How can I improve this many-to-many Django ORM query and model set?

Question

I have a Django query and some Python code that I'm trying to optimize because 1) it's ugly and it's not as performant as some SQL I could use to write it, and 2) because the hierarchical regrouping of the data looks messy to me.

So, 1. Is it possible to improve this to be a single query? 2. How can I improve my Python code to be more Pythonic?

Background

This is for a photo gallery system. The particular view is attempting to display the thumbnails for all photos in a gallery. Each photo is statically sized several times to avoid dynamic resizing, and I would like to also retrieve the URLs and "Size Type" (eg Thumbnail, Medium, Large) of each sizing so that I can Lightbox the alternate sizes without hitting the database again.

Entities

I have 5 models that are of relevance:

class Gallery(models.Model):
    Photos = models.ManyToManyField('Photo', through = 'GalleryPhoto', blank = True, null = True)

class GalleryPhoto(models.Model):
    Gallery = models.ForeignKey('Gallery')
    Photo = models.ForeignKey('Photo')
    Order = models.PositiveIntegerField(default = 1)

class Photo(models.Model):
    GUID = models.CharField(max_length = 32)

class PhotoSize(models.Model):
    Photo = models.ForeignKey('Photo')
    PhotoSizing = models.ForeignKey('PhotoSizing')
    PhotoURL = models.CharField(max_length = 1000)

class PhotoSizing(models.Model):
    SizeName = models.CharField(max_length = 20)
    Width = models.IntegerField(default = 0, null = True, blank = True)
    Height = models.IntegerField(default = 0, null = True, blank = True)
    Type = models.CharField(max_length = 10, null = True, blank = True)

So, the rough idea is that I would like to get all Photos in a Gallery through GalleryPhoto, and for each Photo, I want to get all the PhotoSizes, and I would like to be able to loop through and access all this data through a dictionary.

A rough sketch of the SQL might look like this:

Select PhotoSize.PhotoURL
From PhotoSize
Inner Join Photo On Photo.id = PhotoSize.Photo_id
Inner Join GalleryPhoto On GalleryPhoto.Photo_id = Photo.id
Inner Join Gallery On Gallery.id = GalleryPhoto.Gallery_id
Where Gallery.id = 5
Order By GalleryPhoto.Order Asc

I would like to turn this into a list that has a schema like this:

(
    photo: {
        'guid': 'abcdefg',
        'sizes': {
            'Thumbnail': 'http://mysite/image1_thumb.jpg',
            'Large': 'http://mysite/image1_full.jpg',
            more sizes...
        }
    },
    more photos...
)

I currently have the following Python code (it doesn't exactly mimic the schema above, but it'll do for an example).

gallery_photos = [(photo.Photo_id, photo.Order) for photo in GalleryPhoto.objects.filter(Gallery = gallery)]
photo_list = list(PhotoSize.objects.select_related('Photo', 'PhotoSizing').filter(Photo__id__in=[gallery_photo[0] for gallery_photo in gallery_photos]))

photos = {}
for photo in photo_list:
    order = 1
    for gallery_photo in gallery_photos:
        if gallery_photo[0] == photo.Photo.id:
            order = gallery_photo[1] //this gets the order column value

            guid = photo.Photo.GUID
            if not guid in photos:
                photos[guid] = { 'Photo': photo.Photo, 'Thumbnail': None, 'Sizes': [], 'Order': order }

            photos[guid]['Sizes'].append(photo)

    sorted_photos = sorted(photos.values(), key=operator.itemgetter('Order'))

The Actual Question, Part 1

So, my question is first of all whether I can do my many-to-many query better so that I don't have to do the double query for both gallery_photos and photo_list.

The Actual Question, Part 2

I look at this code and I'm not too thrilled with the way it looks. I sure hope there's a better way to group up a hierarchical queryset result by a column name into a dictionary. Is there?

Answer 1

When you have sql query, that is hard to write using orm - you can use postgresql views. Not sure about mysql. In this case you will have:

Raw SQL like:

CREATE VIEW photo_urls AS
Select
photo.id, --pseudo primary key for django mapper
Gallery.id as gallery_id, 
PhotoSize.PhotoURL as photo_url
From PhotoSize
Inner Join Photo On Photo.id = PhotoSize.Photo_id
Inner Join GalleryPhoto On GalleryPhoto.Photo_id = Photo.id
Inner Join Gallery On Gallery.id = GalleryPhoto.Gallery_id
Order By GalleryPhoto.Order Asc

Django model like:

class PhotoUrls(models.Model):
    class Meta: 
         managed = False 
         db_table = 'photo_urls'
    gallery_id = models.IntegerField()
    photo_url = models.CharField()

ORM Queryset like:

PhotoUrls.objects.filter(gallery_id=5)

Hope it will help.

Answer 2

Django has some built in functions that will clean up the way your code looks. It will result in subqueries, so I guess it depends on performance. https://docs.djangoproject.com/en/dev/ref/models/querysets/#django.db.models.query.QuerySet.values

gallery_photos = GalleryPhoto.objects.filter(Gallery=gallery).values('Photo_id', 'Order')
photo_queryset = PhotoSize.objects.selected_related('Photo', 'PhotoSizing').filter(
                 Photo__id__in=gallery_photos.values_list('Photo_id', flat=True))

calling list() will instantly evaluate the queryset, this might affect performance if you have a lot of data.

Additionally, there should be a rather easy way to get rid of if gallery_photo[0] == photo.Photo.id: This seems like it can be easily resolved with another query, getting gallery_photos for all photos.

Answer 3

You can retrieve all data with a single query, and get a list of data dictionaries. Then you can manage this dictionary or create a new one to form your final dictionary... You can use reverse relations in filtering and selecting specific rows from a table... So:

Let x be your selected Galery...

GalleryPhoto.objexts.filter(Galery=x).values('Order', 'Photo__GUID', 'Photo__Photo__PhotoURL', 'Photo__Photo__PhotoSizing__SizeName', 'Photo__Photo__PhotoSizing__Width', 'Photo__Photo__PhotoSizing__Height', 'Photo__Photo__PhotoSizing__Type')

Using Photo__ will create an inner join to Photo table while Photo__Photo__ will create inner join to PhotoSize (via reverse relation) and Photo__Photo__PhotoSizing__ will inner join to PhotoSizing ....

You get a list of dictionaries:

[{'Order':....,'GUID': ..., 'PhotoURL':....., 'SizeName':...., 'Width':...., 'Height':..., 'Type':...}, {'Order':....,'GUID': ..., 'PhotoURL':....., 'SizeName':...., 'Width':...., 'Height':..., 'Type':...},....]

You can select rows that you need and get all values as a list of dictionaries... Then you can Write a loop function or iterator to loop through this list and create a new dictionary whit grouping your data...