#31967: Resolving an output_field for mixed types crashes in database functions.
-------------------------------------+-------------------------------------
Reporter: Matt Hegarty | Owner: Hasan
| Ramezani
Type: Bug | Status: closed
Component: Documentation | Version: dev
Severity: Normal | Resolution: fixed
Keywords: | Triage Stage: Ready for
| checkin
Has patch: 1 | Needs documentation: 0
Needs tests: 0 | Patch needs improvement: 0
Easy pickings: 0 | UI/UX: 0
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Comment (by retpolanne):
Hello! I see that this is fixed, but I still don't understand how to
workaround this issue.
I'm working on updating a Django app using this library:
https://github.com/annemacedo-tw/django-spillway
I get this error when using it:
{{{
django.core.exceptions.FieldError: Expression contains mixed types:
MultiPolygonField, FloatField. You must set output_field.
}}}
However I don't get the same error on Django 3.1
As far as I debugged, this line here yields these two fields
https://github.com/annemacedo-tw/django-
spillway/blob/fa008ef0db5f3434c1ef438f66231f35a90b1e6d/spillway/query.py#L153
{{{
SimplifyPreserveTopology(F(geometry), Value(0.0439453125))
}}}
The first one being a MultiPolygonField and the other one being a
FloatField. I tried setting the output_field and recreating the
source_expressions but it didn't really work.
{{{
sql = SimplifyPreserveTopology(sql, tilew)
sql_field = sql.source_expressions[0]
sql_value = sql.source_expressions[1]
sql_value.output_field = models.MultiPolygonField()
sql.set_source_expressions([sql_field, sql_value])
}}}
(I understand that I'm setting an output field for MultiPolygonField here
instead of FloatField as should be expected, I set it up to avoid the
mixed types error)
{{{
File "/usr/local/lib/python3.9/site-
packages/django/contrib/gis/db/models/fields.py", line 194, in
get_prep_value
raise ValueError('Cannot use object with type %s for a spatial lookup
parameter.' % type(obj).__name__)
ValueError: Cannot use object with type float for a spatial lookup
parameter.
}}}
TL;DR how can I work around this issue when using GeoDjango?
Another thing I noticed is that ever since GeoManager was deprecated way
back in 1.10, django.contrib.gis.db.models.FloatField resolves to
django.db.models.fields.FloatField. I wonder if the gis FloatField would
play nicely with MultiPolygonField.
{{{
(Pdb) from django.contrib.gis.db import models
(Pdb) models.FloatField
<class 'django.db.models.fields.FloatField'>
}}}
I wish I could open a separate ticket for this, but I'm working on legacy
code that I don't understand so well, so I don't know how to do a minimal
version out of it.
--
Ticket URL: <https://code.djangoproject.com/ticket/31967#comment:13>
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