Pymongo BSON Binary save and retrieve?

问题

I'm working in Python with MongoDB trying to save an array of floats tightly.

I can Create and store correctly *

but I CANNOT RETRIEVE THE DATA IN A USABLE FORMAT.

>>> import random, array, pymongo
>>> from bson.binary import Binary as BsonBinary
>>> con = pymongo.Connection('localhost', 27017)
>>> mm = con['testDatabase']
>>> vals = [random.random() *100 for x in range(1, 5)]
>>> vals
[2.9962593, 64.5582810776, 32.3781311717, 82.0606953423]
>>> varray = array.array('f', vals)
>>> varray
array('f', [2.9962593, 64.5582810776, 32.3781311717, 82.0606953423])
>>> vstring = varray.tostring()
>>> vstring
'\xb7\xc2?@\xd7\x1d\x81B5\x83\x01B\x13\x1f\xa4B'
>>> vbson = BsonBinary(vstring, 5)
>>> vbson
Binary('\xb7\xc2?@\xd7\x1d\x81B5\x83\x01B\x13\x1f\xa4B', 5)
>>> doc1 = { 'something': 1 , 'else' : vbson}
>>> doc1
{'something': 1, 'else': Binary('\xb7\xc2?@\xd7\x1d\x81B5\x83\x01B\x13\x1f\xa4B', 5)}
>>> mm.test1.insert(doc1)
ObjectID('530f7af1d809d80d3db1f635')
>>> gotdoc = mm.test1.find_one()
>>> gotdoc
{u'_id': ObjectId('530f7af1d809d80d3db1f635'), u'something': 3, u'else': Binary('\xb7\xc2?@\xd7\x1d\x81B5\x83\x01B\x13\x1f\xa4B', 5)}
>>> gotfield = gotdoc['else']
>>> gotfield
Binary('\xb7\xc2?@\xd7\x1d\x81B5\x83\x01B\x13\x1f\xa4B', 5)
>>> from bson import BSON
>>> BSON.decode(gotfield)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: unbound method decode() must be called with BSON instance as first argument (got Binary instance instead)
>>> gotfield.decode()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
UnicodeDecodeError: 'ascii' codec can't decode byte 0xb7 in position 0: ordinal not in range(128)
>>>

Once I get my Python string back, I can get my array of random floats back. But how?

回答1:

Let's go through the errors:

The first error appears simply because you need an actual BSON object. Note, that you have never encoded any data - creating bson.binary.Binary object does not mean invoking BSON.encode().
And that is where PyMongo cheats you a bit. The bson.binary.Binary is a runtime-patched str or bytes instance (see source). That is why you get the second error: what you call is actually str.decode(), not BSON.decode(). So, gotfield contains the random float data you've stored initially, but the object itself has some different methods (e.g. repr()) bound to it.

回答2:

I'm coming~ I just find a way. Hope this may help you somehow.

from cStringIO import StringIO
from PIL import Image

save image:

content = StringIO(f.read())

c = dict(
    content=bson.binary.Binary(content.getvalue()),
)
# insert dict into my database, sha1 is primary key
image_data[sha1] = c

retrieve image:

f = image_data[sha1]
image = Image.open(StringIO(f['content']))

----EDIT----

If you want to return an image from web servers.Do like this:

f = image_data[sha1]
# f['mime'] is the type of image, for example 'png'.
resp = Response(f['content'], mimetype='image/' + f['mime'])
return resp

回答3:

Use array.fromstring for the final decoding stage. I can get to the same spot you're at like so:

>>> from bson import Binary
>>> import array
>>> gotstring = Binary('\xb7\xc2?@\xd7\x1d\x81B5\x83\x01B\x13\x1f\xa4B', 5)

And finally:

>>> a = array.array('f')
>>> a.fromstring(gotstring)
>>> a
array('f', [2.9962594509124756, 64.55828094482422, 32.37813186645508, 82.0606918334961])

回答4:

You need to encode the array before storing it, and should not use the array.tostring. Please have a look at the documentation here.

from bson import BSON
bson_string = BSON.encode({"hello": "world"})
>>> bson_string
'\x16\x00\x00\x00\x02hello\x00\x06\x00\x00\x00world\x00\x00'
>>> bson_string.decode()
{u'hello': u'world'}