javascript - Return limited number of records of a certain type, but unlimited number of other records?

Question

I have a query where I need to return 10 of "Type A" records, while returning all other records. How can I accomplish this?

Update: Admittedly, I could do this with two queries, but I wanted to avoid that, if possible, thinking it would be less overhead, and possibly more performant. My query already is an aggregation query that takes both kinds of records into account, I just need to limit the number of the one type of record in the results.

Update: the following is an example query that highlights the problem:

db.books.aggregate([
    {$geoNear: {near: [-118.09771, 33.89244], distanceField: "distance", spherical: true}},
    {$match:    {"type": "Fiction"}},
    {$project:  {
        'title': 1,
        'author': 1,
        'type': 1,
        'typeSortOrder': 
            {$add: [
                {$cond: [{$eq: ['$type', "Fiction"]}, 1, 0]},
                {$cond: [{$eq: ['$type', "Science"]}, 0, 0]},
                {$cond: [{$eq: ['$type', "Horror"]}, 3, 0]}
        ]},
    }},
    {$sort: {'typeSortOrder'}},
    {$limit: 10}
])

db.books.aggregate([
    {$geoNear: {near: [-118.09771, 33.89244], distanceField: "distance", spherical: true}},
    {$match:    {"type": "Horror"}},
    {$project:  {
        'title': 1,
        'author': 1,
        'type': 1,
        'typeSortOrder': 
            {$add: [
                {$cond: [{$eq: ['$type', "Fiction"]}, 1, 0]},
                {$cond: [{$eq: ['$type', "Science"]}, 0, 0]},
                {$cond: [{$eq: ['$type', "Horror"]}, 3, 0]}
        ]},
    }},
    {$sort: {'typeSortOrder'}},
    {$limit: 10}
])

db.books.aggregate([
    {$geoNear: {near: [-118.09771, 33.89244], distanceField: "distance", spherical: true}},
    {$match:    {"type": "Science"}},
    {$project:  {
        'title': 1,
        'author': 1,
        'type': 1,
        'typeSortOrder': 
            {$add: [
                {$cond: [{$eq: ['$type', "Fiction"]}, 1, 0]},
                {$cond: [{$eq: ['$type', "Science"]}, 0, 0]},
                {$cond: [{$eq: ['$type', "Horror"]}, 3, 0]}
        ]},
    }},
    {$sort: {'typeSortOrder'}},
    {$limit: 10}
])

I would like to have all these records returned in one query, but limit the type to at most 10 of any category. I realize that the typeSortOrder doesn't need to be conditional when the queries are broken out like this, I had it there for when the queries were one query, originally (which is where I would like to get back to).

Answer 1

I don't think this is presently (2.6) possible to do with one aggregation pipeline. It's difficult to give a precise argument as to why not, but basically the aggregation pipeline performs transformations of streams of documents, one document at a time. There's no awareness within the pipeline of the state of the stream itself, which is what you'd need to determine that you've hit the limit for A's, B's, etc and need to drop further documents of the same type. $group does bring multiple documents together and allows their field values in aggregate to affect the resulting group document ($sum, $avg, etc.). Maybe this makes some sense, but it's necessarily not rigorous because there are simple operations you could add to make it possible to limit based on the types, e.g., adding a $push x accumulator to $group that only pushes the value if the array being pushed to has fewer than x elements.

Even if I did have a way to do it, I'd recommend just doing two aggregations. Keep it simple.