How to replace root with an array field during MongoDB aggregation pipeline?

Question

Let's say I have a users collections, with a roleIds field containing an array of Role references.

db.users.aggregate([
  {$match:{ _id: ObjectId('5f9453b4484d206714c02a2f') }}, 
  {$project:{ roleIds: 1, _id: 0 }}, 
  {$unwind: "$roleIds"}, 
  {$lookup:{ from: "roles", localField: "roleIds", foreignField: "_id", as: "roles"}}, // <= STEP 4
  {$replaceRoot: "$roles"}
])

After STEP 4, I have something like this:

{ 
  "roles" : [ 
    { "_id" : ObjectId("xxxx"), "name" : "role1" },
    { "_id" : ObjectId("xxxx"), "name" : "role2" },
  ]
}

How can I transform it to this:

[ 
  { "_id" : ObjectId("xxxx"), "name" : "role1" },
  { "_id" : ObjectId("xxxx"), "name" : "role2" },
]

The replaceRoot stage seems to work only if the roles field is a document, not an array, It throws an error in this case.

{ $unwind: "$roles" } deconstruct array before $replaceRoot. — turivishal
– turivishal, Commented Oct 24, 2020 at 17:14

acmoune · Accepted Answer · 2020-10-24 22:53:02Z

5

This works:

db.users.aggregate([
  {$match:{ _id: ObjectId('5f9453b4484d206714c02a2f') }}, 
  {$project:{ roleIds: 1, _id: 0 }}, 
  {$unwind: "$roleIds"}, 
  {$lookup:{ from: "roles", localField: "roleIds", foreignField: "_id", as: "roles"}},
  {$unwind: "$roles"},
  {$replaceRoot: { newRoot: "$roles" }}
])

answered Oct 24, 2020 at 22:53

acmoune

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