python - Removing/Adding an Object from a Collection in SQLAlchemy -

issue @ hand: removing user group , adding user group
many-to-one table relationship
    (many users 1 group)
using: sqlalchemy 1.0.9, python 2.7.9

db operates off of crud operations:
1. create group (e.g. 'admin')
2. retrieve group (using either group name or id)
3. update:
   a) update : add_user group ('joe' 'admin')
   b) update : remove_user group ('bob' 'editor')
4. delete group

i newbie trying hands dirty, smartly here. 1. attempting remove .user group() in function: update_remove_user without deleting user user(). found similar want on stacks here. , in sqlalchemy docs
2. adding user existing group. right now, function creating group , user instead of retrieving group , adding user it.

model: base.py

class group(base):     __tablename__ = 'groups'      id = column(integer, primary_key=true)     group_name = column(string(50), unique=true)     created_on = column(datetime, default=datetime.utcnow)     modified_on = column(datetime, onupdate=datetime.utcnow) #new      user_id = column(integer, foreignkey('users.id'))     user = relationship('user', backref='groups')     def __init__(self, group_name, *args):        self.group_name = group_name        self.args = args      def __repr__(self):         return "<group(group_name='%s')>" % (self.group_name) 

object.py

def create_group(self, group_name):      new_group = group(group_name)     self.session.add(new_group)     self.session.commit()     return new_group     print 'this new group: ', new_group  def update_group_add_user(self, group_name, user):     g = self.session.query(group).\     filter(group.group_name == group_name).one()     if g:         g.append(user) # append not found         self.session.commit()  def update_group_remove_user(self, group_name, user):     remove_user = self.session.query(group).\     filter(group.group_name == group_name).\     filter(group.user == user).one()     if remove_user:         r = remove_user.user         print 'test r : ', r         g = group() # not work          g.remove(r)         self.session.commit() 

---

changes per suggestions below:
works except add_user replaces user new user when added same group (as indicated alex) many-to-one relationship. added many-to-many relationship , same issue occurring. user being replace in many-to-many relationship.

changes reflected # below of working code has resolved issues.code working n-n relationship.

object.py

def update_group_add_user(self, group_name, user):     g = self.session.query(group).\     filter(group.group_name == group_name).one()     if g:         found_name = g.group_name         g.users = [user] # < replaced working: g.users.append(user)         print 'found_name , user: ', found_name, user         self.session.commit()  def update_group_remove_user(self, group_name, user):     remove_user = self.session.query(group).\     filter(group.group_name == group_name).one()     if remove_user:         remove_user.users = none # < replaced working remove_user.users.remove(user)         self.session.commit() 

new many many design:

association_t = table('group_user_link', base.metadata,     column('group_id', integer, foreignkey('groups.id')),     column('user_id', integer, foreignkey('users.id')))  class group(base):     __tablename__ = 'groups'     id = column(integer, primary_key=true)     name = column(string(50), unique=true)     user = relationship('user', secondary='group_user_link', backref='groups')      # more...  class user(base):     __tablename__ = 'users'     id = column(integer, primary_key=true)     username = column(string(40), unique=true)     # more info 

first of all, sure want create many-to-one relationship?

according relationship 1 "group" can contain 1 user. opposite term "group". think, should create many-to-many relationship.

as first issue:

g = self.session.query(group).\ filter(group.group_name == group_name).one() ... g.append(user) # append not found 

when make such query, result instance of group class. , instance doesn't have append method. right way change user group is:

g.user = user 

as second issue:

def update_group_remove_user(self, group_name, user):     remove_user = self.session.query(group).\     filter(group.group_name == group_name).\     filter(group.user == user).one()     if remove_user:         r = remove_user.user         print 'test r : ', r         g = group() # not work          g.remove(r)         self.session.commit() 

again, remove_user instance of group (so, have chosen wrong name it).

when tried g = group(); g.remove(r) created new instance of group , tried call method doesn't exists. so, in order delete user group should:

remove_user.user = none self.session.commit()