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  1. Core Server
  2. SERVER-18794

Add an aggregation expression to convert an object to an array of key, value pairs

    Details

    • Backwards Compatibility:
      Fully Compatible
    • Backport Requested:
      v3.4
    • Sprint:
      Query 2017-03-06

      Description

      In many cases, it would be useful to convert an object to an array. We should add an aggregation expression to do so. For example, it could be called $objectToArray, and do something like this:

      > db.foo.insert({_id: 0, subDoc: {a: 1, b: 3, c: "example"}});
      > db.foo.aggregate([{$project: {expanded: {$objectToArray: "$subDoc"}}}])
      {_id: 0, expanded: [{k: "a", v: 1}, {k: "b", v: 3}, {k: "c", v: "example"}]}
      

      Original Description

      Currently, if a collection contains the following document:

      {_id: 0, data: {a: 1, b: 2, c: 3}}
      

      There is no way to unwind that one document into the following three documents:

      {_id: 0, data: {a: 1}}
      {_id: 0, data: {b: 2}}
      {_id: 0, data: {c: 3}}
      

      One way to accomplish this would be to add a stage to unwind documents, e.g.

      db.coll.aggregate([{$objectToArray: {field: '$data'}}])
      

      Which would yield the following document, which could then be unwound to produce the desired result:

      {_id: 0, data: [{a: 1}, {b: 2}, {c: 3}]}
      

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