[Python] FlushError: new instance conflicts with persistent instance

I have some problems when I try to update information in some tables. For example, I have this table:

class Channel(rdb.Model):
rdb.metadata(metadata)
rdb.tablename("channels")

id = Column("id", Integer, primary_key=True)
title = Column("title", String(100))
hash = Column("hash", String(50))
runtime = Column("runtime", Float)

items = relationship(MediaItem, secondary="channel_items", order_by=MediaItem.position, backref="channels")


And I have this code:

def insertXML(channels, strXml):
channel = Channel()
session = rdb.Session()
result = ""

channel.fromXML(strXml)
fillChannelTemplate(channel, channels)

rChannel = session.query(Channel).get(channel.id)
for chan in channels:
if rChannel.id == channel.id:
rChannel.runtime = channel.runtime
for item in channel.items:
if item.id == 0:
rChannel.items.append(item)


When I do "rChannel.items.append(item)", I got this error:

"FlushError: New instance Channel at 0xaf6e48c with identity key
zeppelinlib.channel.ChannelTest.Channel , (152,) conflicts with
persistent instance Channel at 0xac2e8ac"


However, this instruction is working "rChannel.runtime = channel.runtime".

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