Add field to result

Last 20260123.md journal, I provided an example of duplicate documents when two pipelines are unioned. Now, to identify which document is a result of a particular pipeline, we can use $addFields aggregation stage. In the result, we can see which pipeline a document is associated to. This works by adding a new field to each document. In this case, we’re assigning src property with string literal value.

[
  {
    $match: {
      gender: { $regex: ".*ale" }
    }
  },
  { $addFields: { src: "pipeline1" } },
  {
    $unionWith: {
      coll: "persons",
      pipeline: [
        {
          $match: {
            gender: { $regex: ".*le" }
          },
        },
        { $addFields: { src: "pipeline2" } },
      ]
    }
  },
  { $sort: { _id: 1 } }
]

Sample result

[
	{
		"_id": {
			"$oid": "696e23bbbea06b125b46d15e"
		},
		"id": 2,
		"first_name": "Ashlee",
		"last_name": "Alchin",
		"email": "aalchin1@fema.gov",
		"gender": "Female",
		"ip_address": "157.142.117.33",
		"src": "pipeline2"
	},
	{
		"_id": {
			"$oid": "696e23bbbea06b125b46d15e"
		},
		"id": 2,
		"first_name": "Ashlee",
		"last_name": "Alchin",
		"email": "aalchin1@fema.gov",
		"gender": "Female",
		"ip_address": "157.142.117.33",
		"src": "pipeline1"
	}
]