Django2.0手册:Model



class Options[source]

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 that it provides can be used to:

  • Retrieve all field instances of a model
  • Retrieve a single field instance of a model by name

Field access API¶

Retrieving a single field instance of a model by name¶

Options.get_field(field_name)[source]

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 the related_name defined by the user or the name automatically
generated by Django itself.

Hidden fields cannot be retrieved
by name.

If a field with the given name is not found a
FieldDoesNotExist exception will be
raised.

>>> 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¶

Options.get_fields(include_parents=True, include_hidden=False)[source]

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 that are used to back other field’s functionality. This will
also include any fields that have a related_name (such
as ManyToManyField, or
ForeignKey) that start with a “+”.
>>> 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>)

Migrating from the old API¶

As part of the formalization of the Model._meta API (from the
django.db.models.options.Options class), a number of methods and
properties have been deprecated and will be removed in Django 1.10.

These old APIs can be replicated by either:

Although it’s possible to make strictly equivalent replacements of the old
methods, that might not be the best approach. Taking the time to refactor any
field loops to make better use of the new API – and possibly include fields
that were previously excluded – will almost certainly result in better code.

Assuming you have a model named MyModel, the following substitutions
can be made to convert your code to the new API:

  • MyModel._meta.get_field(name) becomes:

    f = MyModel._meta.get_field(name)
    

    then check if:

    • f.auto_created == False, because the new get_field()
      API will find “reverse” relations, and:
    • f.is_relation and f.related_model is None, because the new
      get_field() API will find
      GenericForeignKey relations.
  • MyModel._meta.get_field_by_name(name) returns a tuple of these four
    values with the following replacements:

    • field can be found by MyModel._meta.get_field(name)

    • model can be found through the
      model attribute on the field.

    • direct can be found by: not field.auto_created or field.concrete

      The auto_created check excludes
      all “forward” and “reverse” relations that are created by Django, but
      this also includes AutoField and OneToOneField on proxy models.
      We avoid filtering out these attributes using the
      concrete attribute.

    • m2m can be found through the
      many_to_many attribute on the field.

  • MyModel._meta.get_fields_with_model() becomes:

    [
        (f, f.model if f.model != MyModel else None)
        for f in MyModel._meta.get_fields()
        if not f.is_relation
            or f.one_to_one
            or (f.many_to_one and f.related_model)
    ]
    
  • MyModel._meta.get_concrete_fields_with_model() becomes:

    [
        (f, f.model if f.model != MyModel else None)
        for f in MyModel._meta.get_fields()
        if f.concrete and (
            not f.is_relation
            or f.one_to_one
            or (f.many_to_one and f.related_model)
        )
    ]
    
  • MyModel._meta.get_m2m_with_model() becomes:

    [
        (f, f.model if f.model != MyModel else None)
        for f in MyModel._meta.get_fields()
        if f.many_to_many and not f.auto_created
    ]
    
  • MyModel._meta.get_all_related_objects() becomes:

    [
        f for f in MyModel._meta.get_fields()
        if (f.one_to_many or f.one_to_one)
        and f.auto_created and not f.concrete
    ]
    
  • MyModel._meta.get_all_related_objects_with_model() becomes:

    [
        (f, f.model if f.model != MyModel else None)
        for f in MyModel._meta.get_fields()
        if (f.one_to_many or f.one_to_one)
        and f.auto_created and not f.concrete
    ]
    
  • MyModel._meta.get_all_related_many_to_many_objects() becomes:

    [
        f for f in MyModel._meta.get_fields(include_hidden=True)
        if f.many_to_many and f.auto_created
    ]
    
  • MyModel._meta.get_all_related_m2m_objects_with_model() becomes:

    [
        (f, f.model if f.model != MyModel else None)
        for f in MyModel._meta.get_fields(include_hidden=True)
        if f.many_to_many and f.auto_created
    ]
    
  • MyModel._meta.get_all_field_names() becomes:

    from itertools import chain
    list(set(chain.from_iterable(
        (field.name, field.attname) if hasattr(field, 'attname') else (field.name,)
        for field in MyModel._meta.get_fields()
        # For complete backwards compatibility, you may want to exclude
        # GenericForeignKey from the results.
        if not (field.many_to_one and field.related_model is None)
    )))
    

    This provides a 100% backwards compatible replacement, ensuring that both
    field names and attribute names ForeignKeys are included, but fields
    associated with GenericForeignKeys are not. A simpler version would be:

    [f.name for f in MyModel._meta.get_fields()]
    

    While this isn’t 100% backwards compatible, it may be sufficient in many
    situations.