django-forcedfields 1.0.1
Django model fields designed to more precisely and semantically define data types.
License: MIT
Keywords: char database django field model timestamp

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django-forcedfields
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*******
Summary
*******

A Python module that provides a set of custom, specialized Django model fields.

While I haved worked with Django's ORM for some time and have enjoyed many of its features for
simple use cases, I find myself increasingly impeded, annoyed, and dissatisfied by its limitations
in complex applications. One glaring problem in my eyes is the ORM's lack of semantic database field
data types and modifiers.

For example, an eight-character varchar field that can be null and that has a default value of
"elegy" will *not* result in the MySQL `DDL
<https://dev.mysql.com/doc/refman/en/glossary.html#glos_ddl>`_::

    VARCHAR(8) DEFAULT 'elegy' NULL

but simply as::

    VARCHAR(8) NULL

While this varchar example may not be the most egregious, it nonetheless illustrates the almost
complete reliance upon the application and its ORM for behavior that should be handled, and indeed
is best handled, by the database management system itself.

Databases should be as self-documenting and semantic as possible, independent of any application
code, ORM models, or documentation. I will not compromise this principle for the sake of an ORM's
conveniences. To this end, I have begun to create these custom Django model fields to force Django
to issue the most specific and complete DDL statements possible. It is my goal with these and future
fields to shift responsibility from the application ORM to the underlying database wherever possible
while maintaining a consistent and complete ORM interface and database backend abstraction.

************
Installation
************
::

    pip install [--user] django-forcedfields

*************
Example Usage
*************
::

    import django_forcedfields as forcedfields

or::

    from django_forcedfields import TimestampField

******
Fields
******

FixedCharField
==============

**class FixedCharField(max_length=None, **options)**

This field extends Django's `CharField
<https://docs.djangoproject.com/en/dev/ref/models/fields/#charfield>`_.

This field inherits all functionality and interfaces from Django's standard CharField but, rather
than producing a ``VARCHAR`` field in the database, the FixedCharField creates a ``CHAR`` field. The
parent CharField class' keyword argument ``max_length`` is retained and, when passed, specifies the
``CHAR`` field's max length just like it does for the ``VARCHAR`` implementation. The ``CHAR`` data
type is supported on all RDBMS in common use with Django.

In addition, if a FixedCharField on a model is not given an explicit value and no default field
value has been explicitly defined, a ``NULL`` value will be inserted on Model.save(). This is in
contrast to Django's standard CharField which incorrectly attempts to insert an empty string in such
a case. Ideally, with no explicit value and no default value, an integrity error would be raised by
the database but Django's ORM absolutely requires a value for all fields in ``INSERT`` operations.
It is impossible to simply omit a database column's value in an ``INSERT`` statement.

A note here on Django's `admonition on null values with text fields
<https://docs.djangoproject.com/en/dev/ref/models/fields/#null>`_: Django is wrong. ``NULL`` means
unknown data, an empty string means an empty string. Their meanings are *semantically different* by
definition. Set ``null=True`` on text fields when your use case warrants it. That is, when you may
have a complete absence of data as well as the need to record an empty string. Google this topic
for more analysis.

TimestampField
==============

**class TimestampField(auto_now=False, auto_now_add=False, auto_now_update=False, **options)**

This field extends Django's `DateTimeField
<https://docs.djangoproject.com/en/dev/ref/models/fields/#datetimefield>`_.

This field supports all `DateTimeField keyword arguments
<https://docs.djangoproject.com/en/dev/ref/models/fields/#datefield>`_ and adds a new
``auto_now_update`` argument.

**TimestampField.auto_now_update**
    ``auto_now_update`` is a boolean that, when True, sets a new timestamp field value on update
    operations *only*, not on insert.

    This option is mutually exclusive with ``auto_now``.

**Warning:** When using the MySQL backend, the database ``TIMESTAMP`` field will also be updated
when ``auto_now`` or ``auto_now_update`` is enabled and when calling QuerySet.update(). In
constrast, Django's DateField and DateTimeField only set current timestamp under ``auto_now`` `when
calling Model.save()
<https://docs.djangoproject.com/en/dev/ref/models/fields/#django.db.models.DateField.auto_now>`_.
This modified behavior is due to the declaration of ``ON UPDATE CURRENT_TIMESTAMP`` in the
TimestampField's column definition DDL.

Like its parent DateTimeField, the TimestampField's options ``auto_now``, ``auto_now_add``, and
``auto_now_update`` will forcibly overwrite any manually-set model field attribute values when
enabled and when their conditions are triggered. The value will be the Django ORM database function
`Now()
<https://docs.djangoproject.com/en/dev/ref/models/database-functions/#now>`_ rather than a datetime
instance since the value will have been generated by the database server and must therefore be
retrieved with a separate query.

Naturally, when designing a system field instead of a user data field, the need to offload
responsibility to the underlying database becomes greater. If the data is for system and metadata
purposes, then it increases consistency and data integrity to delegate field value management to the
system itself.

A timestamp is well-suited to record system and database record metadata such as record insert and
update times. Due to the database data type features, it is also ideal when storing a fixed point in
time, independent of time zone. Although the creation of the TimestampField was largely motivated by
the need for an ORM abstraction for metadata fields, it can also be used just like its parent
DateTimeField as long as one understands the data type's different advantages and limitations.

Instead of DateTimeField's reliance on ``DATETIME`` and similar data types, the TimestampField uses
``TIMESTAMP`` data type and other data types that do not store time zone information. The data type
changes can be seen in the following table:

========== ======================= ===========================
database   DateTimeField data type TimestampField data type
========== ======================= ===========================
MySQL      DATETIME                TIMESTAMP
PostgreSQL TIMESTAMP WITH TIMEZONE TIMESTAMP WITHOUT TIME ZONE
SQLite     DATETIME                DATETIME
========== ======================= ===========================

Also note that standard DDL modifiers such as ``DEFAULT CURRENT TIMESTAMP`` and non-standard ones
such as MySQL's ``ON UPDATE CURRENT_TIMESTAMP`` are used when the corresponding options on a
TimestampField instance are enabled.

******************************
Database Engine Considerations
******************************

When using TimestampField, one must be aware of certain database engine behavior defaults and
configurations. An ORM is usually designed to abstract, as much as is practical and prudent, the
differences between the underlying databases. In this case, however, the abstraction leaks. Consider
the following timestamp column DDL::

    TIMESTAMP NOT NULL

Note the lack of a ``DEFAULT`` clause. One would expect, upon attempting to insert a ``NULL`` value
or failing to provide a value for the column altogether, that some sort of constraint or integrity
exception would be raised. Indeed, this behavior adheres to the principle of least astonishment and
is the standard behavior of both SQLite and PostgreSQL. Both `SQLite
<https://www.sqlite.org/lang_createtable.html>`_ and `PostgreSQL
<https://www.postgresql.org/docs/current/static/ddl-default.html>`_ implicitly assign
``DEFAULT NULL`` to column definitions with no explicit ``DEFAULT`` clause.

MySQL
=====

MySQL requires a specific configuration to achieve the same standard behavior. The following
configuration options affect ``TIMESTAMP`` columns:

- `strict mode <https://dev.mysql.com/doc/refman/en/sql-mode.html#sql-mode-strict>`_
- `NO_ZERO_DATE <https://dev.mysql.com/doc/refman/en/sql-mode.html#sqlmode_no_zero_date>`_
- `NO_ZERO_IN_DATE <https://dev.mysql.com/doc/refman/en/sql-mode.html#sqlmode_no_zero_in_date>`_
- `explicit_defaults_for_timestamp <https://dev.mysql.com/doc/refman/en/server-system-variables.html#sysvar_explicit_defaults_for_timestamp>`_

At minimum, MySQL requires that both strict mode and ``explicit_defaults_for_timestamp`` are
enabled for ``TIMESTAMP`` behavior to conform to standards. If one attempts to omit a value for the
``TIMESTAMP NOT NULL`` column, a "ERROR 1364 (HY000): Field <field_name> doesn't have a default
value" is emitted and if one attempts to insert a ``NULL`` value, a "ERROR 1048 (23000): Column
<field_name> cannot be null" is emitted. As of version MySQL 5.7, strict mode is enabled by default
but ``explicit_defaults_for_timestamp`` is not.

MariaDB
=======

MariaDB, on the other hand, applies the same configuration parameters in a different way and its
logic as it relates to ``TIMESTAMP NOT NULL`` is less clear and, dare I say, erroneous. Assuming
identical configuration (strict mode and ``explicit_defaults_for_timestamp`` enabled), MariaDB
raises "ERROR 1364 (HY000): Field <field_name> doesn't have a default value" on insert value
omission but successfully accepts a ``NULL`` value with no error and stores the results of
``CURRENT_TIMESTAMP()`` in the field instead.

In an attempt to bring MariaDB in line with the standard, I also tested ``NO_ZERO_DATE`` and
``NO_ZERO_IN_DATE``. As long as both ``explicit_defaults_for_timestamp`` and ``NO_ZERO_DATE`` or
``NO_ZERO_IN_DATE`` are enabled, it is impossible to create a table containing the
``TIMESTAMP NOT NULL`` column as the ``CREATE TABLE`` statement fails with "ERROR 1067 (42000):
Invalid default value for <field_name>". This suggests that not only is the ``DEFAULT`` value
validated during DDL statements, but MariaDB is also attempting to implicitly define a zero value
``DEFAULT`` value on the ``TIMESTAMP`` field as the same error is raised when
``DEFAULT '0000-00-00 00:00:00'`` is explicitly defined. This is nonstandard, erroneous behavior and
conflicts with that of MySQL. From the `MySQL documentation
<https://dev.mysql.com/doc/refman/en/server-system-variables.html#sysvar_explicit_defaults_for_timestamp>`_:

    ``TIMESTAMP`` columns explicitly declared with the ``NOT NULL`` attribute and without an
    explicit ``DEFAULT`` attribute are treated as having no default value.

From the same documentation page, the following governs ``INSERT`` operations under these
conditions:

    For inserted rows that specify no explicit value for such a column, the result depends on the
    SQL mode. If strict SQL mode is enabled, an error occurs. If strict SQL mode is not enabled, the
    column is declared with the implicit default of '0000-00-00 00:00:00' and a warning occurs. This
    is similar to how MySQL treats other temporal types such as DATETIME.

The DDL validation failure may have something to do with these ``INSERT`` rules.

It is impossible for MariaDB's ``TIMESTAMP`` fields to behave in a standard way when dealing with
``TIMESTAMP NOT NULL`` columns. I found `this bug report
<https://jira.mariadb.org/browse/MDEV-10802>`_ for MariaDB but it appears that the work has ceased
and the fix has not been merged into the target release. All tests were performed on MariaDB 10.2
and 10.3.

Conclusion
==========

I now have a choice to make: do I cause TimestampField to step aside and let the user more directly
experience the effects of the underlying database engine's configuration or do I attempt to abstract
the behavior differences as much as possible? Given the spirit and goal of this library, I have
opted for less abstraction and have removed any additional, artificial normalization of database
engine behavior in these field classes. I am certainly open to discussion on this point so please
don't hesitate to open communication with me or point out any errors in my testing.

Given MariaDB's deviation from standards, this package's unit tests are performed using MySQL and
testing on MariaDB is disabled until further notice.

As an aside, please note that many inconsistent behaviors between database engines can be mitigated
or even eliminated by explicitly defining field keyword arguments such as ``default``, ``null``,
etc., causing more explicit DDL SQL to be generated by Django in the resulting migrations and SQL.

***********
Development
***********

To set up the development environment, a Vagrantfile is included. Install `Vagrant
<https://www.vagrantup.com/>`_ and::

    vagrant up

Once Vagrant has completed provisioning, ``vagrant ssh`` into the box and start the database servers
against which to run the test suite::

    docker-compose up -d

Finally, run the tests with::

    make tests

The Vagrant machine is provisioned to use the UTC time zone to facilitate tests. If you elect to run
tests outside of the Vagrant machine, be aware that certain tests assume identical time, date, and
time zone settings between all database engines. SQLite defaults to the host's localtime while the
Docker containers use the host's clock and default to the UTC time zone.

In this project, I use PEP8 and `Google's Python style guide
<https://google.github.io/styleguide/pyguide.html>`_. Pylint doesn't play nicely with some of the
styles. A few notes on pylint:

* bad-continuation

    * Ignore most of these. Google style guide allows for a 4-space hanging indent with nothing on
      first line.
    * Example: `indentation
      <https://google.github.io/styleguide/pyguide.html?showone=Indentation#Indentation>`_

**************
Oracle Support
**************

The FixedCharField should work on Oracle but the TimestampField will default to DateTimeField
database field data types when used with Oracle. I neither implemented functionality for nor tested
on Oracle for a few reasons:

#. It is too difficult to get an Oracle server instance against which to test. As one can see, I use
   lightweight Docker containerized services to run the test databases. To use Oracle, one needs to
   provide the Oracle installation binaries. To get the binaries, one needs to sign in to Oracle's
   web site for the privilege of downloading over 2.5 gigabytes. Too much unnecessary pain, not
   enough return. If you use Oracle products, I sympathize and may god have mercy on your soul.

    * https://github.com/oracle/docker-images/tree/master/OracleDatabase

#. Oracle seems to be `rarely used with Django <https://www.djangosites.org/stats/>`_.
#. I hate Oracle products and Oracle as an entity.

*********
Changelog
*********

v1.0
====

* Automatic values from ``auto_now``, ``auto_now_add``, and ``auto_now_update`` are no longer
  generated in the application using ``datetime.datetime.now()`` or ``django.utils.timezone.now()``.
  ``CURRENT_TIMESTAMP`` generation is now performed by the database using the Django database
  function `django.db.models.functions.Now
  <https://docs.djangoproject.com/en/dev/ref/models/database-functions/#now>`_.
* All fields now cause the ORM to issue explicit ``DEFAULT`` clauses in column DDL statements where
  previously the ORM always omitted ``DEFAULT`` clauses from column definitions. ``DEFAULT`` clauses
  will be defined in DDL if Field.has_default() returns True. This behavior naturally includes the
  generation of ``DEFAULT NULL`` in the column DDL if the field's ``default`` option is set
  to ``None``.
* If no kwargs (options) are passed to TimestampField, no ``DEFAULT`` clause is generated in the
  column DDL for MySQL. Previously, a ``DEFAULT NULL`` or ``DEFAULT 0`` clause was output in the DDL
  to disable MySQL's default ``TIMESTAMP`` behavior. Howver, default ``TIMESTAMP`` behavior varies
  according to certain server system variables and, depending upon configuration, it may be
  completely valid to omit a ``DEFAULT`` clause altogether.
* FixedCharField will now attempt to insert ``NULL`` if no value is defined on the model's field
  attribute and no explicit field default value has been defined. This behavior is in contrast to
  Django's standard CharField which always attempts to (incorrectly) store an empty string in such a
  case.