How to make Django ManyToMany relationships explicit on the receiving model's end












2














Relationships, particularly ManyToMany, in Django have always bothered me somewhat. In particular, since the relationship is only defined in one of the models, you can't tell from looking at the paired model what other relationships it might be hiding.



For example, from the Django Documentation:



class Topping(models.Model):

    # ...



class Pizza(models.Model):

    # ...

    toppings = models.ManyToManyField(Topping)


You can tell from looking at the code that I'd find out the relevant toppings for a pizza at Pizza.toppings. But you cannot tell that I would be able to tell what pizzas have a topping at Topping.Pizza_set--you have to look at the Pizza class to see this.



As a result, by looking at Toppings, I don't actually know the full range of fields that it has.



Is there any way around this or to make it more explicit? Or is there something that I'm missing?






python django django-models







share|improve this question






















  • Did you try Topping.Pizza_set.all() ?
    – Victor Castillo Torres
    Aug 8 '13 at 2:50










  • Topping.pizza_set.all() is how to get the actual pizzas, which is not the issue--the question is how to make it clear in the actual model that the way to get pizzas is by calling topping.pizza_set.<something>.
    – jdotjdot
    Aug 8 '13 at 5:07
















2














Relationships, particularly ManyToMany, in Django have always bothered me somewhat. In particular, since the relationship is only defined in one of the models, you can't tell from looking at the paired model what other relationships it might be hiding.



For example, from the Django Documentation:



class Topping(models.Model):

    # ...



class Pizza(models.Model):

    # ...

    toppings = models.ManyToManyField(Topping)


You can tell from looking at the code that I'd find out the relevant toppings for a pizza at Pizza.toppings. But you cannot tell that I would be able to tell what pizzas have a topping at Topping.Pizza_set--you have to look at the Pizza class to see this.



As a result, by looking at Toppings, I don't actually know the full range of fields that it has.



Is there any way around this or to make it more explicit? Or is there something that I'm missing?






python django django-models







share|improve this question






















  • Did you try Topping.Pizza_set.all() ?
    – Victor Castillo Torres
    Aug 8 '13 at 2:50










  • Topping.pizza_set.all() is how to get the actual pizzas, which is not the issue--the question is how to make it clear in the actual model that the way to get pizzas is by calling topping.pizza_set.<something>.
    – jdotjdot
    Aug 8 '13 at 5:07














2












2








2







Relationships, particularly ManyToMany, in Django have always bothered me somewhat. In particular, since the relationship is only defined in one of the models, you can't tell from looking at the paired model what other relationships it might be hiding.



For example, from the Django Documentation:



class Topping(models.Model):

    # ...



class Pizza(models.Model):

    # ...

    toppings = models.ManyToManyField(Topping)


You can tell from looking at the code that I'd find out the relevant toppings for a pizza at Pizza.toppings. But you cannot tell that I would be able to tell what pizzas have a topping at Topping.Pizza_set--you have to look at the Pizza class to see this.



As a result, by looking at Toppings, I don't actually know the full range of fields that it has.



Is there any way around this or to make it more explicit? Or is there something that I'm missing?






python django django-models







share|improve this question













Relationships, particularly ManyToMany, in Django have always bothered me somewhat. In particular, since the relationship is only defined in one of the models, you can't tell from looking at the paired model what other relationships it might be hiding.



For example, from the Django Documentation:



class Topping(models.Model):

    # ...



class Pizza(models.Model):

    # ...

    toppings = models.ManyToManyField(Topping)


You can tell from looking at the code that I'd find out the relevant toppings for a pizza at Pizza.toppings. But you cannot tell that I would be able to tell what pizzas have a topping at Topping.Pizza_set--you have to look at the Pizza class to see this.



As a result, by looking at Toppings, I don't actually know the full range of fields that it has.



Is there any way around this or to make it more explicit? Or is there something that I'm missing?






python django django-models




python django django-models






share|improve this question













share|improve this question











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share|improve this question










asked Aug 8 '13 at 2:45









jdotjdot

9,09474486




9,09474486












  • Did you try Topping.Pizza_set.all() ?
    – Victor Castillo Torres
    Aug 8 '13 at 2:50










  • Topping.pizza_set.all() is how to get the actual pizzas, which is not the issue--the question is how to make it clear in the actual model that the way to get pizzas is by calling topping.pizza_set.<something>.
    – jdotjdot
    Aug 8 '13 at 5:07


















  • Did you try Topping.Pizza_set.all() ?
    – Victor Castillo Torres
    Aug 8 '13 at 2:50










  • Topping.pizza_set.all() is how to get the actual pizzas, which is not the issue--the question is how to make it clear in the actual model that the way to get pizzas is by calling topping.pizza_set.<something>.
    – jdotjdot
    Aug 8 '13 at 5:07
















Did you try Topping.Pizza_set.all() ?
– Victor Castillo Torres
Aug 8 '13 at 2:50




Did you try Topping.Pizza_set.all() ?
– Victor Castillo Torres
Aug 8 '13 at 2:50












Topping.pizza_set.all() is how to get the actual pizzas, which is not the issue--the question is how to make it clear in the actual model that the way to get pizzas is by calling topping.pizza_set.<something>.
– jdotjdot
Aug 8 '13 at 5:07




Topping.pizza_set.all() is how to get the actual pizzas, which is not the issue--the question is how to make it clear in the actual model that the way to get pizzas is by calling topping.pizza_set.<something>.
– jdotjdot
Aug 8 '13 at 5:07












2 Answers
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This seems to be an unavoidable side effect of the DRY principle. I don't know of any way to declaratively show the symmetry in these relations (other than by commenting and such). If you really want to make things explicit you could put the relationship in its own table (which Django is doing behind the scenes anyway), like:



class Topping(models.Model):

    # ...



class Pizza(models.Model):

    # ...



class PizzaToppings(models.Model):

    # '+' disables the reverse relationship

    pizza = models.ForeignKey(Pizza, related_name='+') 

    topping = models.ForeignKey(Topping, related_name='+')


... but of course then you'd lose some of the convenience of the ORM.






share|improve this answer





























    0














    Found a way on Django's forum (lost link, sorry)



    class Topping(models.Model):
    
    explicit_pizza_set = models.ManyToManyField(Pizza, through=Pizza.toppings.through, blank=True)
    

    
class Pizza(models.Model):
    
    toppings = models.ManyToManyField(Topping)





    share|improve this answer





















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      2 Answers
      2






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      2 Answers
      2






      active

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      0














      This seems to be an unavoidable side effect of the DRY principle. I don't know of any way to declaratively show the symmetry in these relations (other than by commenting and such). If you really want to make things explicit you could put the relationship in its own table (which Django is doing behind the scenes anyway), like:



      class Topping(models.Model):
      
    # ...
      

      
class Pizza(models.Model):
      
    # ...
      

      
class PizzaToppings(models.Model):
      
    # '+' disables the reverse relationship
      
    pizza = models.ForeignKey(Pizza, related_name='+') 
      
    topping = models.ForeignKey(Topping, related_name='+')


      ... but of course then you'd lose some of the convenience of the ORM.






      share|improve this answer


























        0














        This seems to be an unavoidable side effect of the DRY principle. I don't know of any way to declaratively show the symmetry in these relations (other than by commenting and such). If you really want to make things explicit you could put the relationship in its own table (which Django is doing behind the scenes anyway), like:



        class Topping(models.Model):
        
    # ...
        

        
class Pizza(models.Model):
        
    # ...
        

        
class PizzaToppings(models.Model):
        
    # '+' disables the reverse relationship
        
    pizza = models.ForeignKey(Pizza, related_name='+') 
        
    topping = models.ForeignKey(Topping, related_name='+')


        ... but of course then you'd lose some of the convenience of the ORM.






        share|improve this answer
























          0












          0








          0






          This seems to be an unavoidable side effect of the DRY principle. I don't know of any way to declaratively show the symmetry in these relations (other than by commenting and such). If you really want to make things explicit you could put the relationship in its own table (which Django is doing behind the scenes anyway), like:



          class Topping(models.Model):
          
    # ...
          

          
class Pizza(models.Model):
          
    # ...
          

          
class PizzaToppings(models.Model):
          
    # '+' disables the reverse relationship
          
    pizza = models.ForeignKey(Pizza, related_name='+') 
          
    topping = models.ForeignKey(Topping, related_name='+')


          ... but of course then you'd lose some of the convenience of the ORM.






          share|improve this answer












          This seems to be an unavoidable side effect of the DRY principle. I don't know of any way to declaratively show the symmetry in these relations (other than by commenting and such). If you really want to make things explicit you could put the relationship in its own table (which Django is doing behind the scenes anyway), like:



          class Topping(models.Model):
          
    # ...
          

          
class Pizza(models.Model):
          
    # ...
          

          
class PizzaToppings(models.Model):
          
    # '+' disables the reverse relationship
          
    pizza = models.ForeignKey(Pizza, related_name='+') 
          
    topping = models.ForeignKey(Topping, related_name='+')


          ... but of course then you'd lose some of the convenience of the ORM.







          share|improve this answer












          share|improve this answer



          share|improve this answer










          answered Aug 8 '13 at 3:07









          Kevin Christopher Henry

          22.5k46361




          22.5k46361

























              0














              Found a way on Django's forum (lost link, sorry)



              class Topping(models.Model):
              
    explicit_pizza_set = models.ManyToManyField(Pizza, through=Pizza.toppings.through, blank=True)
              

              
class Pizza(models.Model):
              
    toppings = models.ManyToManyField(Topping)





              share|improve this answer


























                0














                Found a way on Django's forum (lost link, sorry)



                class Topping(models.Model):
                
    explicit_pizza_set = models.ManyToManyField(Pizza, through=Pizza.toppings.through, blank=True)
                

                
class Pizza(models.Model):
                
    toppings = models.ManyToManyField(Topping)





                share|improve this answer
























                  0












                  0








                  0






                  Found a way on Django's forum (lost link, sorry)



                  class Topping(models.Model):
                  
    explicit_pizza_set = models.ManyToManyField(Pizza, through=Pizza.toppings.through, blank=True)
                  

                  
class Pizza(models.Model):
                  
    toppings = models.ManyToManyField(Topping)





                  share|improve this answer












                  Found a way on Django's forum (lost link, sorry)



                  class Topping(models.Model):
                  
    explicit_pizza_set = models.ManyToManyField(Pizza, through=Pizza.toppings.through, blank=True)
                  

                  
class Pizza(models.Model):
                  
    toppings = models.ManyToManyField(Topping)






                  share|improve this answer












                  share|improve this answer



                  share|improve this answer










                  answered Nov 11 at 23:12









                  Guillaume Lebreton

                  545516




                  545516






























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