Optimize pagination of relay connection fields

Copied from https://github.com/tfoxy/graphene-django-optimizer/issues/1#issuecomment-419340157.

@tfoxy Do you use optimizers with any pagination? We’re also using Apollo as the client and we want to provide cursor-based pagination. But we also have queries with nested connections e.g.:

{
  products(first: 10) {
    edges {
      node {
        name
        variants(first: 5) {
          edges {
            node {
              name
            }
          }
        }
      }
    }
  }
}

Using just the ORM I’m able to fetch the data only with two DB queries:

Product.objects.prefetch_related('variants')

I’m trying to achieve the same in the API but unfortunately any pagination seems to break the prefetches as they do slicing on querysets internally which results in refetching objects that were already cached by prefetch_related.

[maarcingebala]

@nwaxiomatic Here is an example usage of optimization hints in my project. There is a Product model and each product can have multiple images (1-N relationship). From these images, we choose one as a product thumbnail to display in the UI.

Now, if we had a query that fetches first 5 products and a thumbnail for each of them:

{
  products(first: 5) {
    edges {
      node {
        name
        thumbnailUrl
      }
    }
  }
}

• Without optimization hints For each product, Django would do a query to fetch all its images and return one as a thumbnail.

• With optimization hints @gql_optimizer.resolver_hints(prefetch_related='images') tells Graphene to prefetch images relation when the resolve_thumbnail_url resolver is used. As a result, each time the thumbnail field is used, I’m able to take advantage of prefetched images and avoid duplicated database queries.

[maarcingebala]

@tfoxy Yes it does, but I still can benefit from prefetching data in one query instead of doing duplicated queries for each row of data.

Anyway, I managed to use prefetching by using optimization hints and the slightly modified version of DjangoConnectionField, as suggested in this comment. I’ll keep experimenting with it. For now, I’d consider my question resolved, so you can close the issue.