python - Why can't uniqueness be enforced on Django ManyToMany field? -

i'm creating simple binary rating system website, study materials website summaries can rated +1 or -1.

i've defined manytomany fields between summary model , user model so:

users_rated_positive = models.manytomanyfield(     user, blank=true, related_name='summaries_rated_positive') users_rated_negative = models.manytomanyfield(     user, blank=true, related_name='summaries_rated_negative') 

however obvious given user can not rate given summary more once.

i tried setting unique=true django doesn't seem let me that.

is there more optimal way store ratings aligns better intentions?

consider using manytomany through table.

class summary(models.model):     user_ratings = models.manytomanyfield(         'user', blank=true, through='userrating',     )   class userrating(models.model):     user = models.foreignkey('user')     summary = models.foreignkey('summary')     is_positive = models.booleanfield()      class meta:         unique_together = ('user', 'summary') 

then can query users reviewed summary positively like

summary.user_ratings.filter(is_positive=true) summary.user_ratings.filter(is_positive=false) 

the unique_together property ensures user can't review same summary twice.

you retain similar syntax using custom queryset. here's guess @ might like:

class summaryqueryset(models.queryset):      def users_rated_negative(self):         return self.user_ratings.filter(is_positive=false)      def users_rated_positive(self):         return self.user_ratings.filter(is_positive=true)   class summary(models.model):     user_ratings = models.manytomanyfield(         'user', blank=true, through='userrating',     )      objects = summaryqueryset.as_manager() 

now in-use syntax summary.users_rated_positive

a similar custom queryset user give summaries_rated_positive property, if needed too.