数据存储区ndb，在投影内使用ndb.ComputedProperty查询是否安全？

Question

I am storing thumbnails 72x72 encoded base64 in a ndb.TextProperty()

The model structure is something like:

class Article(ndb.Model):
    title = ndb.StringProperty()
    body = ndb.TextProperty()
    tags = ndb.StringProperty(repeated=True, indexed=True)
    thumbnail = ndb.TextProperty()  
    has_thumbnail = ndb.ComputedProperty(
        lambda self: True if self.thumbnail else False) 
    enable = ndb.BooleanProperty(default=True)   

The method for query using a projection is:

@classmethod
def get_articles(cls):
    q = Aricle.query(
        True == Article.enable,
        projection = [
            Article.title
            Article.has_thumbnail
        ])
     return q.get()

Since the ndb.TextProperty() are not indexed I can not get them on the projections, therefore I tried with a ndb.ComputedProperty and notice that was working.

My main question is to know if this is a correct way of doing the query, basically I just want a query to return the title of the article and the thumbnail or to know if the article has thumbnail.

Answer 1

That is correct, the computed property will work with a projection query.

The only caveat to be aware of is that the query will use the computed property value that is persisted to data store. So if you just added the computed property, it will not be present in the data store for previously-added entities; you would need to re-put those entities to persist the new property.