Question 117604 in Mongodb Question posted in
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Mongodb – Check for non-existing field in lookup pipeline

How to check if a field is not existing inside lookup expression? The answers in similar questions are not helpful (e.g. {$eq : null} or $exists:true).

For example, I want to lookup inventory only if disabled is not existing.

db.orders.aggregate([
  {
    $lookup: {
      from: "inventory",
      let: {item: "$item"},
      pipeline: [
        {
          $match: {
            $expr: {
              $and: [
                {
                  $eq: ["$sku", "$$item" ]
                },
                {
                  $eq: [ "$disabled", null ]
                }
              ]
            }
          }
        },
      ],
      as: "inv"
    }
  }
])

A playground sample is here

Tags:

2

Answers


  1. 0

    you could use:

    { $match: { someField: { $exists: true } } }

    before the look up, to filter out the documents that you do not want to look up

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  2. 0

    You can use $exists outside the $expr:

    db.orders.aggregate([
  {
    $lookup: {
      from: "inventory",
      let: {item: "$item"},
      pipeline: [
        {
          $match: {
            $and: [
              {$expr: {$eq: ["$sku", "$$item"]}},
              {disabled: {$exists: false}}
            ]
          }
        }
      ],
      as: "inv"
    }
  }
])

    See how it works on the playground example

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