I forgot to mention:
I'm using:
- Python 2.7.6 on Linux,
- SQLAlchemy version 0.9.9.
- PostgreSQL 9.3
Thanks -
terry
On Monday, June 15, 2015 at 11:12:54 AM UTC-4, T Mark wrote:
>
> Hi there,
>
> I have been pulling my hair out on this one.
>
> I understood that objects make it into the session only due to an explicit
> call to add(). But, I seem to be seeing objects being added without my
> explicitly doing so. Is this to be expected ?
>
> For instance, I want to establish a many-to-many relationship between two
> classes: say, for the purposes here, "Person" and "Kid".
>
>
> test=> create table persons (id SERIAL NOT NULL);
> test=> CREATE TABLE person_to_kids (person_id INT NOT NULL, kid_id INT
> NOT NULL);
> test=> create table kids (id SERIAL NOT NULL, name TEXT NOT NULL);
> test=> insert into kids (name) VALUES ('Fred');
> test=> insert into kids (name) VALUES ('Barney');
>
> person_to_kids = Table('person_to_kids',
> Base.metadata,
> Column('person_id', Integer, ForeignKey('
> persons.id')),
> Column('kid_id', Integer, ForeignKey('kids.id')))
> class Person(Base):
>
> __tablename__ = 'persons'
> id = Column('id', Integer, primary_key = True)
> def __init__(self,
> kids = []):
>
> kids = Kid.get_kids(kid_names = kids)
>
> print("__init__ before kids assignment")
> print(session.new)
>
> """ Assigning to self.kids here seems to add self to session ???
> """
>
> self.kids=kids
> print("After assignment to self.kids")
> print(session.new)
>
>
> class Kid(Base):
> __tablename__ = 'kids'
> id = Column(Integer, primary_key = True)
> name = Column(String)
> parents = relationship("Person",
> secondary = person_to_kids,
> backref="kids")
>
> def __init__(self, name = None):
> self.name = name
>
> @staticmethod
> def get_kids(kid_names = []):
>
> kids = []
>
> for name in kid_names:
> # find first kid
> target_set = session.query(Kid).filter(Kid.name ==
> name).first()
> kids.append(target_set)
>
> return kids
>
>
>
> What is puzzling me is that, if I have a collection of Kid objects, and I
> assign it to the kids collection in a Person, the Person object seems to be
> automatically added to the session and marked as pending, even if I have
> not added it.
>
> For instance, if the Persons table is empty:
>
> test=> select * from persons;
> id
> ----
> (0 rows)
>
>
>
> and I run the following code:
>
> print(session.new)
> obj = Person(kids = ['Barney', 'Fred'])
> print("obj has been created")
> print(session.new)
> session.commit()
>
>
> The output shows that the Person object is added immediately after the
> assignment to obj.kids, without any call to session.add() anywhere in the
> code:
>
> IdentitySet([])
> __init__ before kids assignment
> IdentitySet([])
> After assignment to self.kids
> IdentitySet([<__main__.Person object at 0x7fb6ce447b10>])
> obj has been created
> IdentitySet([<__main__.Person object at 0x7fb6ce447b10>])
>
>
> And indeed, due to the commit() at the end, the person object makes it
> into the database:
>
> test=> select * from persons;
> id
> ----
> 10
> (1 row)
>
>
>
> But, I understood that objects (only) make it into a session by virtue of
> being explicitly added.
>
> So, is this the correct behavior, or am I misunderstanding something ?
>
> If I'm not misunderstanding this all, the complete code is at
> https://github.com/NuggyBuggy/sqlalchemy_question.git .
>
> Thanks for reading,
> terry
>
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