CODEWITHSUNDEEP
mongodbmongooseaggregation-framework
uL1
uL1

Reputation: 2167

MongoDB - Update / Replace array field with subdocument to embed its content

Description

After reworking my data construct, I want to embed a document (or parts of it) from a different collection. Pay your attention to event.tags[]. Currently, there are only their stringified _ids (as plain string, no ObjectId!) stored, but I want to migrate their content, like {tags.name, tags._id}. I want to update/replace the original event.tags with the content of tag-documents.

Data

Collection "events"

db.events.insertMany([{
    "_id" : ObjectId("5f6c5b0594911c0b7231643d"),
    "title" : "Test Event",
    "tags" : [ 
        "5fd8cef2368c625a9d7516cb"
    ]
},
{
    "_id" : ObjectId("5fb3896afc13ae2ed1000000"),
    "title" : "Test Event 2",
    "tags" : [ 
        "5fd8cef2368c625a9d7516cb", 
        "5fd8cfb04a4a6063ab4ca4bf", 
        "5fd8cfb04a4a6063ab4ca4c0"
    ]
}
]);

Collection "tags"

db.tags.insertMany([{
    "_id" : ObjectId("5fd8cef2368c625a9d7516cb"),
    "name" : "Foo",
    "createdAt" : ISODate("2020-12-15T14:57:54.096Z")
},
{
    "_id" : ObjectId("5fd8cfb04a4a6063ab4ca4bf"),
    "name" : "Bar",
    "createdBy" : "[email protected]",
    "createdAt" : ISODate("2020-12-16T14:57:54.096Z")
},
{
    "_id" : ObjectId("5fd8cfb04a4a6063ab4ca4c0"),
    "name" : "Foobar",
    "createdAt" : ISODate("2020-12-17T14:57:54.096Z")
}
]);

Expected result

I want to achieve to embed some/all fields of collection "tags" matched by "events.tags" <-> "tags._id" as an array of embedded/nested documents. This will be part of a migration script, running over mongoDB API >4.0 thru node.js.

Goal: I want to replace an Array of Strings (see collection events) with certain information of a DIFFERENT collection (see collection tags) and save it back to collection events (migrate process). So I assume, I have to fetch/lookup something from tag collection and need a mechanism to replace the string (events.tags[x]) with complex objects ({tags._id, tags.name})

{
    "_id" : ObjectId("5f6c5b0594911c0b7231643d"),
    "title" : "Test Event",
    "tags" : [{
        "_id" : ObjectId("5fd8cef2368c625a9d7516cb"),
        "name" : "Foo",
        "createdAt" : ISODate("2020-12-15T14:57:54.096Z")
    }]
},
{
    "_id" : ObjectId("5fb3896afc13ae2ed1000000"),
    "title" : "Test Event 2",
    "tags" : [{
        "_id" : ObjectId("5fd8cef2368c625a9d7516cb"),
        "name" : "Foo"
    },
    {
        "_id" : ObjectId("5fd8cfb04a4a6063ab4ca4bf"),
        "name" : "Bar"
    },
    {
        "_id" : ObjectId("5fd8cfb04a4a6063ab4ca4c0"),
        "name" : "Foobar"
    }]
}

Baby steps

I tried to use the aggregation pipeline and started by matching and lookup their content to replaceWith in the last step. I am not too familiar to solve this via the aggregation pipeline.

db.getCollection('events').aggregate([
    { $match: { tags: { "$exists" : true } } },
    {
       $lookup:
         {
           from: "tags",
           localField: "tags",
           foreignField: "_id",
           as: "taags"
         }
    },
    ])

Upvotes: 1

Views: 819

Answers (1)

turivishal
turivishal

Reputation: 36104

You can convert tag's id from string to objectId using $toObjectId operator, before $lookup stage,

  • $addFields to add new fields and update/format existing fields
  • $map to iterate loop of tags array
  • $toObjectId to convert string objectId to objectId type
  • put the name tags in as property of $lookup stage
  // $match stage here
  {
    $addFields: {
      tags: {
        $map: {
          input: "$tags",
          in: { $toObjectId: "$$this" }
        }
      }
    }
  },
  {
    $lookup: {
      from: "tags",
      localField: "tags",
      foreignField: "_id",
      as: "tags"
    }
  }

Playground

  • With the aggregation query you can not update current events collection, but you can use $out stage to result in new collection, supported from MongoDB 4.4,
  • put this stage at last stage
  { $out: "updated_events" }

The second option is to prepare a script to do the manual process,

  1. Find all records from the events collection
  2. loop the results of events collection
  3. inside loop execute find query by passing tags ids in tags collection
  4. inside loop update events collection by replacing result from 3) step

Upvotes: 1

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