gistfile1.txt

The index I've created
db.gift_ideas.ensureIndex({"sales_rank":1, "rated_by":1, "random":1});

The query, in it's fast form, but that does not return gift_ideas as ordered by the random field
db.gift_ideas.find({"sales_rank":{$lt:1200}, "rated_by":{$nin:["100001450582916"]}, "random":{$gt:0.6071995101094684}}).limit(40)

The query, in it's dog-slow form, where I enforce the sorting
db.gift_ideas.find({"sales_rank":{$lt:1200}, "rated_by":{$nin:["100001450582916"]}, "random":{$gt:0.6071995101094684}}).limit(40).sort({"random":1})

Field Info for gift_ideas:
sales_rank - Int
rated_by - Array
random - Float

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So suspecting that the problem is with sales_rank, I tried this instead:
db.gift_ideas.ensureIndex({"rated_by":1, "random":1});

Now when I run:
db.gift_ideas.find({"rated_by":{$nin:["100001450582916"]}, "random":{$gt:0.6071995101094684}}).limit(40)

It's fast as hell, but it isn't even using my index.  As a result, still no sorting

The query, in it's dog-slow form, where I enforce the sorting
db.gift_ideas.find({"rated_by":{$nin:["100001450582916"]}, "random":{$gt:0.6071995101094684}}).limit(40).sort({"random":1})

Again, no index usage, just BasicCursor.  So it's still too slow even when not doing that I need it to do.

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So I was able to force the use of the index, which accomplished what I wanted like so:
db.gift_ideas.find({"rated_by":{$nin:["100001450582916"]}, "random":{$gt:0.6071995101094684}})..hint({"rated_by":1,"random":1}).limit(40)

So the next plan I guess will be to just over-select, and then weed out the bad ideas, and just query iteratively until I have what I need.

I can do it in 2 shots, like this, and then do a simple select in memory from there.
db.gift_ideas.find({"rated_by":{$nin:["100001450582916"]}, "random":{$gt:0.6071995101094684}},{"sales_rank":1,"random":1}).limit(100)