SQLAlchemy many-to-many relationship deletion operation on one side - sqlalchemy

Question

I have a many-to-many relationship linked by an association table, like this:
left_right_association_table = Table("left_right_association_table", Base.metadata,
Column('leftside_id', Integer, ForeignKey('leftside_table.id')),
Column('rightside_id', Integer, ForeignKey('rightside_table.id'))
)
class LeftSide(Base):
...
rightside_members = relationship("RightSide", secondary=left_right_association_table, backref="leftside", lazy="joined")
...
class RightSide(Base):
...
Now I have a LeftSide instance, with a list of RightSide instances as its rightside_members attribute:
ls = LeftSide(**kw)
rs1 = RightSide(**kw)
rs2 = RightSide(**kw)
ls.rightside_members.append(rs1)
ls.rightside_members.append(rs2)
Then, I want to drop one of the list member: rs2.
Step 1: I reproduce the rightside_members list:
updated_rightside_members = [RightSide(**rs_attrs) for rs_attrs in ls.rightside_members] # each `rs` object already has primary key
updated_rightside_members.pop() # remove the second item from the list
Step 2: I retrieve the ls from the database:
old_ls = db_session.query(LeftSide).filter(LeftSide_id == ls.id).one()
Step 3: I tack the updated_rightside_members onto the old_ls:
old_ls.rightside_members = updated_rightside_members
Step 4: commit to database.
db_session.commit()
Then I get this Error:
sqlite3.IntegrityError: UNIQUE constraint failed: rightside_table.id
It seems to me that SQLAlchemy is thinking that I'm trying to put a duplicate rs1 back into the database, instead of deleting its brother rs2 from the database.
How should I do this deletion operation?

Answer 1

I've solved this problem. First I really want to point out that reading carefully the documentation helps. Using mapped class to initialize an instance doesn't mean the session knows about this object, at least not until you use Session.add() method.
So, the proper steps should be like this:
decide whether the incoming args contain a id which represent the instance primary key. If it does, then instead of initialize it, the right way to do is to query it from the database. This way the Session will ensure that the queried object has a unique object id in its identity map.
if it doesn't contain an id attribute, then initialize it, and add it to the member list.