On 6/15/15 11:12 AM, 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().
or if they are associated with a parent object that is added to the
Session via add(), or if they are associated with an object that is
already present in a Session via add(); this also will occur for
backrefs, e.g. A is in the session, B.a is referred to A, B.a has a
backref A.bs, therefore B is now added. This is configurable.
But, I seem to be seeing objects being added without my explicitly
doing so. Is this to be expected ?
yes. please see:
http://docs.sqlalchemy.org/en/rel_1_0/orm/cascades.html?highlight=cascades
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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