Add _meta to aggregation results (via $facet support)

[Wytamma]

I think _perform_aggregation could be modified with $facet to return a count value with the aggregation results. I know limitaions says that it's not the intended behaviour, however, I think it would be beneficial.

Here's how I would consider implementing:

def _perform_aggregation(resource, pipeline, options):
    ...
    paginated_results = []
    if req.max_results > 1:
        limit = {"$limit": req.max_results}
        skip = {"$skip": (req.page - 1) * req.max_results}
        paginated_results.append(skip)
        paginated_results.append(limit)
    else:
        # sub-pipeline in $facet stage cannot be empty
        skip = {"$skip": 0}
        paginated_results.append(skip)
    
    facet_pipelines = {}
    facet_pipelines["paginated_results"] = paginated_results
    facet_pipelines["total_count"] = [{"$count": "count"}]

    facet = {"$facet":facet_pipelines}

    req_pipeline.append(facet)

    getattr(app, "before_aggregation")(resource, req_pipeline)

    cursor = app.data.aggregate(resource, req_pipeline, options).next()

    for document in cursor['paginated_results']:
        documents.append(document)

    getattr(app, "after_aggregation")(resource, documents)

    response[config.ITEMS] = documents

    # add pagination info
    if config.DOMAIN[resource]["pagination"]:
        count = cursor['total_count'][0]['count']
        response[config.META] = _meta_links(req, count)

    return response, None, None, 200, []

This modification would change the aggregate pipeline considerably, so maybe a config variable could be used to control this behaviour for those already hooking into "before_aggregation".

Am I missing anything obvious? Should I try implementing this and submitting a PR?

[nicolaiarocci]

Hi,

First, sorry for the late answer. This looks interesting, feel free to submit a PR and I will be happy to review it.