#21273: Add read only support for Oracle XE to django.contrib.gis
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Reporter: vinhussey | Owner: nobody
Type: Cleanup/optimization | Status: new
Component: GIS | Version: master
Severity: Normal | Resolution:
Keywords: | Triage Stage: Accepted
Has patch: 0 | Needs documentation: 1
Needs tests: 1 | Patch needs improvement: 0
Easy pickings: 0 | UI/UX: 0
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Comment (by vinhussey):
Hi Shai,
Thanks for the constructive comments. I have changed from the explicit
use of `__getattribute__` as suggested.
The reason for taking this approach is that Oracle XE does not provide a
WKT function. See these comments in
https://github.com/django/django/blob/master/django/contrib/gis/db/backends/oracle/operations.py
{{{
"""
This module contains the spatial lookup types, and the
`get_geo_where_clause`
routine for Oracle Spatial.
Please note that WKT support is broken on the XE version, and thus
this backend will not work on such platforms. Specifically, XE lacks
support for an internal JVM, and Java libraries are required to use
the WKT constructors.
"""
...
class OracleOperations(DatabaseOperations, BaseSpatialOperations):
compiler_module = "django.contrib.gis.db.backends.oracle.compiler"
name = 'oracle'
oracle = True
valid_aggregates = {'Union', 'Extent'}
Adapter = OracleSpatialAdapter
Adaptor = Adapter # Backwards-compatibility alias.
area = 'SDO_GEOM.SDO_AREA'
gml = 'SDO_UTIL.TO_GMLGEOMETRY'
centroid = 'SDO_GEOM.SDO_CENTROID'
difference = 'SDO_GEOM.SDO_DIFFERENCE'
distance = 'SDO_GEOM.SDO_DISTANCE'
extent = 'SDO_AGGR_MBR'
intersection = 'SDO_GEOM.SDO_INTERSECTION'
length = 'SDO_GEOM.SDO_LENGTH'
num_geom = 'SDO_UTIL.GETNUMELEM'
num_points = 'SDO_UTIL.GETNUMVERTICES'
perimeter = length
point_on_surface = 'SDO_GEOM.SDO_POINTONSURFACE'
reverse = 'SDO_UTIL.REVERSE_LINESTRING'
sym_difference = 'SDO_GEOM.SDO_XOR'
transform = 'SDO_CS.TRANSFORM'
union = 'SDO_GEOM.SDO_UNION'
unionagg = 'SDO_AGGR_UNION'
# We want to get SDO Geometries as WKT because it is much easier to
# instantiate GEOS proxies from WKT than SDO_GEOMETRY(...) strings.
# However, this adversely affects performance (i.e., Java is called
# to convert to WKT on every query). If someone wishes to write a
# SDO_GEOMETRY(...) parser in Python, let me know =)
}}}
Once we have WKT, we can use geos, so thanks for that tip.
Vincent
Replying to [comment:8 shai]:
> Hi Vincent,
>
> I am far from being a !GeoDjango expert; in fact, I only started looking
into it recently. I usually handle Oracle issues, but I don't use Oracle
XE currently, and am not sure exactly how much spatial functionality is
available on XE. I took a look at your latest code, and I have some
comments.
>
> First of all, the general scope of the work: As far as I could see, what
you have added is a class for interpreting the SDO_GEOMETRY type; do you
intend to also support other operations, such as geometric queries? Are
they supported on XE? If not, the best integration into Django seems to be
a custom field type.
>
> Second, technical details:
>
> The most glaring point in your code is the explicit use of
`__getattribute__`: This is a "magic method" in Python, it implements
attribute access. You should be able to replace every occurrence of
`self.geometry.__getattribute__('NAME')` with `self.geometry.NAME`, for
every `NAME`.
>
> The second is -- instead of trying to generate the WKT yourself, it
seems better to use GEOS objects -- WKT generation is already implemented
there, as long as a ton of other functionality. The GEOS objects are
explained [https://docs.djangoproject.com/en/1.6/ref/contrib/gis/geos/
here].
>
> So -- the right thing to achieve, apparently, is a custom field, which
inherits `TextField` and interprets the object it gets as a GEOS object. I
haven't looked into the !GeoDjango (Oracle backend) code, but perhaps you
can extract some code from there which will help you; I'd be a little
surprised if SDO_GEOMETRY objects themselves are much different between
Oracle XE and Standard.
>
> Hope this helps,
> Shai.
--
Ticket URL: <https://code.djangoproject.com/ticket/21273#comment:9>
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