问题
I have a pretty simple N:M relationship in SqlAlchemy 0.6.6. I have a class "attractLoop" that can contain a bunch of Media (Images or Videos). I need to have a list in which the same Media (let's say image) can be appended twice. The relationship is as follows:
The media is a base class with most of the attributes Images and Videos will share.
class BaseMedia(BaseClass.BaseClass, declarativeBase):
__tablename__ = "base_media"
_polymorphicIdentity = Column("polymorphic_identity", String(20), key="polymorphicIdentity")
__mapper_args__ = {
'polymorphic_on': _polymorphicIdentity,
'polymorphic_identity': None
}
_name = Column("name", String(50))
_type = Column("type", String(50))
_size = Column("size", Integer)
_lastModified = Column("last_modified", DateTime, key="lastModified")
_url = Column("url", String(512))
_thumbnailFile = Column("thumbnail_file", String(512), key="thumbnailFile")
_md5Hash = Column("md5_hash", LargeBinary(32), key="md5Hash")
Then the class who is going to use these "media" things:
class TestSqlAlchemyList(BaseClass.BaseClass, declarativeBase):
__tablename__ = "tests"
_mediaItems = relationship("BaseMedia",
secondary=intermediate_test_to_media,
primaryjoin="tests.c.id == intermediate_test_to_media.c.testId",
secondaryjoin="base_media.c.id == intermediate_test_to_media.c.baseMediaId",
collection_class=list,
uselist=True
)
def __init__(self):
super(TestSqlAlchemyList, self).__init__()
self.mediaItems = list()
def getMediaItems(self):
return self._mediaItems
def setMediaItems(self, mediaItems):
if mediaItems:
self._mediaItems = mediaItems
else:
self._mediaItems = list()
def addMediaItem(self, mediaItem):
self.mediaItems.append(mediaItem)
#log.debug("::addMediaItem > Added media item %s to %s. Now length is %d (contains: %s)" % (mediaItem.id, self.id, len(self.mediaItems), list(item.id for item in self.mediaItems)))
def addMediaItemById(self, mediaItemId):
mediaItem = backlib.media.BaseMediaManager.BaseMediaManager.getById(int(mediaItemId))
if mediaItem:
if mediaItem.validityCheck():
self.addMediaItem(mediaItem)
else:
raise TypeError("Media item with id %s didn't pass the validity check" % mediaItemId)
else:
raise KeyError("Media Item with id %s not found" % mediaItem)
mediaItems = synonym('_mediaItems', descriptor=property(getMediaItems, setMediaItems))
And the intermediate class to link both of the tables:
intermediate_test_to_media = Table(
"intermediate_test_to_media",
Database.Base.metadata,
Column("id", Integer, primary_key=True),
Column("test_id", Integer, ForeignKey("tests.id"), key="testId"),
Column("base_media_id", Integer, ForeignKey("base_media.id"), key="baseMediaId")
)
When I append the same Media object (instance) twice to one instances of that TestSqlAlchemyList, it appends two correctly, but when I retrieve the TestSqlAlchemyList instance from the database, I only get one. It seems to be behaving more like a set.
The intermediate table has properly all the information, so the insertion seems to be working fine. Is when I try to load the list from the database when I don't get all the items I had inserted.
mysql> SELECT * FROM intermediate_test_to_media;
+----+---------+---------------+
| id | test_id | base_media_id |
+----+---------+---------------+
| 1 | 1 | 1 |
| 2 | 1 | 1 |
| 3 | 1 | 2 |
| 4 | 1 | 2 |
| 5 | 1 | 1 |
| 6 | 1 | 1 |
| 7 | 2 | 1 |
| 8 | 2 | 1 |
| 9 | 2 | 1 |
| 10 | 2 | 2 |
| 11 | 2 | 1 |
| 12 | 2 | 1 |
As you can see, the "test" instance with id=1 should have the media [1, 1, 2, 2, 1, 1]. Well, it doesn't. When I load it from the DB, it only has the media [1, 2]
I have tried to set any parameter in the relationship that could possibly smell to list... uselist, collection_class = list... Nothing...
You will see that the classes inherit from a BaseClass. That's just a class that isn't actually mapped to any table but contains a numeric field ("id") that will be the primary key for every class and a bunch of other methods useful for the rest of the classes in my system (toJSON, toXML...). Just in case, I'm attaching an excerpt of it:
class BaseClass(object):
_id = Column("id", Integer, primary_key=True, key="id")
def __hash__(self):
return int(self.id)
def setId(self, id):
try:
self._id = int(id)
except TypeError:
self._id = None
def getId(self):
return self._id
@declared_attr
def id(cls):
return synonym('_id', descriptor=property(cls.getId, cls.setId))
If anyone can give me a push, I'll appreciate it a lot. Thank you. And sorry for the huge post... I don't really know how to explain better.
回答1:
I think what you want is a described in the documentation as "Extra Fields in Many-to-Many Relationships". Rather than storing a unique row in the database foreach "link", between attractLoop and Media, you would store a single association and specify (as a part of the link object model) how many times it is referenced and/or in which location(s) in the final list the media should appear. This is a different paradigm from where you started, so it'll certainly require some re-coding, but I think it addresses your issue. You would likely need to use a property to redefine how to add or remove Media from the attactLoop.
来源：https://stackoverflow.com/questions/5656715/can-i-append-twice-the-same-object-to-an-instrumentedlist-in-sqlalchemy