pycql

A pure python CQL parser.

Installation

pip install pycql

Usage

The basic functionality parses the input string to an abstract syntax tree (AST) representation. This AST can then be used to build database filters or similar functionality.

import pycql

ast = pycql.parse(filter_expression)

Testing

The basic functionality can be tested using pytest.

python -m pytest

There is a test project/app to test the Django integration. This is tested using the following command:

python manage.py test testapp

Django integration

For Django there is a default bridging implementation, where all the filters are translated to the Django ORM. In order to use this integration, we need two dictionaries, one mapping the available fields to the Django model fields, and one to map the fields that use choices. Consider the following example models:

from django.contrib.gis.db import models


optional = dict(null=True, blank=True)

class Record(models.Model):
    identifier = models.CharField(max_length=256, unique=True, null=False)
    geometry = models.GeometryField()

    float_attribute = models.FloatField(**optional)
    int_attribute = models.IntegerField(**optional)
    str_attribute = models.CharField(max_length=256, **optional)
    datetime_attribute = models.DateTimeField(**optional)
    choice_attribute = models.PositiveSmallIntegerField(choices=[
                                                                 (1, 'ASCENDING'),
                                                                 (2, 'DESCENDING'),],
                                                        **optional)


class RecordMeta(models.Model):
    record = models.ForeignKey(Record, on_delete=models.CASCADE, related_name='record_metas')

    float_meta_attribute = models.FloatField(**optional)
    int_meta_attribute = models.IntegerField(**optional)
    str_meta_attribute = models.CharField(max_length=256, **optional)
    datetime_meta_attribute = models.DateTimeField(**optional)
    choice_meta_attribute = models.PositiveSmallIntegerField(choices=[
                                                                      (1, 'X'),
                                                                      (2, 'Y'),
                                                                      (3, 'Z')],
                                                             **optional)

Now we can specify the field mappings and mapping choices to be used when applying the filters:

FIELD_MAPPING = {
    'identifier': 'identifier',
    'geometry': 'geometry',
    'floatAttribute': 'float_attribute',
    'intAttribute': 'int_attribute',
    'strAttribute': 'str_attribute',
    'datetimeAttribute': 'datetime_attribute',
    'choiceAttribute': 'choice_attribute',

    # meta fields
    'floatMetaAttribute': 'record_metas__float_meta_attribute',
    'intMetaAttribute': 'record_metas__int_meta_attribute',
    'strMetaAttribute': 'record_metas__str_meta_attribute',
    'datetimeMetaAttribute': 'record_metas__datetime_meta_attribute',
    'choiceMetaAttribute': 'record_metas__choice_meta_attribute',
}

MAPPING_CHOICES = {
    'choiceAttribute': dict(Record._meta.get_field('choice_attribute').choices),
    'choiceMetaAttribute': dict(RecordMeta._meta.get_field('choice_meta_attribute').choices),
}

Finally we are able to connect the CQL AST to the Django database models. We also provide factory functions to parse the timestamps, durations, geometries and envelopes, so that they can be used with the ORM layer:

from pycql.integrations.django import to_filter

cql_expr = 'strMetaAttribute LIKE "%parent%" AND datetimeAttribute BEFORE 2000-01-01T00:00:01Z'

ast = pycql.parse(
    cql_expr, GEOSGeometry, Polygon.from_bbox, parse_datetime,
    parse_duration
)
filters = to_filter(ast, mapping, mapping_choices)

qs = Record.objects.filter(**filters)