allow put and get to work with compressed blob properties

[cguardia]

From #160.

[eyalhei]

Hi, hope it is ok, I had a look.
In case someone changes the compressed flag it is not supported, is that intentional?
In the old version the compression info was somehow saved in the datastore

[chrisrossi]

In case someone changes the compressed flag it is not supported, is that intentional?

True. See below.

In the old version the compression info was somehow saved in the datastore

I looked into this some this morning. The way NDB did this previously was by setting a "meaning uri" field of the protocol buffer to a custom value that was understood by NDB to mean it was a compressed property. Unfortunately, the modern incarnation of Datastore does not have a "meaning uri" field we can set.

So, yes, currently the way NDB knows a value stored in Datastore is compressed is from the Property definition in the Python code. There's no information stored in Datastore to indicate that a property is compressed or not. The consequence of this is that, as you point out, you can't really change a property between compressed and uncompressed without introducing errors when trying to read back out previously written values.

If this is particularly troublesome, we could look into storing BlobProperty values in some kind of wrapper that indicates compressed or uncompressed state. That could get complicated trying to preserve backwards compatibility.

[chrisrossi]

This is fine with caveats noted in comments.

@andrewsg Should we open an issue for the caveats explored in the comments?

[eyalhei]

@chrisrossi Thanks for the quick response.
The support for changing is not critical for me, as I have migration scripts as it is so that will be another case.
When do you think there will be a version with this?

Thanks

[andrewsg]

Hi @eyalhei, this PR will be out in a new version very soon.

As far as the compressed tag caveats, let's not open an issue for it yet; I will make a note to raise this question to the Datastore team internally in our meeting next month and see what they think of it.

[eyalhei]

Thank you