#36612: KeyTextTransform uses ->> on CASTed TextField to JSONField in MySQL,
causing a syntax error
-------------------------------------+-------------------------------------
Reporter: jacobtavenerhulla | Type: Bug
Status: new | Component: Database
| layer (models, ORM)
Version: 4.2 | Severity: Normal
Keywords: | Triage Stage:
| Unreviewed
Has patch: 0 | Needs documentation: 0
Needs tests: 0 | Patch needs improvement: 0
Easy pickings: 0 | UI/UX: 0
-------------------------------------+-------------------------------------
== Background / Context ==
In Django 4.2, '''KeyTextTransform''' is used for JSON key lookups that
return text, such as when performing __icontains on a JSON key.
On MySQL, Django generates SQL using the ->> operator. While this works
for '''native JSONField columns''', it '''fails when the JSON expression
comes from a CAST of a TextField to a JSONField''', because MySQL does not
allow ->> on expressions like CAST(text_column AS JSON). The ->> operator
is only valid on actual JSON columns. Attempting it on a CASTed expression
produces a '''syntax error'''.
== Steps to Reproduce ==
Create a model with a TextField storing JSON:
{{{#!div style="font-size: 80%"
{{{#!python
from django.db import models
class ExampleModel(models.Model):
json_text = models.TextField()
}}}
}}}
Store a sample JSON string in the TextField:
{{{#!div style="font-size: 80%"
{{{#!python
ExampleModel.objects.create(json_text='{"key": "value"}')
}}}
}}}
Perform a JSON key lookup using a cast and string lookup:
{{{#!div style="font-size: 80%"
{{{#!python
from django.db.models import F, JSONField
from django.db.models.functions import Cast
ExampleModel.objects.annotate(
json_data=Cast('json_text', JSONField())
).filter(json_data__key__icontains='val')
}}}
}}}
== Expected Behavior ==
Django should generate valid MySQL SQL that extracts the JSON key as text,
compatible with CASTed TextField columns. Example:
{{{#!div style="font-size: 80%"
{{{#!sql
JSON_UNQUOTE(JSON_EXTRACT(json_text, '$.key')) LIKE '%val%'
}}}
}}}
This SQL should execute without errors.
== Actual Behavior ==
Django generates SQL using the ->> operator on the CASTed expression:
{{{#!div style="font-size: 80%"
{{{#!sql
json_text->>'$.key' LIKE '%val%'
}}}
}}}
This is invalid in MySQL when applied to a CASTed expression.
Results in a syntax error at runtime:
{{{#!div style="font-size: 80%"
{{{#!text
ProgrammingError: (1064, 'You have an error in your SQL syntax; check the
manual that corresponds to your MySQL server version for the right syntax
to use near '->> '$.\"key\"')) LIKE LOWER('%val%') LIMIT 21' at line 1')
}}}
}}}
== Current Workaround ==
A temporary workaround in projects using MySQL is to monkey patch
KeyTextTransform.as_mysql to use JSON_UNQUOTE(JSON_EXTRACT(...)) instead
of the ->> operator. This allows JSON key lookups on CASTed TextField →
JSONField expressions to work correctly.
Example monkey patch:
{{{#!div style="font-size: 80%"
{{{#!python
from django.db.models.fields.json import KeyTextTransform,
compile_json_path
from django.db.models import JSONField
from django.db.models.functions import Cast
original_as_mysql = KeyTextTransform.as_mysql
def safe_as_mysql(self, compiler, connection):
lhs, params, key_transforms = self.preprocess_lhs(compiler, connection)
json_path = compile_json_path(key_transforms)
sql, params = "JSON_EXTRACT(%s, %%s)" % lhs, tuple(params) + (json_path,)
return "JSON_UNQUOTE(%s)" % sql, params
KeyTextTransform.as_mysql = safe_as_mysql
}}}
}}}
Notes:
This patch only affects MySQL connections.
It applies globally and must be applied once at project startup (e.g., in
an AppConfig.ready() method).
This is a temporary workaround; a proper fix in Django core is preferred.
== Notes / Additional Context ==
This issue occurs only on MySQL.
Native JSONField columns are not affected; ->> works fine on them.
The problem arises specifically when a TextField containing JSON is cast
to JSONField for ORM JSON lookups.
--
Ticket URL: <https://code.djangoproject.com/ticket/36612>
Django <https://code.djangoproject.com/>
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