原始代码：
# 邻区多对多关系表
class ZJadjacent(models.Model):
cellfrom = models.ForeignKey(ZJcell, on_delete=models.CASCADE)
cellto = models.ForeignKey(ZJcell, on_delete=models.CASCADE)
# 两个邻区间的距离
distance = models.IntegerField()
# 站在源小区，看目标小区的方位角
azi1 = models.IntegerField()
# 站在目标小区，看源小区的方位角
azi2 = models.IntegerField()执行py manage.py makemigrations时报错：
(venv) C:\Users\Administrator\PycharmProjects\cellsmap>py manage.py makemigrations
SystemCheckError: System check identified some issues:
ERRORS:
dbbackend.ZJadjacent.cellfrom: (fields.E304) Reverse accessor for 'ZJadjacent.cellfrom' clashes with reverse accessor for 'ZJadjacent.cellto'.
HINT: Add or change a related_name argument to the definition for 'ZJadjacent.cellfrom' or 'ZJadjacent.cellto'.
dbbackend.ZJadjacent.cellto: (fields.E304) Reverse accessor for 'ZJadjacent.cellto' clashes with reverse accessor for 'ZJadjacent.cellfrom'.
HINT: Add or change a related_name argument to the definition for 'ZJadjacent.cellto' or 'ZJadjacent.cellfrom'.
官网解释：
ManyToManyField.related_name¶
Same as ForeignKey.related_name.
ForeignKey.related_name¶
The name to use for the relation from the related object back to this one. It's also the default value for related_query_name (the name to use for the reverse filter name from the target model). See the related objects documentation for a full explanation and example. Note that you must set this value when defining relations on abstract models; and when you do so some special syntax is available.
如果想关闭反查或者反向关系，可将related_name 设为 '+'。
注意：这与多对多字段的对称属性symmetrical=False不同，之前我将多对多字段的对称属性symmetrical=False误认为是关闭反查关系。详见：
https://blog.csdn.net/qq_27361945/article/details/83181900
If you'd prefer Django not to create a backwards relation, set related_name to '+' or end it with '+'. For example, this will ensure that the User model won't have a backwards relation to this model:
user = models.ForeignKey(
User,
on_delete=models.CASCADE,
related_name='+',
)
虽然没有ManyToManyField的例子，但是也是一样的，ManyToManyField相当于两个ForeignKey。
定义：
from django.db import models
class Blog(models.Model):
name = models.CharField(max_length=100)
tagline = models.TextField()
def __str__(self):
return self.name
class Author(models.Model):
name = models.CharField(max_length=200)
email = models.EmailField()
def __str__(self):
return self.name
class Entry(models.Model):
blog = models.ForeignKey(Blog, on_delete=models.CASCADE)
headline = models.CharField(max_length=255)
body_text = models.TextField()
pub_date = models.DateField()
mod_date = models.DateField()
authors = models.ManyToManyField(Author)
n_comments = models.IntegerField()
n_pingbacks = models.IntegerField()
rating = models.IntegerField()
def __str__(self):
return self.headlineFollowing relationships "backward"¶
If a model has a ForeignKey, instances of the foreign-key model will have access to a Manager that returns all instances of the first model. By default, this Manager is named FOO_set, where FOO is the source model name, lowercased. This Manager returns QuerySets, which can be filtered and manipulated as described in the "Retrieving objects" section above.
举例：
>>> b = Blog.objects.get(id=1)
>>> b.entry_set.all() # Returns all Entry objects related to Blog.
# b.entry_set is a Manager that returns QuerySets.
>>> b.entry_set.filter(headline__contains='Lennon')
>>> b.entry_set.count()
注意：没有定义related_name的外键，反查时用entry_set，即原模型的名字 Entry的小写（class Entry(models.Model):）
一单定义了related_name='entries'，反查时用entries代替entry_set。
但是多对多怎么理解？
在反查时，如果指明ZJadjacent关系表中的两个models.ForeignKey(ZJcell, on_delete=models.CASCADE)哪个是源小区，哪个是目标小区，通过ZJcell对象反查邻区关系时：
ZJcell.objects.first().zjadjacent_set
就会发生混淆，因为ZJcell.objects.first()这个实例既可以是源小区，也可以是目标小区，我们到底要从它是源小区的关系表中提取数据，还是从目标小区的关系表中提取数据，从ZJcell.objects.first().zjadjacent_set中看不出来。程序就会混淆。
当增加related_name='from_cell_id'后，就可以这样查：
ZJcell.objects.first().from_cell_id
这样就知道是把ZJcell.objects.first()当做源小区，从邻区表中提取数据，而不会提取邻区表中ZJcell.objects.first()是目标小区的数据。
You can override the FOO_set name by setting the related_name parameter in the ForeignKey definition. For example, if the Entry model was altered to blog = ForeignKey(Blog, on_delete=models.CASCADE, related_name='entries'), the above example code would look like this:
>>> b = Blog.objects.get(id=1)
>>> b.entries.all() # Returns all Entry objects related to Blog.
# b.entries is a Manager that returns QuerySets.
>>> b.entries.filter(headline__contains='Lennon')
>>> b.entries.count()
修改 后的代码：
# 邻区多对多关系表
class ZJadjacent(models.Model):
cellfrom = models.ForeignKey(ZJcell, on_delete=models.CASCADE, related_name='from_cell_id')
# 因为有两个ZJcell外键，必须对这两个外键命名为不同的名字，否则出现报错fields.E304
cellto = models.ForeignKey(ZJcell, on_delete=models.CASCADE, related_name='to_cell_id')
# 两个邻区间的距离
distance = models.IntegerField()
# 站在源小区，看目标小区的方位角
azi1 = models.IntegerField()
# 站在目标小区，看源小区的方位角
azi2 = models.IntegerField()当增加related_name='from_cell_id'后，就可以这样查：
ZJcell.objects.first().from_cell_id
这样就知道是把ZJcell.objects.first()当做源小区，从邻区表中提取数据，而不会提取邻区表中ZJcell.objects.first()是目标小区的数据。
https://blog.csdn.net/meylovezn/article/details/46924893
(venv) C:\Users\Administrator\PycharmProjects\cellsmap>py manage.py makemigrations
You are trying to add a non-nullable field 'azi1' to zjadjacent without a default; we can't do that (the database needs something to populate existing rows).
Please select a fix:
1) Provide a one-off default now (will be set on all existing rows with a null value for this column)
2) Quit, and let me add a default in models.py
Select an option: 2
# 邻区多对多关系表
class ZJadjacent(models.Model):
cellfrom = models.ForeignKey(ZJcell, on_delete=models.CASCADE, related_name='from_cell_id')
# 因为有两个ZJcell外键，必须对这两个外键命名为不同的名字，否则出现报错fields.E304
cellto = models.ForeignKey(ZJcell, on_delete=models.CASCADE, related_name='to_cell_id')
# 两个邻区间的距离
distance = models.IntegerField(default=None)
# 站在源小区，看目标小区的方位角
azi1 = models.IntegerField(default=None)
# 站在目标小区，看源小区的方位角
azi2 = models.IntegerField(default=None)