CODEWITHSUNDEEP
pythonsqlalchemymany-to-many
alfredodeza
alfredodeza

Reputation: 5198

Updating a field in an Association Object in SQLAlchemy

I have an association object in SQLAlchemy that has some extra information (actually a single field) for 2 other objects.

The first object is a Photo model, the second object is a PhotoSet and the association object is called PhotoInSet which holds the position attribute which tells us in what position is the Photo in the current PhotoSet.

class Photo(Base):

    __tablename__ = 'photos'
    id = Column(Integer, primary_key=True)
    filename = Column(String(128), index=True)
    title = Column(String(256))
    description = Column(Text)
    pub_date = Column(SADateTime)

class PhotoInSet(Base):

    __tablename__ = 'set_order'
    photo_id = Column(Integer, ForeignKey('photos.id'), primary_key=True)
    photoset_id = Column(Integer, ForeignKey('photo_set.id'), primary_key=True)
    position = Column(Integer)
    photo = relationship('Photo', backref='sets')

    def __repr__(self):
        return '<PhotoInSet %r>' % self.position


class PhotoSet(Base):

    __tablename__ = 'photo_set'
    id = Column(Integer, primary_key=True)
    name = Column(String(256))
    description = Column(Text)
    timestamp = Column(SADateTime)
    user_id = Column(Integer, ForeignKey('users.id'))
    user = relationship('User', backref=backref('sets', lazy='dynamic'))
    photo_id = Column(Integer, ForeignKey('photos.id'))
    photos = relationship('PhotoInSet', backref=backref('set', lazy='select'))

I have no problems creating a new PhotoSet saving the position and creating the relationship, which is (roughly) done like this:

    # Create the Set
    new_set = PhotoSet(name, user)

    # Add the photos with positions applied in the order they came
    new_set.photos.extend(
        [
            PhotoInSet(position=pos, photo=photo)
            for pos, photo in
            enumerate(photo_selection)
        ]
    )

But I am having a lot of trouble attempting to figure out how to update the position when the order changes.

If I had, say, 3 Photo objects with ids: 1, 2, and 3, and positions 1, 2, and 3 respectively, would look like this after creation:

>>> _set = PhotoSet.get(1)
>>> _set.photos
[<PhotoInSet 1>, <PhotoInSet 2>, <PhotoInSet 3>]

If the order changes, (lets invert the order for this example), is there anyway SQLAlchemy can help me update the position value? So far I am not happy with any of the approaches I can come up with.

What would be the most concise way to do this?

Upvotes: 2

Views: 4142

Answers (1)

Mark Hildreth
Mark Hildreth

Reputation: 43111

Take a look at the Ordering List extension:

orderinglist is a helper for mutable ordered relationships. It will intercept list operations performed on a relationship()-managed collection and automatically synchronize changes in list position onto a target scalar attribute.

I believe you could change your schema to look like:

from sqlalchemy.ext.orderinglist import ordering_list

# Photo and PhotoInSet stay the same...

class PhotoSet(Base):
    __tablename__ = 'photo_set'
    id = Column(Integer, primary_key=True)
    name = Column(String(256))
    description = Column(Text)
    photo_id = Column(Integer, ForeignKey('photos.id'))
    photos = relationship('PhotoInSet',
       order_by="PhotoInSet.position",
       collection_class=ordering_list('position'),
       backref=backref('set', lazy='select'))

# Sample usage...
session = Session()

# Create two photos, add them to the set...
p_set = PhotoSet(name=u'TestSet')

p = Photo(title=u'Test')
p2 = Photo(title='uTest2')

p_set.photos.append(PhotoInSet(photo=p))
p_set.photos.append(PhotoInSet(photo=p2))
session.add(p_set)

session.commit()
print 'Original list of titles...'
print [x.photo.title for x in p_set.photos]
print ''

# Change the order...
p_set.photos.reverse()
# Any time you change the order of the list in a way that the existing
# items are in a different place, you need to call "reorder". It will not
# automatically try change the position value for you unless you are appending
# an object with a null position value.
p_set.photos.reorder()    
session.commit()

p_set = session.query(PhotoSet).first()
print 'List after reordering...'
print [x.photo.title for x in p_set.photos]

The results of this script...

Original list of titles...
[u'Test', u'uTest2']

List after reordering...
[u'uTest2', u'Test']

In your comment, you said...

So this would mean that if I assign a new list to _set.photos I get the positioning for free?

I doubt this is the case.

Upvotes: 3

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