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Editorial Team
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Asked: 2026-06-13T17:20:58+00:00

How to build a query that returns all entities sorted desc by count embedded

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How to build a query that returns all entities sorted desc by count embedded post ids?

Sample data:

{ "_id" : ObjectId( "5090f8061e7bf28095000012" ),
  "name" : "test",
  "post_ids" : 
    [ ObjectId( "5090f8061e7bf28095000010" ) ],
    [ ObjectId( "5090f8061e7bf28095000011" ) ] },
{ "_id" : ObjectId( "5090f8061e7bf28095000012" ),
  "name" : "test2",
  "post_ids" : [ 
    ObjectId( "5090f8061e7bf28095000010" ), 
    ObjectId( "509104ec1e7bf2989b00000b" ), 
    ObjectId( "509105001e7bf2989b00000f" ) ] }

#my actual query returns only count:
tags_by_count = Tag.desc(:post_ids).count
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1 Answer

  1. Editorial Team

    I don’t think this is possible in MongoDB. Everything I’m reading suggests creating a second field for the length of the array and indexing that.

    This is a little old, but might still be relevant:

    https://groups.google.com/forum/?fromgroups=#!topic/mongodb-user/zSSnj6ldfLE

    Also, even if it were possible, it would be slow because it would require using javascript instead of an index sort, since you definitely can’t index array length.

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