count hasMany relation in mongodb/mongoid -

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enter code herei"m working on rails app running mongodb via mongoid. let's have 2 collections , posts , comments, , they're linked habtm relation, i'd select posts have comments, , think way count comments each post , order comments.count desc. don't know how count number of comments each posts, add column posts results count(comments) comments_count in sql, , order "pseudo-field".

any thoughts on ?

thanks

edit, looking @ others answers related this, i've tried:

db.posts.aggregate([     { $group: {       _id: { post_id: '$post_id', comment_id: '$comment_id' }     }},     { $group: {       _id: '$_id.post_id',       comments_count: { $sum: 1 }     }}]     , function(err, result){        console.log(result);     }   );

i'm getting

{ "result" : [{         "_id" : null,         "datasets_count" : 1     }], "ok" : 1 }

the aggregation framework in mongodb has operators grouping, summing, , sorting. following working example of how group, count, , sort posts number of comments. hope want , helps.

models/post.rb

class post   include mongoid::document   has_many :comments   field :text, type: string end

models/comment.rb

class comment   include mongoid::document   belongs_to :post   field :text, type: string end

test/unit/post_test.rb

require 'test_helper' require 'pp' class posttest < activesupport::testcase   def setup     post.delete_all     comment.delete_all   end   test "posts ordered comment count"     [ [ "now time men come aid of country.",         [ "tally ho!" ] ],       [ "twas brillig, , slithy toves did gyre , gimble in wabe.",         [ "off heads!",           "the time has come, walrus said.",           "curiouser , curiouser." ] ],       [ "the quick brown fox jumped on lazy dog.",         [ "the typewriter mightier sword.",           "dachshund badger dog." ] ]     ].each |post, comments|       post = post.create(text: post)       comments.each{|comment| post.comments << comment.create(text: comment)}     end     pipeline = [         { '$group' => {             '_id' => '$post_id',             'count' => { '$sum' => 1 }           }         },         { '$sort' => { 'count' => -1}         }     ]     result = comment.collection.aggregate(pipeline).to_a.collect |post_id_count|       post.find(post_id_count['_id'])     end     puts     pp result   end end

rake test

running tests:

[1/1] posttest#test_posts_ordered_by_comment_count [#<post _id: 52717e7f7f11ba52d8000003, text: "twas brillig, , slithy toves did gyre , gimble in wabe.">,  #<post _id: 52717e7f7f11ba52d8000007, text: "the quick brown fox jumped on lazy dog.">,  #<post _id: 52717e7f7f11ba52d8000001, text: "now time men come aid of country.">] finished tests in 0.050494s, 19.8043 tests/s, 0.0000 assertions/s. 1 tests, 0 assertions, 0 failures, 0 errors, 0 skips

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