Sqlalchemy orm dynamic query filtering from given input

Question

I came across a need to create dynamic orm filtering on a given query.
I.e, given some input dictionary:
{"orm_entitiy":"City", attribute":"id","op":">","value":"1"}
and an existing query object query

Will need to evaluate to: new_filtered_query = query.filter(City.id > 1)

My questions are:
1. Are you familiar with some mature library which solves this need?
2. I saw a solution in SO and a blog, but I have a difficulty to understand the section:

 attr = list(filter(
                        lambda e: hasattr(column, e % op),
                        ['%s', '%s_', '__%s__']
                    ))[0] % op

Could someone please describe the logic behind this in details?

leovp · Accepted Answer · 2017-12-02 15:26:31Z

2

Let's uncomplicate this expression by making it more verbose.

existing_attrs = list(filter(<filter_func>, potential_attributes))
attr = existing_attrs[0] % op

The first line produces a list of objects from potential_attributes passing the check of <filter_func>.
The second line is simpler: we take the first existing attribute and apply string formatting to it.

<filter_func> is lambda e: hasattr(column, e % op): it returns True if column has an attribute named e % op, which is a current potential_attribute with string formatting applied.

For example, if potential_attributes is ['%s', '%s_', '__%s__'] and op is gt, the following attributes will be checked: column.gt, column.gt_ and column.__gt__.

Let's say we have a column.gt_ attribute. The value that passes the check is therefore %s_ and will be put in a list of existing_attrs. Then, the second line (attr = '%s_' % 'gt') will produce a string 'gt_'.

answered Dec 2, 2017 at 15:26

leovp

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2 Comments

JavaSa Over a year ago

What is the meaning of e % op?

leovp Over a year ago

It's a string formatting operator. docs.python.org/2/library/stdtypes.html#string-formatting Example: "%02d" % 9 is "09"

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