Composite Fields

Composite fields allow you to create fields that are combinations of multiple other fields. Suppose you’re creating a table where you plan to store a collection of social media items (tweets, facebook posts, instagram pics, etc). If you make the hash key the id of the item, there is the remote possiblity that a tweet id will collide with a facebook id. Here is the solution:

class SocialMediaItem(Model):
    userid = Field(hash_key=True)
    type = Field()
    id = Field()
    uid = Composite('type', 'id', range_key=True)

This will automatically generate a uid field from the values of type and id. For example:

>>> item = SocialMediaItem(type='facebook', id='12345')
>>> print item.uid
facebook:12345

Note that setting a Composite field just doesn’t work:

>>> item.uid = 'ILikeThisIDBetter'
>>> print item.uid
facebook:12345

By default, a Composite field simply joins its subfields with a ':'. You can change that behavior for fancier applications:

def score_merge(likes, replies, deleted):
    if deleted:
        return None
    return likes + 5 * replies

class Post(Model):
    userid = Field(hash_key=True)
    id = Field(range_key=True)
    likes = Field(type=int)
    replies = Field(type=int)
    deleted = Field(type=bool)
    score = Composite('likes', 'replies', 'deleted', type=int,
                      merge=score_merge, index='score-index')

So now you can update the likes or replies count, and the score will automatically change. Which will re-arrange it in the index that you created. Then, if you mark the post as “deleted”, it will remove the score field which removes it from the index.

Whooooaaahh…

The last neat little thing about Composite fields is how you can query them. For numeric Composite fields you probably want to query directly on the score like any other field. But if you’re merging strings like with SocialMediaItem, it can be cleaner to refer to the component fields themselves:

>>> fb_post = engine.query(SocialMediaItem).filter(userid='abc123',
...     type='facebook', id='12345').first()

The engine will automatically detect that you’re trying to query on the range key, and construct the uid from the pieces you provided.