django-model-helpers 3.2

Helpful functions and classes for your django app's models

Model helpers are small collection of django functions and classes that make working with models easier. All functions here are compliant with pylint and has test cases with over 95% code coverage. This doc describe each of these helpers.

upload_to

Pass this function to your FileField as upload_to argument

cached_model_property

Decorate a model function with that decorator to turn it into a property that caches the result.

cached_function

Decorate any function or class method with that decorator to enable out of the box caching.

Choices

A feature rich solution for implementing choice field

KeyValueField

A field that can store multiple key/value entries in a human readable form

model_helpers.upload_to

Pass model_helpers.upload_to as upload_to parameter for any FileField or ImageField. This will - by default - generate slugified version of the file name. By default each model get its own storage folder named after model’s name.

upload_to function also block files with certain harmful extensions like “php” or “py” from being uploaded.

Sample usage:

import model_helpers

class Profile(models.model):
    name = CharField(max_length=100)
    picture = ImageField(upload_to=model_helpers.upload_to)

uploaded images for this model will be stored in: media/Profile/<current_year>/<slugified_original_filename>.

settings

settings for upload_to function should be placed in UPLOAD_TO_OPTIONS inside your settings.py file These are the default settings

settings.UPLOAD_TO_OPTIONS = {
    "black_listed_extensions": ["php", "html", "htm", "js", "vbs", "py", "pyc", "asp", "aspx", "pl"],
    "max_filename_length": 40,
    "file_name_template": "{model_name}/%Y/{filename}.{extension}"
}

• black_listed_extensions prevent any file with any of these extensions from being saved.

• max_filename_length trim filename if it exceeds certain length to mitigate DB errors when user upload long filename

• file_name_template controls where the file should be saved.

specifying ``file_name_template``

file_name_template pass your string to strftime() function; '%Y' in the example above is the four-digit year. other accepted variables are:

• model_name: name of the model which the file is being uploaded for.

• filename: name of the file - without extension - after it has been processed by upload_to (trimmed and slugified)

• extension: file’s extension

• instance: the model instance passed to upload_to function

For example to save uploaded files to a directory like this

model name/current year/current month/instance's name(dot)file's extension

you do

UPLOAD_TO_OPTIONS = {"file_name_template": "{model_name}/%Y/%m/{instance.name}.{extension}" }

customizing ``upload_to`` per model

If you want to have different upload_to options for different models, use UploadTo class instead. For example to have ImageField that allow all file extensions, You can do this:

my_image = models.ImageField(upload_to=models_helper.UploadTo(black_listed_extensions=[])

UploadTo class accepts all upload_to settings documented above. You can also inherit from this class if you want to have very custom file naming schema (like if you want file name be based on its md5sum)

cached_model_property decorator

cached_model_property is a decorator for model functions that takes no arguments. The decorator convert the function into a property that support caching out of the box

Note: cached_model_property is totally different from django’s cached_property the later is not true caching but rather memorizing function’s return value. cache_timeout: number of seconds before cache expires.

Sample usage:

class Team(models.Model):
    @cached_model_property
    def points(self):
        # Do complex DB queries
        return result

    @cached_model_property(readonly=False)
    def editable_points(self):
        # get result
        return result

    @cached_model_property(cache_timeout=1)
    def one_second_cache(self):
        # get result
        return result

Now try

team = Team.objects.first()

• team.points <– complex DB queries will happen, result will be returned

• team.points <– this time result is returned from cache (points function is not called

• del team.points <– points value has been removed from cache

• team.points <– complex DB queries will happen, result will be returned

How does it work?: first time the decorator store the function output in the cache with key = "<model_class>_<instance.pk>_<function_name>" so if you have two models with same name, or have model that provide no primary key you can’t use this decorator.

set readonly parameter to False to make the property writeable

team.editable_points = 88

In this case the assigned value will replace the value stored in the cache

team.editable_points returns 88

I personally don’t use the writable cached property option but might be useful to someone else

cached_function decorator

cached_function is a decorator for any functions. The decorator automatically cache function’s result for a defined amount of time. The caching takes into account function arguments and - for class methods - class properties.

cache_timeout: number of seconds before cache expires. key_parameters: Function parameters which the cached value depends on key_class_attrs: Class attributes which the cached value depends on

Sample Usage:

class ExampleClass:

    example_field = 0

    @cached_function(cache_timeout=1, key_parameters=["arg_a", "arg_b"], key_class_attrs=["example_field"])
    def example_function(self, arg_a, arg_b, print_result=False):
        if print_result:
            print("Result is ", arg_a + arg_b)
        return arg_a + arg_b

This output from example_function will be cached for exactly 1 second. The cache would depend on value of function’s arg_a and arg_b parameters and class’s example_field value.

Choices class (version 2.0)

Dealing with Django’s choices attribute is a pain. Here is a proper way of implementing choice field in Django

class Student(models.Model):
    FRESHMAN = 'FR'
    SOPHOMORE = 'SO'
    JUNIOR = 'JR'
    SENIOR = 'SR'
    YEAR_IN_SCHOOL_CHOICES = (
        (FRESHMAN, 'Freshman'),
        (SOPHOMORE, 'Sophomore'),
        (JUNIOR, 'Junior'),
        (SENIOR, 'Senior'),
    )
    year_in_school = models.CharField(
                        max_length=2,
                        choices=YEAR_IN_SCHOOL_CHOICES,
                        default=FRESHMAN)

Then you can do

student = Student.objects.first()
if student.year_in_school == Student.SENIOR:
      # do some senior stuff

With Choices class this becomes

class SchoolYearChoices(Choices):
    # attributes must be uppercase or it will be ignoredS
    FRESHMAN = 'FR'
    SOPHOMORE = 'SO'
    JUNIOR = 'JR'
    SENIOR = 'SR'

YEAR_IN_SCHOOL_CHOICES = SchoolYearChoices()


class Student(models.Model):
    year_in_school = models.CharField(
                        max_length=2,
                        choices=YEAR_IN_SCHOOL_CHOICES(),
                        default=YEAR_IN_SCHOOL_CHOICES.FRESHMAN)

Then you can do

student = Student.objects.first()
if student.year_in_school == YEAR_IN_SCHOOL_CHOICES.SENIOR:
      # do some senior stuff

Choices class is more flexible because it allow you to specify 3 values (or more!). The standard ones are:

• choice attribute name (must be uppercase)

• choice db value (aka choice_id)

• choice display name.

The example above can be better written like that

class SchoolYearChoices(Choices):
     FRESHMAN: {"id": 0, "display": "New comer"},
     SOPHOMORE: 1,
     JUNIOR: 2,
     SENIOR: 3

 YEAR_IN_SCHOOL_CHOICES = SchoolYearChoices()


class Student(models.Model):
    year_in_school = models.SmalllIntegerField(
        choices=YEAR_IN_SCHOOL_CHOICES(),
        default=YEAR_IN_SCHOOL_CHOICES.FRESHMAN
    )

Then you can do something like this

Student.objects.filter(
    year_in_school__gt=YEAR_IN_SCHOOL_CHOICES.SOPHOMORE)

To return all students in grades higher than Sophomore

• A choice can be defined as attribute/value SOPHOMORE = 1 in which case display name will be code name capitalized "Sophomore" and will be saved in DB as number 1

• A choice value can be fully defined as a dict FRESHMAN = {"id": 0, "display": "New comer"} in which case display name will be "New comer" and id will be 0

• NOTE: attribute name must be in uppercase or it will be ignored (i.e FRESHMEN not freshman)

Defining extra keys to use in your code.

Choices attributes are dictionaries so they can hold additional keys. Those keys can be used in your code to define some settings or constants associated with that choice. For example:

class SettingsChoices(Choices):
    MAX_PAGE_WIDTH = {"id": 0, "display": "Maximum page width in pixels", "default": 100}

    def get_default_value(self, choice_id):
        try:
           return self.get_choice(choice_id)["default"]
        except KeyError:
           return None

AVAILABLE_SETTINGS = SettingsChoices()

then in your code you can do

try:
    return Settings.objects.get(name=AVAILABLE_SETTINGS.MAX_PAGE_WIDTH).value
except Settings.DoesNotExist:
    return AVAILABLE_SETTINGS.get_default_value(AVAILABLE_SETTINGS.MAX_PAGE_WIDTH)

Inheriting Choices

One choice class can inherit from another choice class. Example:

class SchoolYearChoices(Choices):
    FRESHMAN = {"id": 0, "display": "New comer"},
    SOPHOMORE = 1,
    JUNIOR = 2,
    SENIOR = 3

class SchoolYearChoicesWithGraduate(SchoolYearChoices):
    GRADUATE = 4

YEAR_IN_SCHOOL = SchoolYearChoicesWithGraduate()
YEAR_IN_SCHOOL() == [
    {"id": 0, "display": "New comer"},
    {"id": 1, "display": "Sophomore"},
    {"id": 2, "display": "Junior"},
    {"id": 3, "display": "Senior"},
    {"id": 4, "display": "Graduate"},
]

Useful functions of Choices class

• get_display_name: given choice id (value), return the display name of that id. same as model’s get_<field_name>_display()

• get_choice_name: Given choice id same as get_display_name but return code name

• get_choice: Given choice id, return a dictionary of all choice attributes

Example:

class ExampleChoice(Choices):
    MY_KEY = {"id": 0, "display": "Display Of My Key", "additional_key": 1234}

CHOICES_EXAMPLE = ExampleChoice()


>>> CHOICES_EXAMPLE.get_display_name(CHOICES_EXAMPLE.MY_KEY)
"Display Of My Key"
>>> CHOICES_EXAMPLE.get_choice_name(CHOICES_EXAMPLE.MY_KEY)
"my_key"
>>> CHOICES_EXAMPLE.get_choice(CHOICES_EXAMPLE.MY_KEY)
{"id": 0, "display": "Display Of My Key", "additional_key": 1234}

model_helpers.KeyValueField

Sometimes you need to have a simple key/value field. most developers would rely on JsonField which is good for some use cases but people using django admin may not like to modify json object that look like this

{"key1": "value of some sort", "key2": "value containing \" character"}

KeyValueField serialize objects in a more readable way. the dictionary above would be stored and displayed like this.

key1 = value of some sort
key2 = value containing " character

That’s it. For you as a developer you will access your KeyValueField as a dictionary.

Example:

class MyModel(models.Model):
     options = KeyValueField(separator=":")

>> my_model.options = "key1 : val1 \n key2 : val2"
>> my_model.options
{"key1": "val1", "key2": "val2"}
>>> str(my_model.options)
"key1 : val1 \n key2 : val2"

You can find more examples in the test file tests/test_key_value_field.py

``KeyValueField`` is NOT good for:

• Maintain original value’s datatype. all values are converted to unicode strings

• Store a multiline value