# Django 6.0.7 documentation

# 模型 `_meta` API

class Options

模型 `_meta` 的 API 是 Django ORM 的核心。它使系统的其他部分，如查找、查询、表单和管理能够理解每个模型的功能。API 可以通过每个模型类的 `_meta` 属性来访问，它是 `django.db.models.options.Options` 对象的一个实例。

Methods and attributes that it provides can be used to:

- 检索一个模型的所有字段实例

- 按名称检索模型的单个字段实例

- Retrieve all fields that compose the primary key of a model

## 字段访问 API

### 按名称检索模型的单个字段实例

Options.get_field(field_name)

  返回给定字段名的字段实例。

  `field_name` 可以是模型上的字段名，也可以是抽象或继承的模型上的字段名，或者是指向模型的另一个模型上定义的字段名。在后一种情况下，`field_name` 将是（按优先级排序）用户设置的 `related_query_name`，用户设置的 `related_name`，或者 Django 自动生成的名称。

  Hidden fields, fields with `hidden=True`, cannot be retrieved by name.

  如果没有找到指定名称的字段，将引发一个 `FieldDoesNotExist` 异常。

      >>> 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'

### 检索一个模型的所有字段实例

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

  返回与模型相关联的字段的元组。`get_fields()` 接受两个参数，可以用来控制返回哪些字段：

  `include_parents`

    默认为 `True`。递归地包括父类上定义的字段。如果设置为 `False`，`get_fields()` 将只搜索直接在当前模型上声明的字段。直接继承自抽象模型或代理类的模型的字段被认为是本地的，而不是在父类上。

  `include_hidden`

    `False` by default. If set to `True`, `get_fields()` will include fields with `hidden=True`.

      >>> 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

Options.pk_fields

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

  When a `CompositePrimaryKey` is defined on a model it will contain all the fields referenced by it.

      from django.db import models

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

      >>> 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 `primary_key=True` or implicitly as the automatic primary key.

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