django 的model是如何把字段加入到meta中的

def contribute_to_class(self, cls, name):
        self.set_attributes_from_name(name)
        self.model = cls
        cls._meta.add_field(self)
        if self.choices:
            setattr(cls, 'get_%s_display' % self.name,
                    curry(cls._get_FIELD_display, field=self))

上面在Field类中定义的contribute_to_class(self, cls, name)，该类调用_meta.add_field(self)

def add_field(self, field):
        # Insert the given field in the order in which it was created, using
        # the "creation_counter" attribute of the field.
        # Move many-to-many related fields from self.fields into
        # self.many_to_many.
        if field.rel and isinstance(field.rel, ManyToManyRel):
            self.local_many_to_many.insert(bisect(self.local_many_to_many, field), field)
            if hasattr(self, '_m2m_cache'):
                del self._m2m_cache
        else:
            self.local_fields.insert(bisect(self.local_fields, field), field)
            self.setup_pk(field)
            if hasattr(self, '_field_cache'):
                del self._field_cache
                del self._field_name_cache

        if hasattr(self, '_name_map'):
            del self._name_map

add_field会把field加到meta对象中。

fields = property(_fields)

    def get_fields_with_model(self):
        """
        Returns a sequence of (field, model) pairs for all fields. The "model"
        element is None for fields on the current model. Mostly of use when
        constructing queries so that we know which model a field belongs to.
        """
        try:
            self._field_cache
        except AttributeError:
            self._fill_fields_cache()
        return self._field_cache

    def _fill_fields_cache(self):
        cache = []
        for parent in self.parents:
            for field, model in parent._meta.get_fields_with_model():
                if model:
                    cache.append((field, model))
                else:
                    cache.append((field, parent))
        cache.extend([(f, None) for f in self.local_fields])
        self._field_cache = tuple(cache)
        self._field_name_cache = [x for x, _ in cache]

调用meta的fields会返回model的所有field

model实例化要取得所有的field