Multiple M2M in django

Question

I have two models in Django for a 2d map based game:

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)

Now I just want associate these two models using ONE table:

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

After I set the through field in ShopBuilding model, Django yiled multiple fails when syncdb, like

    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'.

If I set db_table field Django says:

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.

How could I fix this? I really want to make sure those shop-block relations are using the sharing the same intermediate associate model with the same db_table

score 4 · Accepted Answer · answered Mar 09 '12 at 08:43

4

The problem is that on model Block it tries to create backwards links. Django names those links based on Model, in your case for all ManyToMany it tries to create shopbuilding_set and names collide. To avoid this add related_name, for example:

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")

Then you'll be able to access ShopBuilding from Block instance like this:

block.shopbuilding_npc_set.all()

answered Mar 09 '12 at 08:43

Marius Grigaitis

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yes thanks, but how do I make sure these fields using the same intermediate model? – est Mar 09 '12 at 08:54
if you want one intermediate model you don't create 5 ManyToMany relations, you create just one. like blocks = models.ManyToManyField(Block) and that's it. Problem is that when you have one intermediate model you can't have "types" – Marius Grigaitis Mar 09 '12 at 08:57
so basically Django ORM doesn't support multiple same m2m with one assoc model? – est Mar 09 '12 at 09:00
You can create Model, add "type" field and specify that model in Through. But it's harder to create such objects because you'll always have to specify "type" field and etc. Django doesn't merge many manytomany relationships into one. You can always create some "accessors" and managers that helps you deal with your custom relationship (DRY). – Marius Grigaitis Mar 09 '12 at 09:09
Anyhow in question you specified your `ShopBlockAssoc` model that is standart manytomany relationship. Basically it's the same as writing `blocks = models.ManyToManyField(Block)`. You can select all blocks from ShopBuilding and you can select all shops from block but you don't have types like house, street, river. – Marius Grigaitis Mar 09 '12 at 09:13

Multiple M2M in django

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