django中的多个M2M答案

【问题标题】：Multiple M2M in djangodjango中的多个M2M
【发布时间】：2012-03-09 08:24:24
【问题描述】：

我在 Django 中有两个模型用于基于 2d 地图的游戏：

class Block(models.Model):
   type = models.IntegerField()

class ShopBuilding(models.Model):
   house_blocks = models.ManyToManyField(Block)
   street_blocks = models.ManyToManyField(Block)
   river_blocks = models.ManyToManyField(Block)
   decoration_blocks = models.ManyToManyField(Block)
   npc_blocks = models.ManyToManyField(Block)

现在我只想使用一张表关联这两个模型：

class ShopBlockAssoc(models.Model):
    block = models.ForeignKey(Block)
    shop = models.foreignKey(Shop)

在ShopBuilding模型中设置through字段后，Django在syncdb时多次失败，比如

    Error: One or more models did not validate:
tnew.shopbuilding: Accessor for m2m field 'house_blocks' clashes with related m2m field 'Block.shopbuilding_set'. Add a related_name argument to the definition for 'house_blocks'.
tnew.shopbuilding: Accessor for m2m field 'house_blocks' clashes with related m2m field 'Block.shopbuilding_set'. Add a related_name argument to the definition for 'house_blocks'.
tnew.shopbuilding: Accessor for m2m field 'house_blocks' clashes with related m2m field 'Block.shopbuilding_set'. Add a related_name argument to the definition for 'house_blocks'.
tnew.shopbuilding: Accessor for m2m field 'house_blocks' clashes with related m2m field 'Block.shopbuilding_set'. Add a related_name argument to the definition for 'house_blocks'.
tnew.shopbuilding: Accessor for m2m field 'street_blocks' clashes with related m2m field 'Block.shopbuilding_set'. Add a related_name argument to the definition for 'street_blocks'.
tnew.shopbuilding: Accessor for m2m field 'street_blocks' clashes with related m2m field 'Block.shopbuilding_set'. Add a related_name argument to the definition for 'street_blocks'.
tnew.shopbuilding: Accessor for m2m field 'street_blocks' clashes with related m2m field 'Block.shopbuilding_set'. Add a related_name argument to the definition for 'street_blocks'.
tnew.shopbuilding: Accessor for m2m field 'street_blocks' clashes with related m2m field 'Block.shopbuilding_set'. Add a related_name argument to the definition for 'street_blocks'.
tnew.shopbuilding: Accessor for m2m field 'river_blocks' clashes with related m2m field 'Block.shopbuilding_set'. Add a related_name argument to the definition for 'river_blocks'.
tnew.shopbuilding: Accessor for m2m field 'river_blocks' clashes with related m2m field 'Block.shopbuilding_set'. Add a related_name argument to the definition for 'river_blocks'.
tnew.shopbuilding: Accessor for m2m field 'river_blocks' clashes with related m2m field 'Block.shopbuilding_set'. Add a related_name argument to the definition for 'river_blocks'.
tnew.shopbuilding: Accessor for m2m field 'river_blocks' clashes with related m2m field 'Block.shopbuilding_set'. Add a related_name argument to the definition for 'river_blocks'.
tnew.shopbuilding: Accessor for m2m field 'decoration_blocks' clashes with related m2m field 'Block.shopbuilding_set'. Add a related_name argument to the definition for 'decoration_blocks'.
tnew.shopbuilding: Accessor for m2m field 'decoration_blocks' clashes with related m2m field 'Block.shopbuilding_set'. Add a related_name argument to the definition for 'decoration_blocks'.
tnew.shopbuilding: Accessor for m2m field 'decoration_blocks' clashes with related m2m field 'Block.shopbuilding_set'. Add a related_name argument to the definition for 'decoration_blocks'.
tnew.shopbuilding: Accessor for m2m field 'decoration_blocks' clashes with related m2m field 'Block.shopbuilding_set'. Add a related_name argument to the definition for 'decoration_blocks'.
tnew.shopbuilding: Accessor for m2m field 'npc_blocks' clashes with related m2m field 'Block.shopbuilding_set'. Add a related_name argument to the definition for 'npc_blocks'.
tnew.shopbuilding: Accessor for m2m field 'npc_blocks' clashes with related m2m field 'Block.shopbuilding_set'. Add a related_name argument to the definition for 'npc_blocks'.
tnew.shopbuilding: Accessor for m2m field 'npc_blocks' clashes with related m2m field 'Block.shopbuilding_set'. Add a related_name argument to the definition for 'npc_blocks'.
tnew.shopbuilding: Accessor for m2m field 'npc_blocks' clashes with related m2m field 'Block.shopbuilding_set'. Add a related_name argument to the definition for 'npc_blocks'.

如果我设置 db_table 字段 Django 会说：

The model has two manually-defined m2m relations through the model, which is not permitted. Please consider using an extra field on your intermediary model instead.

我该如何解决这个问题？我真的很想确保那些商店街区关系使用具有相同 db_table 的共享相同中间关联模型

【问题讨论】：

标签： django model m2m

【解决方案1】：

问题是在模型Block 上它试图创建反向链接。 Django 根据模型命名这些链接，在您的情况下，对于所有 ManyToMany，它会尝试创建 shopbuilding_set 并且名称会发生冲突。为了避免这种情况，请添加related_name，例如：

class ShopBuilding(models.Model):
   house_blocks = models.ManyToManyField(Block, related_name="shopbuilding_house_set")
   street_blocks = models.ManyToManyField(Block, related_name="shopbuilding_street_set")
   river_blocks = models.ManyToManyField(Block, related_name="shopbuilding_river_set")
   decoration_blocks = models.ManyToManyField(Block, related_name="shopbuilding_decoration_set")
   npc_blocks = models.ManyToManyField(Block, related_name="shopbuilding_npc_set")

然后您就可以像这样从Block 实例访问ShopBuilding：

block.shopbuilding_npc_set.all()

【讨论】：

是的，谢谢，但是如何确保这些字段使用相同的中间模型？
如果您想要一个中间模型，您不需要创建 5 个多对多关系，您只需创建一个。像 blocks = models.ManyToManyField(Block) 就是这样。问题是当你有一个中间模型时，你不能有“类型”
所以基本上 Django ORM 不支持多个相同的 m2m 和一个 assoc 模型？
您可以创建模型，添加“类型”字段并在通过中指定该模型。但是创建这样的对象更难，因为您总是必须指定“类型”字段等。Django 不会将许多多对多关系合并为一个。您始终可以创建一些“访问器”和管理器来帮助您处理自定义关系 (DRY)。
无论如何，您指定的ShopBlockAssoc 模型是标准的多对多关系。基本上和写blocks = models.ManyToManyField(Block)是一样的。您可以从 ShopBuilding 中选择所有街区，也可以从街区中选择所有商店，但您没有房屋、街道、河流等类型。