Understanding Django GenericForeignKey and GenericRelation

Question

I\'m building vocabulary and have following models:

class Word(Model):
 name = CharField(max_length=75)

class EnNoun(Model):
 word = OneToOneField(Word)

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Answer 1

GenericForeignKey try to give you a ForeignKey behavior but instead to be against one type of object, they do it for a set of object types (thats why they are defined with 2 columns, 1 to keep the primary_key and another to keep the contenty_type).

GenericRelation is the reverse relation of a GenericForeignKey, because Django do not automatically create reverse relations for GenericForeignKeys (unlike ForeignKeys) you have to setup them manually.

I'm not very familiar with the best-practices in translations/vocabulary staffs, but if you want to approach the problem with GenericRelations and GenericForeignKeys, one way to do it would be:

class Word(Model):
    name = CharField(max_length=75)
    nouns = GenericRelation('WordNoun', content_type_field='noun_ct', object_id_field='noun_id')

class WordNoun(Model):
    word = ForeignKey(Word)
    noun_ct = ForeignKey(ContentType,
        on_delete=models.CASCADE,
        #this is optional
        limit_choices_to = {"model__in": ('EnNoun', 'FrNoun')}
    )
    noun_id = PositiveIntegerField()
    noun = GenericForeignKey('noun_ct', 'noun_id')

class EnNoun(Model):
    word = OneToOneField(Word)

class FrNoun(Model):
    word = ForeignKey(Word)
    gender = CharField()

We are basically creating a model keeping word-noun relations, this give is the following

# Having some word
word = Word.objects.get(pk=1)

# With 1 query you can get a list with
# all WordNoun objects for this word.
word_nouns = word.nouns.all()

The problem with this approach is that after you get the word_nouns list, accessing a single noun instance will make a new query.

for word_noun in word.nouns.all():
    print word_noun.noun #this will make a query

One way to slightly optimize this is to use prefetch_related, so if a single word has 3 word_nouns (lets say 1 EnNoun and 2 FrNoun).

Then instead of 4 queries - 1 for the word_nouns and 3 for each noun, we optimize it to 3 queries - 1 for the word_nouns and 2 for each contenty_type (EnNoun and FrNoun)

for word_noun in word.nouns.all().prefetch_related('noun'):
    print word_noun.noun #this will not make a query

The difference is that the number of queries will now depends on the number of different ContentTypes, rather than the number of related WordNoun objects.

This scales nice when you have several Nouns from one contenty_type in the list you are prefetching, but will make no difference if you have 1 Noun percontenty_type`.

Another approach which I can think of is to use the following model structure:

class Word(Model):
    name = CharField(max_length=75)

class WordNoun(Model):
    LANG_CHOICES = (
        ('en', 'English'),
        ('fr', 'French'),
    )
    word = ForeignKey(Word)
    lang = models.CharField(max_length=2, choices=LANG_CHOICES)
    gender = CharField(max_length=2, blank=True, default='')


#Now accessing all word_nouns would as easy as:
word_nouns = word.wordnoun_set.all()

Answer 2

It is possible to add several GenericForeignKey to the same model. This would allow you to create a "link" between objects with generic type. I give an example latter. I modified your example to create something more useful in my sense. The table Translation create links between French and English words. Words are stored in classes EnVerb, FrVerb, EnNoun and FrNoun. Without GenericForeignKey, you would have to create two translation models: TranslationVerb and TranslationNoun. But in the following code, I show a generic translation model working for both verbs, and nouns. I hope it makes sense!

More technically, you forgot to add the GenericRelation field. Also, they must specify the new names of the fields content_type and object_id in the GenericRelation field of the related objects.

EXAMPLE

from django.db import models
from django.contrib.contenttypes.fields import GenericRelation
from django.contrib.contenttypes.fields import GenericForeignKey
from django.contrib.contenttypes.models import ContentType

Translation(models.Model):
    fr_content_type = models.ForeignKey(ContentType, on_delete=models.CASCADE, related_name="fr_content_type")
    fr_object_id = models.PositiveIntegerField()
    fr_word = GenericForeignKey('fr_content_type', 'fr_object_id')

    en_content_type = models.ForeignKey(ContentType, on_delete=models.CASCADE, related_name="en_content_type")
    en_object_id = models.PositiveIntegerField()
    en_word = GenericForeignKey('en_content_type', 'en_object_id')

class FrVerb(models.Model):
    name = models.CharField(max_length=75)
    translation = GenericRelation(Translation, content_type_field='fr_content_type', object_id_field='fr_object_id')

class FrNoun(models.Model):
    name = models.CharField(max_length=75)
    gender = models.CharField(max_length=75)
    translation = GenericRelation(Translation, content_type_field='fr_content_type', object_id_field='fr_object_id')

class EnVerb(models.Model):
    name = models.CharField(max_length=75)
    translation = GenericRelation(Translation, content_type_field='en_content_type', object_id_field='en_object_id')

class EnNoun(models.Model):
    name = models.CharField(max_length=75)
    translation = GenericRelation(Translation, content_type_field='en_content_type', object_id_field='en_object_id')

You can use this to create generic translations between models :

from polls.models import *      

EnNoun(name='tree').save()                                                                                                                       
FrNoun(name='arbre').save()                                                                                                                      
EnVerb(name='be').save()                                                                                                                         
FrVerb(name='etre').save()                                                                                                                       

trans1 = Translation(fr_word=FrNoun.objects.first(), en_word=EnNoun.objects.first())                                                              
trans2 = Translation(fr_word=FrVerb.objects.first(), en_word=EnVerb.objects.first())

Now trans1 is used to link 'tree' and 'arbre' and trans2 to link 'be' et 'être' while these objects belongs to different models !