Select MongoDB documents with custom random probability distribution - mongodb

Question

I have a collection that looks something like this:
[
{
"id": 1,
"tier": 0
},
{
"id": 2,
"tier": 1
},
{
"id": 3
"tier": 2
},
{
"id": 4,
"tier": 0
}
]
Is there a standard way to select n elements where the probabilty of choosing an element of the lowest tier is p, the next lowest tier is (1-p)*p, and so on, with standard random selection of element?
So for example, if the most likely thing happens and I run the query against the above example with n = 2 and any p > .5 (which I think will always be true), then I'd get back [{"id": 1, ...}, {"id": 4}]; with n = 3, then [{"id": 4}, {"id": 1}, {"id": 2}], etc.
E.g. here's some pseudo-Python code given a dictionary like that as objs:
def f(objs, p, n):
# get eligible tiers
tiers_set = set()
for o in objs:
eligible_tiers.add(o["tier"])
tiers_list = sorted(list(tiers_set))
# get the tier for each index of results
tiers = []
while len(tiers) < min(n, len(obis)):
tiers.append(select_random_with_initial_p(eligible_tiers, p))
# get res
res = []
for tier in tiers:
res.append(select_standard_random_in_tier(objs, tier)
return res

Answer 1

First, enable geospatial indexing on a collection:
db.docs.ensureIndex( { random_point: '2d' } )
To create a bunch of documents with random points on the X-axis:
for ( i = 0; i < 10; ++i ) {
db.docs.insert( { key: i, random_point: [Math.random(), 0] } );
}
Then you can get a random document from the collection like this:
db.docs.findOne( { random_point : { $near : [Math.random(), 0] } } )
Or you can retrieve several document nearest to a random point:
db.docs.find( { random_point : { $near : [Math.random(), 0] } } ).limit( 4 )
This requires only one query and no null checks, plus the code is clean, simple and flexible. You could even use the Y-axis of the geopoint to add a second randomness dimension to your query.
To make your custom random selection, you can change that part [Math.random(), 0], so it best suits your random distribution
Source: Random record from MongoDB