#34944: Missing or misinferred attributes in output fields of generated fields
-------------------------------------+-------------------------------------
Reporter: Paolo | Owner: nobody
Melchiorre |
Type: Bug | Status: new
Component: Database | Version: 5.0
layer (models, ORM) |
Severity: Release | Keywords: field, database,
blocker | generated, output_field
Triage Stage: | Has patch: 0
Unreviewed |
Needs documentation: 0 | Needs tests: 0
Patch needs improvement: 0 | Easy pickings: 0
UI/UX: 0 |
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Continuing with my experiments with the generated fields I found these two
error situations which I reported together because they are very connected
and could lead to a single solution.
I do not rule out that there are other examples of use of the generated
fields that could lead to the same errors and if they come to mind, please
add them in the comments.
== Misinferred attributes
In the common example of a field generated as a concatenation of two
CharField in a Model:
{{{
#!python
class Person(models.Model):
first_name = models.CharField(max_length=255)
last_name = models.CharField(max_length=255)
full_name = models.GeneratedField(
expression=Concat(
"first_name", models.Value(" "), "last_name"
),
db_persist=True,
)
}}}
The SQL code for the generated column has an automatically inferred max
length of only 255 characters (`varchar(255)`), while the field should be
able to contain strings of 511 characters (`varchar(511)`):
{{{
#!sql
CREATE TABLE "community_person" (
"id" integer NOT NULL PRIMARY KEY AUTOINCREMENT,
"first_name" varchar(255) NOT NULL,
"last_name" varchar(255) NOT NULL,
"full_name" varchar(255) GENERATED ALWAYS AS (
COALESCE("first_name", '') ||
COALESCE(COALESCE(' ', '') ||
COALESCE("last_name", ''), '')
) STORED
);
}}}
==== Proposal
To solve the problem you could alternatively:
a. make the maximum length extraction process smarter for the
automatically created output fields
b. mandatorily require specifying the output field when some of the fields
involved could lead to situations like the ones above
== Missing attributes
If in the previous example, I explicitly specify the output field without
attributes, the migration is generated without generating errors:
{{{
#!python
class Person(models.Model):
first_name = models.CharField(max_length=255)
last_name = models.CharField(max_length=255)
full_name = models.GeneratedField(
expression=Concat(
"first_name", models.Value(" "), "last_name"
),
db_persist=True,
output_field=models.CharField(),
)
}}}
The SQL code contains an incorrect `varchar(None)` type:
{{{
#!sql
CREATE TABLE "community_person" (
"id" integer NOT NULL PRIMARY KEY AUTOINCREMENT,
"first_name" varchar(255) NOT NULL,
"last_name" varchar(255) NOT NULL,
"full_name" varchar(None) GENERATED ALWAYS AS (
COALESCE("first_name", '') ||
COALESCE(COALESCE(' ', '') ||
COALESCE("last_name", ''), '')
) STORED
);
}}}
which will generate an error when trying to apply the migration to the
database:
{{{
#!console
sqlite3.OperationalError: near "None": syntax error
}}}
==== Proposal
To solve the problem you could alternatively:
a. make the SQL code generation process smarter so as to generate working
SQL code
b. request to specify mandatory attributes for the output fields raising
an error in the migration creation phase
== Cumulative proposal
To solve both problems shown above, given the little time remaining before
the release of the stable version of Django 5, the two most drastic
solutions from both of the above proposals could be adopted.
==== Required output field
Always require specifying the output field ''(except when you are sure
that the extracted type cannot generate error situations?)''
Example of error message:
{{{
#!console
django.core.exceptions.FieldError: Expression doesn't contain an explicit
type. You must set output_field.
}}}
==== Required attributes
Request to specify mandatory attributes for the output fields raising an
error in the migration creation phase.
{{{
#!console
$ python3 -m manage makemigrations
SystemCheckError: System check identified some issues:
ERRORS:
community.Person.full_name: (fields.E120) CharFields must define a
'max_length' attribute.
}}}
--
Ticket URL: <https://code.djangoproject.com/ticket/34944>
Django <https://code.djangoproject.com/>
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