Proper way to bulk_create for ManyToMany field, Django?

Question

I have this code for table populating.

def add_tags(count):
    print "Add tags"
    insert_list = []
    photo_pk_lower_bound = Photo.objects.all().order_by("id")[0].pk
    photo_pk_upper_bound = Photo.objects.all().order_by("-id")[0].pk
    for i in range(count):
        t = Tag( tag = 'tag' + str(i) )
        insert_list.append(t)
    Tag.objects.bulk_create(insert_list)
    for i in range(count):
        random_photo_pk = randint(photo_pk_lower_bound, photo_pk_upper_bound)
        p = Photo.objects.get( pk = random_photo_pk )
        t = Tag.objects.get( tag = 'tag' + str(i) )
        t.photos.add(p)

And this is the model:

class Tag(models.Model):
    tag = models.CharField(max_length=20,unique=True)
    photos = models.ManyToManyField(Photo)

As I understand this answer : Django: invalid keyword argument for this function I have to save tag objects first (due to ManyToMany field) and then attach photos to them through add(). But for large count this process takes too long. Are there any ways to refactor this code to make it faster?

In general I want to populate Tag model with random dummy data.

EDIT 1 (model for photo)

class Photo(models.Model):
    photo = models.ImageField(upload_to="images")
    created_date = models.DateTimeField(auto_now=True)
    user = models.ForeignKey(User)

    def __unicode__(self):
       return self.photo.name

Answer 1

TL;DR Use the "through" model to bulk insert m2m relationships.

"Tag.photos.through" => Django generated Model with 3 fields [ id, photo, tag ]
photo_tag_1 = Tag.photos.through(photo_id=1, tag_id=1)
photo_tag_2 = Tag.photos.through(photo_id=1, tag_id=2)
Tag.photos.through.objects.bulk_insert([photo_tag_1, photo_tag_2, ...])

---

This is the fastest way that I know of, I use this all the time to create test data. I can generate millions of records in minutes.

Edit from Georgy:

def add_tags(count):
    Tag.objects.bulk_create([Tag(tag='tag%s' % t) for t in range(count)])

    tag_ids = list(Tag.objects.values_list('id', flat=True))
    photo_ids = Photo.objects.values_list('id', flat=True)
    tag_count = len(tag_ids)
       
    for photo_id in photo_ids:
        tag_to_photo_links = []
        shuffle(tag_ids)

        rand_num_tags = randint(0, tag_count)
        photo_tags = tag_ids[:rand_num_tags]

        for tag_id in photo_tags:
            # through is the model generated by django to link m2m between tag and photo
            photo_tag = Tag.photos.through(tag_id=tag_id, photo_id=photo_id)
            tag_to_photo_links.append(photo_tag)

        Tag.photos.through.objects.bulk_create(tag_to_photo_links, batch_size=7000)

I didn't create the model to test, but the structure is there you might have to tweaks some stuff to make it work. Let me know if you run into any problems.

[edited]

Answer 2

As shown in Du D's answer, Django ManyToMany fields use a table called through that contains three columns: the ID of the relation, the ID of the object linked to and the ID of the object linked from. You can use bulk_create on through to bulk create ManyToMany relations.

As a quick example, you could bulk create Tag to Photo relations like this:

tag1 = Tag.objects.get(id=1)
tag2 = Tag.objects.get(id=2)
photo1 = Photo.objects.get(id=1)
photo2 = Photo.objects.get(id=2)


through_objs = [
    Tag.photos.through(
        photo_id=photo1.id,
        tag_id=tag1.id,
    ),
    Tag.photos.through(
        photo_id=photo1.id,
        tag_id=tag2.id,
    ),
    Tag.photos.through(
        photo_id=photo2.id,
        tag_id=tag2.id,
    ),
]
Tag.photos.through.objects.bulk_create(through_objs)

General solution

Here is a general solution that you can run to set up ManyToMany relations between any list of object pairs.

from typing import Iterable
from collections import namedtuple


ManyToManySpec = namedtuple(
    "ManyToManySpec", ["from_object", "to_object"]
)


def bulk_create_manytomany_relations(
    model_from,
    field_name: str,
    model_from_name: str,
    model_to_name: str,
    specs: Iterable[ManyToManySpec]
):
    through_objs = []
    for spec in specs:
        through_objs.append(
            getattr(model_from, field_name).through(
                **{
                    f"{model_from_name.lower()}_id": spec.from_object.id,
                    f"{model_to_name.lower()}_id": spec.to_object.id,
                }
            )
        )
    getattr(model_from, field_name).through.objects.bulk_create(through_objs)

Example usage

tag1 = Tag.objects.get(id=1)
tag2 = Tag.objects.get(id=2)
photo1 = Photo.objects.get(id=1)
photo2 = Photo.objects.get(id=2)

bulk_create_manytomany_relations(
    model_from=Tag,
    field_name="photos",
    model_from_name="tag",
    model_to_name="photo",
    specs=[
        ManyToManySpec(from_object=tag1, to_object=photo1),
        ManyToManySpec(from_object=tag1, to_object=photo2),
        ManyToManySpec(from_object=tag2, to_object=photo2),
    ]
)