It isn't clear from the documentation how does the "collection_class" attribute work in a many-to-many relationship

From the documentation on the site, it isn't clear how to create a many-to-many relation where both sides hold their relations in a user defined "collection_class". The doc doesn't mention if the "collection_class" attribute affects only one side of the relation or both.

For example, let's look on the following code:

import sqlalchemy
from sqlalchemy import *
from sqlalchemy.orm import *
engine = create_engine('sqlite://')
metadata = MetaData()
metadata.bind = engine

Session = scoped_session(sessionmaker(bind = engine, autoflush = True,
autocommit = False))

# TABLES

owners_table = Table('owners', metadata, 
    Column('owner_id', Integer, primary_key=True),
)

dogs_table = Table('dogs', metadata, 
    Column('dog_id', Integer, primary_key=True),
)

owners2dogs_table = Table('owners2dogs', metadata, 
    Column('owner_id', Integer, ForeignKey(owners_table.columns.owner_id)),
    Column('dog_id', Integer, ForeignKey(dogs_table.columns.dog_id)))

metadata.create_all()

# POJOS
class Owner(object):
    pass

class Dog(object):
    pass

# MAPPERS
owners2dogs_relation = relation(Dog,
                              backref = "owners",
                              secondary = owners2dogs_table,
                              collection_class = set)
mapper(Owner, owners_table, properties = {"dogs" : owners2dogs_relation})
mapper(Dog, dogs_table)

owner = Owner()
dog = Dog()
print type(owner.dogs)
print type(dog.owners)

The printed result is:

<class 'sqlalchemy.orm.collections.InstrumentedSet'>
<class 'sqlalchemy.orm.collections.InstrumentedList'>

and not:

<class 'sqlalchemy.orm.collections.InstrumentedSet'>
<class 'sqlalchemy.orm.collections.InstrumentedSet'>

I am not sure what is the anticipated result for most users. I can say for myself, that I thought that both sides would turn out to be a set, and I was wrong. Either way, it isn't mentioned anywhere - not in the site documentation and not in the code documentation.

The only way I could think of to make both sides keep their relation in a set is to do something like:

owners2dogs_relation = relation(Dog,
                              back_populates = "owners",
                              secondary = owners2dogs_table,
                              collection_class = set)
mapper(Owner, owners_table, properties = {"dogs" : owners2dogs_relation})
dogs2owners_relation = relation(Owner,
                              back_populates = "dogs",
                              secondary = owners2dogs_table,
                              collection_class = set)
mapper(Dog, dogs_table, properties = {"owners" : dogs2owners_relation})

If it's the answer, it should be documented too.

kobipe3@gmail.com

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