Release: 1.2.18 legacy version | Release Date: February 15, 2019

SQLAlchemy 1.2 Documentation

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Source code for examples.large_collection.large_collection

from sqlalchemy import Column
from sqlalchemy import create_engine
from sqlalchemy import ForeignKey
from sqlalchemy import Integer
from sqlalchemy import MetaData
from sqlalchemy import String
from sqlalchemy import Table
from sqlalchemy.orm import mapper
from sqlalchemy.orm import relationship
from sqlalchemy.orm import sessionmaker


meta = MetaData()

org_table = Table(
    "organizations",
    meta,
    Column("org_id", Integer, primary_key=True),
    Column("org_name", String(50), nullable=False, key="name"),
    mysql_engine="InnoDB",
)

member_table = Table(
    "members",
    meta,
    Column("member_id", Integer, primary_key=True),
    Column("member_name", String(50), nullable=False, key="name"),
    Column(
        "org_id",
        Integer,
        ForeignKey("organizations.org_id", ondelete="CASCADE"),
    ),
    mysql_engine="InnoDB",
)


class Organization(object):
    def __init__(self, name):
        self.name = name


class Member(object):
    def __init__(self, name):
        self.name = name


mapper(
    Organization,
    org_table,
    properties={
        "members": relationship(
            Member,
            # Organization.members will be a Query object - no loading
            # of the entire collection occurs unless requested
            lazy="dynamic",
            # Member objects "belong" to their parent, are deleted when
            # removed from the collection
            cascade="all, delete-orphan",
            # "delete, delete-orphan" cascade does not load in objects on
            # delete, allows ON DELETE CASCADE to handle it.
            # this only works with a database that supports ON DELETE CASCADE -
            # *not* sqlite or MySQL with MyISAM
            passive_deletes=True,
        )
    },
)

mapper(Member, member_table)

if __name__ == "__main__":
    engine = create_engine(
        "postgresql://scott:tiger@localhost/test", echo=True
    )
    meta.create_all(engine)

    # expire_on_commit=False means the session contents
    # will not get invalidated after commit.
    sess = sessionmaker(engine, expire_on_commit=False)()

    # create org with some members
    org = Organization("org one")
    org.members.append(Member("member one"))
    org.members.append(Member("member two"))
    org.members.append(Member("member three"))

    sess.add(org)

    print("-------------------------\nflush one - save org + 3 members\n")
    sess.commit()

    # the 'members' collection is a Query.  it issues
    # SQL as needed to load subsets of the collection.
    print("-------------------------\nload subset of members\n")
    members = org.members.filter(member_table.c.name.like("%member t%")).all()
    print(members)

    # new Members can be appended without any
    # SQL being emitted to load the full collection
    org.members.append(Member("member four"))
    org.members.append(Member("member five"))
    org.members.append(Member("member six"))

    print("-------------------------\nflush two - save 3 more members\n")
    sess.commit()

    # delete the object.   Using ON DELETE CASCADE
    # SQL is only emitted for the head row - the Member rows
    # disappear automatically without the need for additional SQL.
    sess.delete(org)
    print(
        "-------------------------\nflush three - delete org, "
        "delete members in one statement\n"
    )
    sess.commit()

    print("-------------------------\nno Member rows should remain:\n")
    print(sess.query(Member).count())
    sess.close()

    print("------------------------\ndone.  dropping tables.")
    meta.drop_all(engine)