Question 115327 in Mongodb Question posted in
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Mongodb – Why $lookup requires $expr if I add pipeline and make a variable?

I’ve been trying to get data from collection without using $expr inside $match pipeline stage because it’s less performant and compared to simple $match

Here’s a piece of code I’ve been working on:

        from: 'messages',
        let: { "iId": "$_id" },
        pipeline: [
          {
            $match: {
              $expr: {
                $and: [
                  { $eq: ["$interactionId", '$$iId'] },
                  { $eq: ["$author.role", 'agent'] }
                ]
              }
            }
          }
        ],
        as: "messages"
      }
    },

I am expecting to change this condition to simply this:

    {
      $lookup: {
        from: 'messages',
        pipeline: [
          {
            $match: {
              "interactionId": "$_id",
              "author.role": "agent"
            }
          },
        ],
        as: "messages"
      }
    },
Tags:

2

Answers


  1. 0

    One option might be:

    ...
{
  $lookup: {
    from: 'messages',
    localField: "_id",
    foreignField: "interactionId",
    pipeline: [
      {
        $match: {
          "author.role": "agent"
        }
      }
    ],
    as: "messages"
  }
},
...
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  2. 0

    pipeline inside $lookup doesn’t have any reference to parent collection data, it only has the reference to current data.
    so you always have to use let variables to access any data from the parent

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