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The error

TemplateSyntaxError: Caught ReferencePropertyResolveError while rendering: ReferenceProperty failed to be resolved: [u'Image', 12345678L]

Models 

#ModelBase class extends from db.Model

class Article(TaggableModel):
   ...
  image = db.ReferenceProperty(Image, collection_name='image_set')
  image_blob = blobstore.BlobReferenceProperty()

class BlobStoreImage(ModelBase):
   ...
  title = db.StringProperty()
  alt = db.TextProperty(default="")
  blobkey = blobstore.BlobReferenceProperty()

class Image(ModelBase):
   ...
  title = db.StringProperty()
  original = db.BlobProperty()

From what I understand, there is an article that is referring to a non-existent Image with id='12345678L'. However, it seems that you can't query individual fields in app engine. Seeing as this is the case, how can I resolve this error by either removing the Article associated with the non-existent image, or alternatively, creating an image with the above id.

I have already tried the latter, and the server returned a 200 code, but perhaps I have the wrong syntax.

Community
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snakesNbronies
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1 Answers1

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The linked question is both a) out of date and b) completely irrelevant to your question. Of course you can query for an individual image by ID, that isn't what Nick says there at all (FYI he was saying that you can't do a query that just fetches single fields rather than whole records, which was true at the time but in fact you now can by using projection queries).

Using the normal db query syntax, you can find the article with the relevant image id simply as follows:

from google.appengine.ext import db
image_key = db.Key.from_path(u'Image', 12345678L)
article = Article.all().filter('image', image_key).get()

If this is confusing you, you should go and read the datastore query documentation, which is comprehensive.

Daniel Roseman
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  • Firstly - thanks for the quick response! One note: I don't think the ```.get()``` at the end is needed. I haven't gone through the full docs yet, but this makes sense. I would find it quite odd if querying by field weren't available for some reason. – snakesNbronies Apr 22 '13 at 10:22
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    the get, or equvalent is required other wise article will be assigned a query object. You can then iterate over such an object, but given you want a single entity that is inefficient and unwieldy. – Tim Hoffman Apr 22 '13 at 11:08