Opened 4 weeks ago

Last modified 4 weeks ago

#36612 closed Bug

KeyTextTransform uses ->> on CASTed TextField to JSONField in MySQL, causing a syntax error — at Initial Version

Reported by: Jacob Tavener Owned by:
Component: Database layer (models, ORM) Version: 4.2
Severity: Normal Keywords:
Cc: Jacob Tavener Triage Stage: Ready for checkin
Has patch: yes Needs documentation: no
Needs tests: no Patch needs improvement: no
Easy pickings: no UI/UX: no

Description

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:

from django.db import models

class ExampleModel(models.Model):
json_text = models.TextField()

Store a sample JSON string in the TextField:

ExampleModel.objects.create(json_text='{"key": "value"}')

Perform a JSON key lookup using a cast and string lookup:

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:

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:

json_text->>'$.key' LIKE '%val%'

This is invalid in MySQL when applied to a CASTed expression.

Results in a syntax error at runtime:

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:

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.

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