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How to unset field(array) if its length after $pull operation equals 0 in MongoDB?

I need to do it using only 1 query. Is it possible?

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1 Answer 1

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There is no way in a regular update query, but You can try update with aggregation pipeline starting from MongoDB 4.2,

Consider this is a sample document and you want to pull "1" from array field:

{
  "array": [1]
}
  • $filter to iterate loop of array and filter array that is not equal to "1"
  • $cond to check after pull, is array field empty then remove by $$REMOVE otherwise nothing to do
db.collection.update(
  { "array": 1 },
  [{
    $set: {
      array: {
        $filter: {
          input: "$array",
          cond: { $ne: ["$$this", 1] }
        }
      }
    }
  },
  {
    $set: {
      array: {
        $cond: [
          { $eq: ["$array", []] },
          "$$REMOVE",
          "$array"
        ]
      }
    }
  }]
)

Playground

The second alternate option,

  • $cond to check is array equal to the single element that we are going to pull [1] then remove array field otherwise go to $filter operation to remove single element
db.collection.update(
  { "array": 1 },
  [{
    $set: {
      array: {
        $cond: [
          { $eq: ["$array", [1]] },
          "$$REMOVE",
          {
            $filter: {
              input: "$array",
              cond: { $ne: ["$$this",1] }
            }
          }
        ]
      }
    }
  }]
)

Playground

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