Hi,
I do not know if this is that I am missing something in how SQLAlchemy
works or an issue; I have read the doc and searched for an explanation
but could not find an answer.
When appending via an intermediate table, a M2M relationship behaves
differently wether I use the back_populates argument or not ( or use
backref)
* Without using back_populates or backref is works fine
```
from sqlalchemy import Column, Integer, String, ForeignKey, Table
from sqlalchemy.orm import relationship
from sqlalchemy.ext.declarative import declarative_base
Base = declarative_base()
class User(Base):
__tablename__ = 'user'
id = Column(Integer, primary_key=True)
name = Column(String(64))
keywords = relationship("UsersKeywords")
def __init__(self, name):
self.name = name
class Keyword(Base):
__tablename__ = 'keyword'
id = Column(Integer, primary_key=True)
keyword = Column('keyword', String(64))
def __init__(self, keyword):
self.keyword = keyword
class UsersKeywords(Base):
__tablename__ = 'userskeywords'
user_id = Column( Integer, ForeignKey("user.id"), primary_key=True)
keyword_id = Column(Integer, ForeignKey("keyword.id"),
primary_key=True)
user = relationship(User, back_populates="keywords")
keyword = relationship(Keyword)
>>>u = User(‘John’)
>>>u.keywords.append(Keyword(‘Python’)
>>>u.keywords
[<Keyword object at 0x10d09f860>]
```
* With back_populates or backref in one of the ‘M’ sides of the
relation, ‘User’ in this case, I get a KeyError exception:
```
class User(Base):
__tablename__ = 'user'
id = Column(Integer, primary_key=True)
name = Column(String(64))
keywords = relationship('UsersKeywords', back_populates='user') #
added back_populates argument
def __init__(self, name):
self.name = name
>>>u = User(‘John’)
>>>u.keywords.append(Keyword(‘Python’)
Traceback (most recent call last):
File "<input>", line 1, in <module>
File
"/Users/xxx/Development/python_ws/flask_env/lib/python3.6/site-packages/sqlalchemy/orm/collections.py",
line 1044, in append
item = __set(self, item, _sa_initiator)
File
"/Users/xxx/Development/python_ws/flask_env/lib/python3.6/site-packages/sqlalchemy/orm/collections.py",
line 1016, in __set
item = executor.fire_append_event(item, _sa_initiator)
File
"/Users/xxx/Development/python_ws/flask_env/lib/python3.6/site-packages/sqlalchemy/orm/collections.py",
line 680, in fire_append_event
item, initiator)
File
"/Users/xxx/Development/python_ws/flask_env/lib/python3.6/site-packages/sqlalchemy/orm/attributes.py",
line 943, in fire_append_event
state, value, initiator or self._append_token)
File
"/Users/xxx/Development/python_ws/flask_env/lib/python3.6/site-packages/sqlalchemy/orm/attributes.py",
line 1210, in emit_backref_from_collection_append_event
child_impl = child_state.manager[key].impl
KeyError: 'user'
```
Why is the behavior different?
Thanks in advance
Juan
[SQLAlchemy version: 1.2.9]
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