Databases
StackOverflow2013
Postsdbo
POIs NDB model _post_delete_hook called after transaction? OR Best way to clean a blob from the blobstore when its referencing entity is deleted
 

Note that there are some explanatory texts on larger screens.

plurals
  1. POIs NDB model _post_delete_hook called after transaction? OR Best way to clean a blob from the blobstore when its referencing entity is deleted
    primarykey
    data
    text
    <p>I am implementing a file storage system where multiple people can share a file uploaded to the blobstore, but once they have all deleted the file, it is removed from the blobstore. </p> <p>I am doing this using a reference counting entity, which is deleted when its count is set to 0. I am also using the BlobInfo ndb model defined in <code>google.appengine.ext.ndb.blobstore</code> as I need to get a bunch of BlobInfos at a few points and wanted to be able to do that asynchronously.</p> <p>When the reference counting entity is deleted, its _post_delete_hook method is called, which I have calling blobstore.delete_async with the BlobKey and it is also calling BlobInfo._key.delete_async() on the ndb BlobInfo instance. </p> <pre><code>class FileCounter(ndb.Model): count = ndb.IntegerProperty(default=1) @classmethod def _post_delete_hook(cls, key, future): blob_key = cls.blob_key_for_key(key) blobinfo_extension.delete(blobstore.BlobInfo.get(blob_key)) </code></pre> <p>and blobinfo_extension.delete is defined as:</p> <pre><code>@ndb.tasklet def delete_async(blobinfo): """ Delete both the blob info and the blob in the blobstore it refers to. """ yield blobstore.delete_async(blobinfo.key()), blobinfo._key.delete_async() def delete(blobinfo): delete_async(blobinfo).wait() </code></pre> <p>I am getting the count entity, decrementing its count and then checking if it is 0. If it is 0 then it is deleted. This is all done in a transaction. Does the _post_delete_hook method get called outside of the transaction? As you can see, it is accessing the BlobInfo entity and deleting that which would require a cross group transaction.</p> <p>I am going to try this and see if it works, but I posted it here due to not being able to find any answers, so it may help someone else in the future.</p> <p>This this a good solution for this problem, or should I just clean up the blobstore/blobinfo 'manually' when the count is decremented? I think perhaps I am abusing the post delete hook!</p>
    singulars
    1. PO
      singulars
      1. PTAnswer
    1. This table or related slice is empty.
    1. PTQuestion
    1. USDylan
    1. USDylan
    plurals
    1. This table or related slice is empty.
    1. This table or related slice is empty.
    1. This table or related slice is empty.
    1. PO
      singulars
      1. PTAnswer
    1. This table or related slice is empty.
    1. This table or related slice is empty.
 

Querying!

 
Guidance
A row detail

Detail views are divided into sections. All the information in the data section comes from columns in the selected row. The other sections display data from other, related rows.

Related data can be related in a to-one or a to-many fashion. Captions of data related in a to-many fashion link to a list view showing a filtered view of the table.

Try moving around until you find a non-empty to-many entry and click on the label to get to one. You can move back to the root by clicking on the database name in the header.

SQuiL has stopped working due to an internal error.

If you are curious you may find further information in the browser console, which is accessible through the devtools (F12).

Reload