Python PynamoDB - Creating dynamic queries with Flask

Question

I am using PynamoDB and trying to genrate a dynamic query/scan on attributes from a flask request. Assuming the following model:

class Test(Model):
    class Meta:
        table_name = "test"
        region = "us-west-2"
    a = NumberAttribute(hash_key=True)
    b = NumberAttribute(range_key=True)
    c = UnicodeAttribute()

I have this bolted to Flask, so that when a user sends a request with query arguments such as ?a=123&b=456 I can turn this into a filter within PynamoDB to return results which match records with those two filters.

The following works for a single attribute:

>>> query = operator.eq(Test.a, 123)
>>> list(Test.scan(query))
[Test<4426599961>]

I am looking to be able to bolt this onto a dynamic number of attributes (beyond a,b and c for example)

I tried turning this into a tuple, but that seems to break with the following error:

>>> query = (operator.eq(Test.a, 123), operator.eq(Test.b, 456))
>>> list(Test.scan(*query))

but I get the following error:

Invalid type for parameter Segment, value: b = {'N': '1'}, type: <class 'pynamodb.expressions.condition.Comparison'>, valid types: <type 'int'>, <type 'long'>

Any ideas on how I can construct a dynamic filter to pass into the PynamoDB scan method?

pjz · Accepted Answer · 2018-03-16 23:45:53Z

2

How about:

q = { 'a': 123, 'b': 456 }

query = None
for fieldname in q:
    field = getattr(Test, fieldname, None)
    if field is None: continue
    clause = field == q[fieldname]
    query = clause if query is None else query & clause

print(list(Test.scan(cond)))

answered Mar 16, 2018 at 23:45

pjz

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Comments

Betamoo · Accepted Answer · 2018-03-01 17:55:59Z

0

Have you tried:

query = (Test.a == 123) | (Test.b == 456)
list( Test.scan(query))

answered Mar 1, 2018 at 17:55

Betamoo

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