# Dokumentasi Django 4.2.9.dev20231204082947

# Acuan contoh model

This document describes the details of the `Model` API. It builds on the material presented in the model and database query guides, so you'll probably want to read and understand those documents before reading this one.

Throughout this reference we'll use the example blog models presented in the database query guide.

## membuat obyek

To create a new instance of a model, instantiate it like any other Python class:

class `Model`(**kwargs)

The keyword arguments are the names of the fields you've defined on your model. Note that instantiating a model in no way touches your database; for that, you need to `save()`.

Catatan

You may be tempted to customize the model by overriding the `__init__` method. If you do so, however, take care not to change the calling signature as any change may prevent the model instance from being saved. Additionally, referring to model fields within `__init__` may potentially result in infinite recursion errors in some circumstances. Rather than overriding `__init__`, try using one of these approaches:

1. Add a classmethod on the model class:

       from django.db import models

       class Book(models.Model):
           title = models.CharField(max_length=100)

           @classmethod
           def create(cls, title):
               book = cls(title=title)
               # do something with the book
               return book

       book = Book.create("Pride and Prejudice")

2. Tambah sebuat metode pada pengelola penyesuaian (biasanya disukai):

       class BookManager(models.Manager):
           def create_book(self, title):
               book = self.create(title=title)
               # do something with the book
               return book

       class Book(models.Model):
           title = models.CharField(max_length=100)

           objects = BookManager()

       book = Book.objects.create_book("Pride and Prejudice")

### Menyesuaikan memuat model

classmethod `Model.``from_db`(db, field_names, values)

The `from_db()` method can be used to customize model instance creation when loading from the database.

The `db` argument contains the database alias for the database the model is loaded from, `field_names` contains the names of all loaded fields, and `values` contains the loaded values for each field in `field_names`. The `field_names` are in the same order as the `values`. If all of the model's fields are present, then `values` are guaranteed to be in the order `__init__()` expects them. That is, the instance can be created by `cls(*values)`. If any fields are deferred, they won't appear in `field_names`. In that case, assign a value of `django.db.models.DEFERRED` to each of the missing fields.

In addition to creating the new model, the `from_db()` method must set the `adding` and `db` flags in the new instance's `_state` attribute.

Below is an example showing how to record the initial values of fields that are loaded from the database:

    from django.db.models import DEFERRED

    @classmethod
    def from_db(cls, db, field_names, values):
        # Default implementation of from_db() (subject to change and could
        # be replaced with super()).
        if len(values) != len(cls._meta.concrete_fields):
            values = list(values)
            values.reverse()
            values = [
                values.pop() if f.attname in field_names else DEFERRED
                for f in cls._meta.concrete_fields
            ]
        instance = cls(*values)
        instance._state.adding = False
        instance._state.db = db
        # customization to store the original field values on the instance
        instance._loaded_values = dict(
            zip(field_names, (value for value in values if value is not DEFERRED))
        )
        return instance

    def save(self, *args, **kwargs):
        # Check how the current values differ from ._loaded_values. For example,
        # prevent changing the creator_id of the model. (This example doesn't
        # support cases where 'creator_id' is deferred).
        if not self._state.adding and (
            self.creator_id != self._loaded_values["creator_id"]
        ):
            raise ValueError("Updating the value of creator isn't allowed")
        super().save(*args, **kwargs)

The example above shows a full `from_db()` implementation to clarify how that is done. In this case it would be possible to use a `super()` call in the `from_db()` method.

## Menyegarkan kembali obyek dari basisdata

If you delete a field from a model instance, accessing it again reloads the value from the database:

    >>> obj = MyModel.objects.first()
    >>> del obj.field
    >>> obj.field  # Loads the field from the database

`Model.``refresh_from_db`(using=None, fields=None)

`Model.``arefresh_from_db`(using=None, fields=None)

Asynchronous version: `arefresh_from_db()`

If you need to reload a model's values from the database, you can use the `refresh_from_db()` method. When this method is called without arguments the following is done:

1. All non-deferred fields of the model are updated to the values currently present in the database.
2. Any cached relations are cleared from the reloaded instance.

Hanya bidang-bidang dari model dimuat kembali dari basisdata. Nilai-nilai bergantung-basisdata lain seperti keterangan tidak dimuat kembali. Atribut `@cached_property` apapun tidak dibersihkan juga.

The reloading happens from the database the instance was loaded from, or from the default database if the instance wasn't loaded from the database. The `using` argument can be used to force the database used for reloading.

Itu memungkinkan untuk memaksa kumpulan dari bidang-bidang dimuat menggunakan argumen `field`.

Sebagai contoh, untuk mencoba bahwa sebuah panggilan `update()` dihasilkan dalam pembaharuan diharapkan, anda dapat menulis sebuah percobaan miriip seperti ini:

    def test_update_result(self):
        obj = MyModel.objects.create(val=1)
        MyModel.objects.filter(pk=obj.pk).update(val=F("val") + 1)
        # At this point obj.val is still 1, but the value in the database
        # was updated to 2. The object's updated value needs to be reloaded
        # from the database.
        obj.refresh_from_db()
        self.assertEqual(obj.val, 2)

Catat bahwa ketika bidang ditangguhkan diakses, memuat nilai bidang tertangguh terjadi melalui metode ini. dengan demikian itu adalah mungkin untuk menyesuaian cara penangguhan memuat terjadi. Contoh dibawah menunjukkan bagaimana satu dapat dimuat kembali dari bidang-bidang instance ketika bidang tertangguhkan dimuat:

    class ExampleModel(models.Model):
        def refresh_from_db(self, using=None, fields=None, **kwargs):
            # fields contains the name of the deferred field to be
            # loaded.
            if fields is not None:
                fields = set(fields)
                deferred_fields = self.get_deferred_fields()
                # If any deferred field is going to be loaded
                if fields.intersection(deferred_fields):
                    # then load all of them
                    fields = fields.union(deferred_fields)
            super().refresh_from_db(using, fields, **kwargs)

`Model.``get_deferred_fields`()

A helper method that returns a set containing the attribute names of all those fields that are currently deferred for this model.

Changed in Django 4.2:

`arefresh_from_db()` method was added.

## Mensahkan obyek-obyek

There are four steps involved in validating a model:

1. Mensahkan bidang-bidang model - `Model.clean_fields()`
2. Mensahkan model sebagai sebuah keseluruhan - `Model.clean_fields()`
3. Mensahkan keunikan bidang - `Model.validate_unique()`
4. Validate the constraints - `Model.validate_constraints()`

All four steps are performed when you call a model's `full_clean()` method.

Ketika anda menggunakan `ModelForm`, panggilan pada `is_valid()` akan melakukan langkah-langkah pengesahan ini untuk semua bidang yang disertakan pada formulir. Lihat ModelForm documentation ` untuk informasi lebih. Anda hanya butuh memanggil metode :meth:`~Model.full_clean() model jika anda berencana menangani kesalahan pengesahan anda sendiri, atau jika anda telah tidak menyertakan bidang-bidang dari `ModelForm` yang membutuhkan pengesahan.

Peringatan

Constraints containing `JSONField` may not raise validation errors as key, index, and path transforms have many database-specific caveats. This may be fully supported later.

You should always check that there are no log messages, in the `django.db.models` logger, like "Got a database error calling check() on …" to confirm it's validated properly.

Changed in Django 4.1:

In older versions, constraints were not checked during the model validation.

`Model.``full_clean`(exclude=None, validate_unique=True, validate_constraints=True)

This method calls `Model.clean_fields()`, `Model.clean()`, `Model.validate_unique()` (if `validate_unique` is `True`), and `Model.validate_constraints()` (if `validate_constraints` is `True`) in that order and raises a `ValidationError` that has a `message_dict` attribute containing errors from all four stages.

The optional `exclude` argument can be used to provide a `set` of field names that can be excluded from validation and cleaning. `ModelForm` uses this argument to exclude fields that aren't present on your form from being validated since any errors raised could not be corrected by the user.

Catata bahwa `full_clean()` tidak akan dipanggil otomatis ketika anda memanggil metode `save()` model anda. Anda akan butuh memanggil itu secara manual ketika anda ingin menjalankan pengesahan model satu-langkah untuk anda sendiri secara manual model dibuat. Sebagai contoh:

    from django.core.exceptions import ValidationError

    try:
        article.full_clean()
    except ValidationError as e:
        # Do something based on the errors contained in e.message_dict.
        # Display them to a user, or handle them programmatically.
        pass

Langkah pertama `full_clean()` lakukan adalah membersihkan setiap bidang masing-masing.

Changed in Django 4.1:

The `validate_constraints` argument was added.

Changed in Django 4.1:

An `exclude` value is now converted to a `set` rather than a `list`.

`Model.``clean_fields`(exclude=None)

This method will validate all fields on your model. The optional `exclude` argument lets you provide a `set` of field names to exclude from validation. It will raise a `ValidationError` if any fields fail validation.

Langkah kedua `full_clean()` lakukan adalah memanggil `Model.clean()`. Metode ini harus ditimpa untuk melakukan penyesuaian keabsahan pada model anda.

`Model.``clean`()

Metode ini harus digunakan untuk menyediakan pengesahan model penyesuaian, dan untuk merubah atribut pada model anda jika diinginkan. Sebagai contoh, anda dapat menggunakan itu secara otomatis menyediakan nilai untuk bidang, atau melakukan pengesahan yang membutuhkan akses pada lebih dari bidang tunggal:

    import datetime
    from django.core.exceptions import ValidationError
    from django.db import models
    from django.utils.translation import gettext_lazy as _

    class Article(models.Model):
        ...

        def clean(self):
            # Don't allow draft entries to have a pub_date.
            if self.status == "draft" and self.pub_date is not None:
                raise ValidationError(_("Draft entries may not have a publication date."))
            # Set the pub_date for published items if it hasn't been set already.
            if self.status == "published" and self.pub_date is None:
                self.pub_date = datetime.date.today()

Catat, bagaimanapun, itu seeprti `Model.full_clean()`, sebuah metode `clean()` model tidak dipanggil ketika anda memanggil metode `save()` model anda.

Dalam contoh diatas, pengecualian `ValidationError` dimunculkan oleh `Model.clean()` telah dipakai dengan string, sehingga itu akan tetap disimpan dalam kunci dictionary khusus, `NON_FIELD_ERRORS`. Kunci ini digunakan untuk kesalahan-kesalahan yang diikat pada keseluruhan model daripada bidang tertentu:

    from django.core.exceptions import NON_FIELD_ERRORS, ValidationError

    try:
        article.full_clean()
    except ValidationError as e:
        non_field_errors = e.message_dict[NON_FIELD_ERRORS]

Untuk memberikan pengecualian pada bidang khusus, pemakaian `ValidationError` dengan sebuah dictionary, dimana kunci-kunci adalah nama-nama bidang. Kami dapat memperbaharui contoh sebelumnya untuk memberikan kesalahan pada bidang `pub_date`:

    class Article(models.Model):
        ...

        def clean(self):
            # Don't allow draft entries to have a pub_date.
            if self.status == "draft" and self.pub_date is not None:
                raise ValidationError(
                    {"pub_date": _("Draft entries may not have a publication date.")}
                )
            ...

Jika anda mengetahui kesalahan dalam banyak bidang selama `Model.clean()`, anda dapat juga melewatkan nama-nama bidang pemetaan dictionary pada kesalahan:

    raise ValidationError(
        {
            "title": ValidationError(_("Missing title."), code="required"),
            "pub_date": ValidationError(_("Invalid date."), code="invalid"),
        }
    )

Then, `full_clean()` will check unique constraints on your model.

How to raise field-specific validation errors if those fields don't appear in a `ModelForm`

Anda tidak dapat memunculkan kesalahan pengesana dalam `Model.clean()` untuk bidang yang tidak muncul dalam sebuah formulir model (formulir mungkin membatasi bidang-bidangnya menggunakan `Meta.fields` atau `Meta.exclude`). Melakukan itu akan memunculkan `ValueError` karena kesalahan pengesahan tidak dapat dikaitkan dengan bidang tidak disertakan.

Untuk bekerja pada pilihan ini, daripada menimpa `Model.clean_fields()` ketika itu menerima daftar dari bidang-bidang yang tidak disertakan dari pengesahan. Sebagai contoh:

    class Article(models.Model):
        ...

        def clean_fields(self, exclude=None):
            super().clean_fields(exclude=exclude)
            if self.status == "draft" and self.pub_date is not None:
                if exclude and "status" in exclude:
                    raise ValidationError(
                        _("Draft entries may not have a publication date.")
                    )
                else:
                    raise ValidationError(
                        {
                            "status": _(
                                "Set status to draft if there is not a " "publication date."
                            ),
                        }
                    )

`Model.``validate_unique`(exclude=None)

This method is similar to `clean_fields()`, but validates uniqueness constraints defined via `Field.unique`, `Field.unique_for_date`, `Field.unique_for_month`, `Field.unique_for_year`, or `Meta.unique_together` on your model instead of individual field values. The optional `exclude` argument allows you to provide a `set` of field names to exclude from validation. It will raise a `ValidationError` if any fields fail validation.

`UniqueConstraint`s defined in the `Meta.constraints` are validated by `Model.validate_constraints()`.

Catat bahwa jika anda menyediakan sebuah argumen `exclude` pada `validate_unique()`, batasan `unique_together` apapun melibatkan satu dari bidang-bidang anda sediakan tidak akan diperiksa.

Finally, `full_clean()` will check any other constraints on your model.

Changed in Django 4.1:

In older versions, `UniqueConstraint`s were validated by `validate_unique()`.

`Model.``validate_constraints`(exclude=None)

New in Django 4.1.

This method validates all constraints defined in `Meta.constraints`. The optional `exclude` argument allows you to provide a `set` of field names to exclude from validation. It will raise a `ValidationError` if any constraints fail validation.

## Menyimpan obyek

Untuk menyimpan sebuah obyek kembali ke basisdata, panggil `save()`:

`Model.``save`(force_insert=False, force_update=False, using=DEFAULT_DB_ALIAS, update_fields=None)

`Model.``asave`(force_insert=False, force_update=False, using=DEFAULT_DB_ALIAS, update_fields=None)

Asynchronous version: `asave()`

For details on using the `force_insert` and `force_update` arguments, see Memaksa sebuah INSERT atau UPDATE. Details about the `update_fields` argument can be found in the Menentukan bidang-bidang mana yang disimpan section.

If you want customized saving behavior, you can override this `save()` method. See Menimpa metode model yang detetapkan for more details.

The model save process also has some subtleties; see the sections below.

Changed in Django 4.2:

`asave()` method was added.

### Peningkatan-otomatis primary key

If a model has an `AutoField` — an auto-incrementing primary key — then that auto-incremented value will be calculated and saved as an attribute on your object the first time you call `save()`:

    >>> b2 = Blog(name="Cheddar Talk", tagline="Thoughts on cheese.")
    >>> b2.id  # Returns None, because b2 doesn't have an ID yet.
    >>> b2.save()
    >>> b2.id  # Returns the ID of your new object.

There's no way to tell what the value of an ID will be before you call `save()`, because that value is calculated by your database, not by Django.

Untuk kenyamanan, setiap model memiliki sebuah `AutoField` bernama `id` secara awalan meskipun anda secara tegas menentukan `primary_key=True` pada bidang dalam model anda. Lihat dokumentasi untuk `AutoField` untuk rincian lebih.

#### Sifat `pk`

`Model.``pk`

Meskipun apakah anda menentukan bidang primary key anda sendiri, atau membiarkan Django mendukung satu untuk anda, setiap model akan memiliki sifat dipanggil `pk`. Itu berperilaku seperti atribut biasa pada model, tetapi sebenarnya sebuah nama lain untuk atribut mana saja adalah bidang primary key untuk model. Anda dapat membaca dan mensetel nilai ini, sama seperti anda untuk atribut lain, dan itu akan memperbaharui bidang benar dalam model.

#### Explicitly specifying auto-primary-key values

If a model has an `AutoField` but you want to define a new object's ID explicitly when saving, define it explicitly before saving, rather than relying on the auto-assignment of the ID:

    >>> b3 = Blog(id=3, name="Cheddar Talk", tagline="Thoughts on cheese.")
    >>> b3.id  # Returns 3.
    >>> b3.save()
    >>> b3.id  # Returns 3.

If you assign auto-primary-key values manually, make sure not to use an already-existing primary-key value! If you create a new object with an explicit primary-key value that already exists in the database, Django will assume you're changing the existing record rather than creating a new one.

Diberikan contoh blog `'Cheddar Talk'` diatas, contoh ini akan menimpa rekaman sebelumnya di basisdata:

    b4 = Blog(id=3, name="Not Cheddar", tagline="Anything but cheese.")
    b4.save()  # Overrides the previous blog with ID=3!

Lihat How Django knows to UPDATE vs. INSERT, dibawah, untuk alasan ini terjadi.

Explicitly specifying auto-primary-key values is mostly useful for bulk-saving objects, when you're confident you won't have primary-key collision.

If you're using PostgreSQL, the sequence associated with the primary key might need to be updated; see Manual-menentukan nilai-nilai dari primary key peningkatan-otomatis.

### Apa yang terjadi ketika anda menyimpan?

Ketika anda menyimpan sebuah obyek, Django melakukan langkah-langkah berikut:

1. Emit a pre-save signal. The `pre_save` signal is sent, allowing any functions listening for that signal to do something.

2. Preprocess the data. Each field's `pre_save()` method is called to perform any automated data modification that's needed. For example, the date/time fields override `pre_save()` to implement `auto_now_add` and `auto_now`.

3. Prepare the data for the database. Each field's `get_db_prep_save()` method is asked to provide its current value in a data type that can be written to the database.

   Most fields don't require data preparation. Simple data types, such as integers and strings, are 'ready to write' as a Python object. However, more complex data types often require some modification.

   For example, `DateField` fields use a Python `datetime` object to store data. Databases don't store `datetime` objects, so the field value must be converted into an ISO-compliant date string for insertion into the database.

4. Insert the data into the database. The preprocessed, prepared data is composed into an SQL statement for insertion into the database.

5. Emit a post-save signal. The `post_save` signal is sent, allowing any functions listening for that signal to do something.

### Bagaimana Django mengetahui untuk UPDATE lawan INSERT

You may have noticed Django database objects use the same `save()` method for creating and changing objects. Django abstracts the need to use `INSERT` or `UPDATE` SQL statements. Specifically, when you call `save()` and the object's primary key attribute does not define a `default`, Django follows this algorithm:

- If the object's primary key attribute is set to a value that evaluates to `True` (i.e., a value other than `None` or the empty string), Django executes an `UPDATE`.
- If the object's primary key attribute is not set or if the `UPDATE` didn't update anything (e.g. if primary key is set to a value that doesn't exist in the database), Django executes an `INSERT`.

If the object's primary key attribute defines a `default` then Django executes an `UPDATE` if it is an existing model instance and primary key is set to a value that exists in the database. Otherwise, Django executes an `INSERT`.

The one gotcha here is that you should be careful not to specify a primary-key value explicitly when saving new objects, if you cannot guarantee the primary-key value is unused. For more on this nuance, see Explicitly specifying auto-primary-key values above and Forcing an INSERT or UPDATE below.

In Django 1.5 and earlier, Django did a `SELECT` when the primary key attribute was set. If the `SELECT` found a row, then Django did an `UPDATE`, otherwise it did an `INSERT`. The old algorithm results in one more query in the `UPDATE` case. There are some rare cases where the database doesn't report that a row was updated even if the database contains a row for the object's primary key value. An example is the PostgreSQL `ON UPDATE` trigger which returns `NULL`. In such cases it is possible to revert to the old algorithm by setting the `select_on_save` option to `True`.

#### Memaksa sebuah INSERT atau UPDATE

In some rare circumstances, it's necessary to be able to force the `save()` method to perform an SQL `INSERT` and not fall back to doing an `UPDATE`. Or vice-versa: update, if possible, but not insert a new row. In these cases you can pass the `force_insert=True` or `force_update=True` parameters to the `save()` method. Passing both parameters is an error: you cannot both insert and update at the same time!

It should be very rare that you'll need to use these parameters. Django will almost always do the right thing and trying to override that will lead to errors that are difficult to track down. This feature is for advanced use only.

Menggunakan `update_fields` akan memaksa sebuah perbaharuan mirip pada `force_update`.

### Memperbaharui atribut berdasarkan pada bidang yang ada

Sometimes you'll need to perform a simple arithmetic task on a field, such as incrementing or decrementing the current value. One way of achieving this is doing the arithmetic in Python like:

    >>> product = Product.objects.get(name="Venezuelan Beaver Cheese")
    >>> product.number_sold += 1
    >>> product.save()

If the old `number_sold` value retrieved from the database was 10, then the value of 11 will be written back to the database.

The process can be made robust, avoiding a race condition, as well as slightly faster by expressing the update relative to the original field value, rather than as an explicit assignment of a new value. Django provides `F expressions` for performing this kind of relative update. Using `F expressions`, the previous example is expressed as:

    >>> from django.db.models import F
    >>> product = Product.objects.get(name="Venezuelan Beaver Cheese")
    >>> product.number_sold = F("number_sold") + 1
    >>> product.save()

For more details, see the documentation on `F expressions` and their use in update queries.

### Menentukan bidang-bidang mana yang disimpan

If `save()` is passed a list of field names in keyword argument `update_fields`, only the fields named in that list will be updated. This may be desirable if you want to update just one or a few fields on an object. There will be a slight performance benefit from preventing all of the model fields from being updated in the database. For example:

    product.name = "Name changed again"
    product.save(update_fields=["name"])

The `update_fields` argument can be any iterable containing strings. An empty `update_fields` iterable will skip the save. A value of `None` will perform an update on all fields.

Menentukan `update_fields` akan memaksa sebuah pembaharuan.

When saving a model fetched through deferred model loading (`only()` or `defer()`) only the fields loaded from the DB will get updated. In effect there is an automatic `update_fields` in this case. If you assign or change any deferred field value, the field will be added to the updated fields.

`Field.pre_save()` and `update_fields`

If `update_fields` is passed in, only the `pre_save()` methods of the `update_fields` are called. For example, this means that date/time fields with `auto_now=True` will not be updated unless they are included in the `update_fields`.

## menghapus obyek

`Model.``delete`(using=DEFAULT_DB_ALIAS, keep_parents=False)

`Model.``adelete`(using=DEFAULT_DB_ALIAS, keep_parents=False)

Asynchronous version: `adelete()`

Issues an SQL `DELETE` for the object. This only deletes the object in the database; the Python instance will still exist and will still have data in its fields, except for the primary key set to `None`. This method returns the number of objects deleted and a dictionary with the number of deletions per object type.

Untuk rincian lebih, termasuk bagaimana menghapus obyek-obyek dalam kumpulan, lihat menghapus obyek.

Jika anda ingin menyesuaikan perilaku hapus, anda dapat menimpa metode `delete()`. Lihat Menimpa metode model yang detetapkan untuk lebih rinci.

Sometimes with multi-table inheritance you may want to delete only a child model's data. Specifying `keep_parents=True` will keep the parent model's data.

Changed in Django 4.2:

`adelete()` method was added.

## Pickling objects

When you `pickle` a model, its current state is pickled. When you unpickle it, it'll contain the model instance at the moment it was pickled, rather than the data that's currently in the database.

You can't share pickles between versions

Pickles of models are only valid for the version of Django that was used to generate them. If you generate a pickle using Django version N, there is no guarantee that pickle will be readable with Django version N+1. Pickles should not be used as part of a long-term archival strategy.

Since pickle compatibility errors can be difficult to diagnose, such as silently corrupted objects, a `RuntimeWarning` is raised when you try to unpickle a model in a Django version that is different than the one in which it was pickled.

## Other model instance methods

A few object methods have special purposes.

### `__str__()`

`Model.``__str__`()

The `__str__()` method is called whenever you call `str()` on an object. Django uses `str(obj)` in a number of places. Most notably, to display an object in the Django admin site and as the value inserted into a template when it displays an object. Thus, you should always return a nice, human-readable representation of the model from the `__str__()` method.

Sebagai contoh:

    from django.db import models

    class Person(models.Model):
        first_name = models.CharField(max_length=50)
        last_name = models.CharField(max_length=50)

        def __str__(self):
            return f"{self.first_name} {self.last_name}"

### `__eq__()`

`Model.``__eq__`()

The equality method is defined such that instances with the same primary key value and the same concrete class are considered equal, except that instances with a primary key value of `None` aren't equal to anything except themselves. For proxy models, concrete class is defined as the model's first non-proxy parent; for all other models it's simply the model's class.

Sebagai contoh:

    from django.db import models

    class MyModel(models.Model):
        id = models.AutoField(primary_key=True)

    class MyProxyModel(MyModel):
        class Meta:
            proxy = True

    class MultitableInherited(MyModel):
        pass

    # Primary keys compared
    MyModel(id=1) == MyModel(id=1)
    MyModel(id=1) != MyModel(id=2)
    # Primary keys are None
    MyModel(id=None) != MyModel(id=None)
    # Same instance
    instance = MyModel(id=None)
    instance == instance
    # Proxy model
    MyModel(id=1) == MyProxyModel(id=1)
    # Multi-table inheritance
    MyModel(id=1) != MultitableInherited(id=1)

### `__hash__()`

`Model.``__hash__`()

The `__hash__()` method is based on the instance's primary key value. It is effectively `hash(obj.pk)`. If the instance doesn't have a primary key value then a `TypeError` will be raised (otherwise the `__hash__()` method would return different values before and after the instance is saved, but changing the `__hash__()` value of an instance is forbidden in Python.

### `get_absolute_url()`

`Model.``get_absolute_url`()

Define a `get_absolute_url()` method to tell Django how to calculate the canonical URL for an object. To callers, this method should appear to return a string that can be used to refer to the object over HTTP.

Sebagai contoh:

    def get_absolute_url(self):
        return "/people/%i/" % self.id

While this code is correct and simple, it may not be the most portable way to to write this kind of method. The `reverse()` function is usually the best approach.

Sebagai contoh:

    def get_absolute_url(self):
        from django.urls import reverse

        return reverse("people-detail", kwargs={"pk": self.pk})

One place Django uses `get_absolute_url()` is in the admin app. If an object defines this method, the object-editing page will have a "View on site" link that will jump you directly to the object's public view, as given by `get_absolute_url()`.

Similarly, a couple of other bits of Django, such as the syndication feed framework, use `get_absolute_url()` when it is defined. If it makes sense for your model's instances to each have a unique URL, you should define `get_absolute_url()`.

Peringatan

You should avoid building the URL from unvalidated user input, in order to reduce possibilities of link or redirect poisoning:

    def get_absolute_url(self):
        return "/%s/" % self.name

If `self.name` is `'/example.com'` this returns `'//example.com/'` which, in turn, is a valid schema relative URL but not the expected `'/%2Fexample.com/'`.

It's good practice to use `get_absolute_url()` in templates, instead of hard-coding your objects' URLs. For example, this template code is bad:

    <!-- BAD template code. Avoid! -->
    <a href="/people/{{ object.id }}/">{{ object.name }}</a>

Kode cetakan ini jauh lebih baik:

    <a href="{{ object.get_absolute_url }}">{{ object.name }}</a>

The logic here is that if you change the URL structure of your objects, even for something small like correcting a spelling error, you don't want to have to track down every place that the URL might be created. Specify it once, in `get_absolute_url()` and have all your other code call that one place.

Catatan

The string you return from `get_absolute_url()` must contain only ASCII characters (required by the URI specification, RFC 3986#section-2) and be URL-encoded, if necessary.

Code and templates calling `get_absolute_url()` should be able to use the result directly without any further processing. You may wish to use the `django.utils.encoding.iri_to_uri()` function to help with this if you are using strings containing characters outside the ASCII range.

## Extra instance methods

In addition to `save()`, `delete()`, a model object might have some of the following methods:

`Model.``get_FOO_display`()

For every field that has `choices` set, the object will have a `get_FOO_display()` method, where `FOO` is the name of the field. This method returns the "human-readable" value of the field.

Sebagai contoh:

    from django.db import models

    class Person(models.Model):
        SHIRT_SIZES = [
            ("S", "Small"),
            ("M", "Medium"),
            ("L", "Large"),
        ]
        name = models.CharField(max_length=60)
        shirt_size = models.CharField(max_length=2, choices=SHIRT_SIZES)

    >>> p = Person(name="Fred Flintstone", shirt_size="L")
    >>> p.save()
    >>> p.shirt_size
    'L'
    >>> p.get_shirt_size_display()
    'Large'

`Model.``get_next_by_FOO`(**kwargs)

`Model.``get_previous_by_FOO`(**kwargs)

For every `DateField` and `DateTimeField` that does not have `null=True`, the object will have `get_next_by_FOO()` and `get_previous_by_FOO()` methods, where `FOO` is the name of the field. This returns the next and previous object with respect to the date field, raising a `DoesNotExist` exception when appropriate.

Both of these methods will perform their queries using the default manager for the model. If you need to emulate filtering used by a custom manager, or want to perform one-off custom filtering, both methods also accept optional keyword arguments, which should be in the format described in Field lookups.

Note that in the case of identical date values, these methods will use the primary key as a tie-breaker. This guarantees that no records are skipped or duplicated. That also means you cannot use those methods on unsaved objects.

Overriding extra instance methods

In most cases overriding or inheriting `get_FOO_display()`, `get_next_by_FOO()`, and `get_previous_by_FOO()` should work as expected. Since they are added by the metaclass however, it is not practical to account for all possible inheritance structures. In more complex cases you should override `Field.contribute_to_class()` to set the methods you need.

## Atribut-atribut lain

### `_state`

`Model.``_state`

  The `_state` attribute refers to a `ModelState` object that tracks the lifecycle of the model instance.

  The `ModelState` object has two attributes: `adding`, a flag which is `True` if the model has not been saved to the database yet, and `db`, a string referring to the database alias the instance was loaded from or saved to.

  Newly instantiated instances have `adding=True` and `db=None`, since they are yet to be saved. Instances fetched from a `QuerySet` will have `adding=False` and `db` set to the alias of the associated database.