.hybrid_type_name =
                    type(self).__mapper_args__['polymorphic_identity']
                            super(HybridModel, self).__init__(**kwargs)

                        @hybrid_property
                        def hybrid_type_name(self):
                            return self.hybrid_type.name
        <http://self.hybrid_type.name>
                <http://self.hybrid_type.name>
        <http://self.hybrid_type.name>

                        @hybrid_type_name.setter
                        def hybrid_type_name(self, value):
                            self.hybrid_type_id = (
                                select([HybridType.id]).
                                where(HybridType.name == value)
                            )


                    class HybridAlpha(HybridModel):
                        __mapper_args__ = {'polymorphic_identity': 'alpha'}


                    class HybridBeta(HybridModel):
                        __mapper_args__ = {'polymorphic_identity': 'beta'}


                    engine = create_engine('sqlite:///:memory:', echo=True)
                    Base.metadata.create_all(engine)
                    Session = sessionmaker(bind=engine)
                    session = Session()
                    session.add(HybridType(name='alpha'))
                    session.add(HybridType(name='beta'))
                    session.add(HybridAlpha(name='alpha_instance'))
                    session.add(HybridBeta(name='beta_instance'))
                    print(session.query(HybridAlpha).all())



                        On Tue, Nov 29, 2016 at 7:16 AM, mike bayer
                        <mike...@zzzcomputing.com
        <mailto:mike...@zzzcomputing.com>
                <mailto:mike...@zzzcomputing.com
        <mailto:mike...@zzzcomputing.com>>
                <mailto:mike...@zzzcomputing.com
        <mailto:mike...@zzzcomputing.com>
        <mailto:mike...@zzzcomputing.com <mailto:mike...@zzzcomputing.com>>>
                        <mailto:mike...@zzzcomputing.com
        <mailto:mike...@zzzcomputing.com>
                <mailto:mike...@zzzcomputing.com
        <mailto:mike...@zzzcomputing.com>>

                        <mailto:mike...@zzzcomputing.com
        <mailto:mike...@zzzcomputing.com>
                <mailto:mike...@zzzcomputing.com
        <mailto:mike...@zzzcomputing.com>>>>> wrote:



                            On 11/28/2016 05:58 PM, Tucker Beck wrote:

                                Hello, I'm writing today about an
        interesting
                problem we ran
                                into with
                                our sqlalchemy based data store api.

                                Our schema is based on the star-schema idea
                where we have a
                                large 'fact
                                table' with lots of rows. Within that table,
                each row has a
                                foreign key
                                to a small 'dimension table' in which
        each row
                has a unique
                                name. Thus,
                                the type of each row can be defined by the
                relationship
                        between
                                the fact
                                row and the dimension row.

                                We wanted to be able to use SQLAlchemy
        to add
                some custom
                                functionality
                                for the different types of rows we have
        in our 'fact
                        table'. After
                                learning about SQLAlchemy's inheritance
        models, I
                        decided to see
                                if we
                                could support the schema that we had already
                devised for our
                                project.
                                The single-inheritance pattern seemed to
        fit the
                best, but I
                                couldn't
                                find a single case where someone was using a
                star-schema and
                                needed the
                                type discriminator to be derived from the
                foreign key to the
                                dimension
                                table.

                                Further, I found as I was digging into the
                mechanics of the
                                thing that
                                you could not create a row in the fact table
                that was
                        typed by the
                                derived class at creation time. And, you
        cannot
                limit
                        queries
                                from the
                                fact table by creating the queries
        against the
                derived
                        classes.
                                Suppose
                                that (using declarative base) I have the
        fact table
                        represented by a
                                model called HybridModel. This model has two
                derived classes
                                HybridAlpha
                                and HybridBeta. I would like to be able to
                create a new
                        row in
                                the table
                                wrapped by HybridModel by calling
        something like
                                HybridAlpha(**kwargs)
                                and have the type of the new row reflect the
                inheritance
                        model I've
                                described above. Next I wanted to be able to
                formulate a
                        query
                                against
                                one of the derived models and have it
        limited by the
                        type associated
                                with the derived class. So, calling
        something like
                                `session.query(HybridAlpha).all()` would
        only return
                        rows with a
                                type
                                associated with the HybridAlpha model.

                                After a lot of tinkering and
        experimentation,
                I've come
                        up with the
                                following solution:




        https://gist.github.com/dusktreader/2658dedb8ca8e2fd941f8075c75b3c89
        <https://gist.github.com/dusktreader/2658dedb8ca8e2fd941f8075c75b3c89>

        <https://gist.github.com/dusktreader/2658dedb8ca8e2fd941f8075c75b3c89
        <https://gist.github.com/dusktreader/2658dedb8ca8e2fd941f8075c75b3c89>>


        <https://gist.github.com/dusktreader/2658dedb8ca8e2fd941f8075c75b3c89
        <https://gist.github.com/dusktreader/2658dedb8ca8e2fd941f8075c75b3c89>

        <https://gist.github.com/dusktreader/2658dedb8ca8e2fd941f8075c75b3c89
        <https://gist.github.com/dusktreader/2658dedb8ca8e2fd941f8075c75b3c89>>>



        <https://gist.github.com/dusktreader/2658dedb8ca8e2fd941f8075c75b3c89
        <https://gist.github.com/dusktreader/2658dedb8ca8e2fd941f8075c75b3c89>

        <https://gist.github.com/dusktreader/2658dedb8ca8e2fd941f8075c75b3c89
        <https://gist.github.com/dusktreader/2658dedb8ca8e2fd941f8075c75b3c89>>


        <https://gist.github.com/dusktreader/2658dedb8ca8e2fd941f8075c75b3c89
        <https://gist.github.com/dusktreader/2658dedb8ca8e2fd941f8075c75b3c89>

        <https://gist.github.com/dusktreader/2658dedb8ca8e2fd941f8075c75b3c89
        
<https://gist.github.com/dusktreader/2658dedb8ca8e2fd941f8075c75b3c89>>>>

                                I would appreciate any thoughts and
        feedback on the
                        matter. I'm
                                not sure
                                that my approach to this solution has been
                sound, and I
                        would
                                appreciate
                                feedback.


                            I'd try to do things much more simply than
        this.   Most
                        simply, just
                            assign polymorphic_identity to be the integer
                foreign key value.
                            Otherwise, you should be able to do
        polymorphic_on on a
                        correlated
                            subquery, which itself you set up as a
                column_property().
                         There
                            should be no need to modify Query or
        anything like that.


                            I'm out of time today but if you need more
        help I
                can try to
                        work up
                            an example later on.






                                Thanks, and keep up the great work!
        SQLAlchemy
                is just
                        magic!

                                --
                                SQLAlchemy -
                                The Python SQL Toolkit and Object
        Relational Mapper

                                http://www.sqlalchemy.org/

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        --
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Re: [sqlalchemy] Limiting queries from single-table inheritance declarative models

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