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Helpful functions and classes for your django app's models

Project Description

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 cache function’s result
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.

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

Choices class (inspired by Django Choices. )

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

YEAR_IN_SCHOOL_CHOICES = Choices({
    "freshman": "FR",
    "sophomore": "SO",
    "junior": "JR",
    "Senior": "SR"
})


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

YEAR_IN_SCHOOL_CHOICES is a readonly OrderedDict and you can treat it as such. for example: YEAR_IN_SCHOOL_CHOICES.keys() or YEAR_IN_SCHOOL_CHOICES.iteritems()

Choices class is more flexible because it allow you to specify 3 values. choice name, choice db value, choice display name. The example above can be better written like that

 YEAR_IN_SCHOOL_CHOICES = Choices({
     "freshman": {"id": 0, "display": "New comer"},
     "sophomore": 1,
     "junior": 2,
     "Senior": 3
  }, order_by="id")


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 key/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 can be fully defined as key/dict "freshman": {"id": 0, "display": "New comer"} in which case display name will be "New comer" and id will be 0

Defining extra keys to use in your code.

As mentioned before Choices can be treated as an OrderedDictionary and so you should feel free to use the free functionality, for example adding extra keys

AVAILABLE_SETTINGS = Choices({
    "max_page_width": {"id": 0, "display": "Maximum page width in pixels", "default": 100})

then in your code you can do

settings = Settings.objects.filter(name=AVAILABLE_SETTINGS.max_page_width).first()
if settings:
    return settings.value
return AVAILABLE_SETTINGS["max_page_width"]["default"]

Ordering your Choices

Assuming you have a big list of choices you might prefer to ask Choices class to order them for you.

Example:

Choices({
     "usa": {"display": "United States", "id": 0},
     "egypt": 1,
     "uk": {"display": "United Kingdom", "id": 2},
     "ua": {"display": "Ukraine", "id": 3}
    }, order_by="display")

The fields will be in the order “Egypt”, “Ukraine”, “United Kingdom”, “United States”.

order_by="id" will order the list by id

If you don’t want any sort of ordering then set order_by=None and in this case its better that you pass your choices as tuple of dictionaries to maintain order

Choices((
     ("uk", {"display": "United Kingdom", "id": 2),
     ("usa", {"display": "United States", "id": 0),
     ("egypt", 1),
     ("ua": {"display": "Ukraine", "id": 3})
    ), order_by=None)

Note: By default choices are ordered by display name

Useful functions of Choices class

  • get_display_name: given choice id, return the display name of that id. same as model’s get_<field_name>_display()
  • get_code_name: Given choice id same as get_display_name but return code name
  • get_value: Given choice id, return value of any key defined inside choice entry

Example:

CHOICES_EXAMPLE = Choices({"my_key": {"id": 0, "display": "Display Of My Key", "additional_key": 1234})
>>> CHOICES_EXAMPLE.get_display_name(0)
"Display Of My Key"
>>> CHOICES_EXAMPLE.get_code_name(0)
"my_key"
>>> CHOICES_EXAMPLE.get_value(0, "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
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