django 得models关系讨论：
版本：1.11.4
选择1.11.x系列得原因是同时支持py2.7 和py3，比较稳定，后期django2.0变化主要在path（目前没有看到，不做讨论）
小白得成长之路：路上遇见得问题或者思考想到得在次记录一下，如有高见请大佬解释一下，有疑问共同讨论进步。
ManyToManyFiled：django源码简单拷贝
class ManyToManyField(RelatedField):
"""
Provide a many-to-many relation by using an intermediary model that
holds two ForeignKey fields pointed at the two sides of the relation.
Unless a ``through`` model was provided, ManyToManyField will use the
create_many_to_many_intermediary_model factory to automatically generate
the intermediary model.
"""
# Field flags
many_to_many =True
many_to_one =False
one_to_many =False
one_to_one =False
rel_class = ManyToManyRel
description = _("Many-to-many relationship")
def __init__(self, to, related_name=None, related_query_name=None,
limit_choices_to=None, symmetrical=None, through=None,
through_fields=None, db_constraint=True, db_table=None,
swappable=True, **kwargs):
try:
to._meta
except AttributeError:
assert isinstance(to, six.string_types), (
"%s(%r) is invalid. First parameter to ManyToManyField must be "
"either a model, a model name, or the string %r" %
(self.__class__.__name__, to, RECURSIVE_RELATIONSHIP_CONSTANT)
)
# Class names must be ASCII in Python 2.x, so we forcibly coerce it
# here to break early if there's a problem.
to =str(to)
采用中介模式，提供一个多对多得关系，使拥有两个外键字段指向关系的双方，除非提供了一个“through”模式，多对多域将使用create_many_to_many_intermediary_model厂自动生成中介模型。
ManyToManyRel
class ManyToManyRel(ForeignObjectRel):
"""
Used by ManyToManyField to store information about the relation.
``_meta.get_fields()`` returns this class to provide access to the field
flags for the reverse relation.
"""
采用多对多域存储联系信息，``_meta.get_fields()``返回这个类提供的对反向关系的字段标志的访问
个人认为后者是关系的表述，日常调用前者更多，挖个坑有读源码能力了回来填
后者调用很少，大概在官方文档溜了一下，model的关系处，没看到对后者的介绍
