===================
Model ``_meta`` API
===================

.. module:: django.db.models.options
   :synopsis: Model meta-class layer

.. class:: Options

The model ``_meta`` API is at the core of the Django ORM. It enables other
parts of the system such as lookups, queries, forms, and the admin to
understand the capabilities of each model. The API is accessible through
the ``_meta`` attribute of each model class, which is an instance of an
``django.db.models.options.Options`` object.

Methods and attributes that it provides can be used to:

* Retrieve all field instances of a model
* Retrieve a single field instance of a model by name
* Retrieve all fields that compose the primary key of a model

.. _model-meta-field-api:

Field access API
================

Retrieving a single field instance of a model by name
-----------------------------------------------------

.. method:: Options.get_field(field_name)

    Returns the field instance given a name of a field.

    ``field_name`` can be the name of a field on the model, a field on an
    abstract or inherited model, or a field defined on another model that
    points to the model. In the latter case, the ``field_name`` will be (in
    order of preference) the :attr:`~.ForeignKey.related_query_name` set by the
    user, the :attr:`~.ForeignKey.related_name` set by the user, or the name
    automatically generated by Django.

    Hidden fields, fields with :attr:`hidden=True
    <django.db.models.Field.hidden>`, cannot be retrieved by name.

    If a field with the given name is not found a
    :class:`~django.core.exceptions.FieldDoesNotExist` exception will be
    raised.

    .. code-block:: pycon

        >>> from django.contrib.auth.models import User

        # A field on the model
        >>> User._meta.get_field("username")
        <django.db.models.fields.CharField: username>

        # A field from another model that has a relation with the current model
        >>> User._meta.get_field("logentry")
        <ManyToOneRel: admin.logentry>

        # A non existent field
        >>> User._meta.get_field("does_not_exist")
        Traceback (most recent call last):
            ...
        FieldDoesNotExist: User has no field named 'does_not_exist'

Retrieving all field instances of a model
-----------------------------------------

.. method:: Options.get_fields(include_parents=True, include_hidden=False)

    Returns a tuple of fields associated with a model. ``get_fields()`` accepts
    two parameters that can be used to control which fields are returned:

    ``include_parents``
        ``True`` by default. Recursively includes fields defined on parent
        classes. If set to ``False``, ``get_fields()`` will only search for
        fields declared directly on the current model. Fields from models that
        directly inherit from abstract models or proxy classes are considered
        to be local, not on the parent.

    ``include_hidden``
        ``False`` by default. If set to ``True``, ``get_fields()`` will include
        fields with :attr:`hidden=True <django.db.models.Field.hidden>`.

    .. code-block:: pycon

        >>> from django.contrib.auth.models import User
        >>> User._meta.get_fields()
        (<ManyToOneRel: admin.logentry>,
         <django.db.models.fields.AutoField: id>,
         <django.db.models.fields.CharField: password>,
         <django.db.models.fields.DateTimeField: last_login>,
         <django.db.models.fields.BooleanField: is_superuser>,
         <django.db.models.fields.CharField: username>,
         <django.db.models.fields.CharField: first_name>,
         <django.db.models.fields.CharField: last_name>,
         <django.db.models.fields.EmailField: email>,
         <django.db.models.fields.BooleanField: is_staff>,
         <django.db.models.fields.BooleanField: is_active>,
         <django.db.models.fields.DateTimeField: date_joined>,
         <django.db.models.fields.related.ManyToManyField: groups>,
         <django.db.models.fields.related.ManyToManyField: user_permissions>)

        # Also include hidden fields.
        >>> User._meta.get_fields(include_hidden=True)
        (<ManyToOneRel: auth.user_groups>,
         <ManyToOneRel: auth.user_user_permissions>,
         <ManyToOneRel: admin.logentry>,
         <django.db.models.fields.AutoField: id>,
         <django.db.models.fields.CharField: password>,
         <django.db.models.fields.DateTimeField: last_login>,
         <django.db.models.fields.BooleanField: is_superuser>,
         <django.db.models.fields.CharField: username>,
         <django.db.models.fields.CharField: first_name>,
         <django.db.models.fields.CharField: last_name>,
         <django.db.models.fields.EmailField: email>,
         <django.db.models.fields.BooleanField: is_staff>,
         <django.db.models.fields.BooleanField: is_active>,
         <django.db.models.fields.DateTimeField: date_joined>,
         <django.db.models.fields.related.ManyToManyField: groups>,
         <django.db.models.fields.related.ManyToManyField: user_permissions>)

Retrieving fields composing the primary key of a model
------------------------------------------------------

.. attribute:: Options.pk_fields

    Returns a list of the fields composing the primary key of a model.

    When a :class:`~django.db.models.CompositePrimaryKey` is defined on a model
    it will contain all the fields referenced by it.

    .. code-block:: python

        from django.db import models


        class TenantUser(models.Model):
            pk = models.CompositePrimaryKey("tenant_id", "id")
            tenant_id = models.IntegerField()
            id = models.IntegerField()

    .. code-block:: pycon

        >>> TenantUser._meta.pk_fields
        [
            <django.db.models.fields.IntegerField: tenant_id>,
            <django.db.models.fields.IntegerField: id>
        ]

    Otherwise it will contain the single field declared as the primary key of
    the model, either explicitly with :attr:`primary_key=True
    <django.db.models.Field.primary_key>` or implicitly as the :ref:`automatic
    primary key <automatic-primary-key-fields>`.

    .. code-block:: pycon

        >>> User._meta.pk_fields
        [<django.db.models.fields.AutoField: id>]
