How to check if an array field is a part of another array in MongoDB?

Question

In this question, Jeff the Bear explained how to search documents with an array

contains \'tag1\'
contains [\'tag1\',\'tag2\'],
contains any of [\'tag3\', \'         


        

Answer 1

You can use aggregation framework to do this. Given the following data

> db.post.find()
{ "_id" : 1, "tags" : [ "tag1" ] }
{ "_id" : 2, "tags" : [ "tag1", "tag3" ] }
{ "_id" : 3, "tags" : [ "tag2", "tag4" ] }
{ "_id" : 4, "tags" : [ "tag1", "tag2", "tag3", "tag4" ] }

the aggregation query

db.post.aggregate({
  $project: {
    _id: 1,
    tags: 1,
    killFlag: {
      $const: [true, false]
    }
  }
}, {
  $unwind: "$tags"
}, {
  $unwind: "$killFlag"
}, {
  $match: {
    $nor: [{
        tags: {
          $in: ['tag1', 'tag2', 'tag4']
        },
        killFlag: true
      }
    ]
  }
}, {
  $group: {
    _id: "$_id",
    tags: {
      $addToSet: "$tags"
    },
    killFlag: {
      $max: "$killFlag"
    }
  }
}, {
  $match: {
    killFlag: false
  }
}, {
  $project: {
    _id: 1,
    tags: 1
  }
})

would give you

{
  "result": [{
      "_id": 3,
      "tags": [
          "tag4",
          "tag2"
      ]
    }, {
      "_id": 1,
      "tags": [
          "tag1"
      ]
    }
  ],
  "ok": 1
}