How do I use mongo aggregation to loop through array and return needed documents?

Question

I have a query where I need to find a document by ID, I then need to only return items inside of an array that match certain conditions.

In the code below you will see that I need to only "match" one document by ID, I am then putting a "limit" of one (there should only be one). This is were I really get confused...I am then "unwind"ing the fragments field which is the array, inside the array are subdocuments. I need to filter out these subdocuments by the last item of an array inside of it, so I "project" a "slice" of the assignments_array. This is were i only need the documents where the assignments_array is EMPTY, or where assignment_history.to is null

db.getCollection('territories').aggregate([
    {
      "$match": {
        "congregation": ObjectId("5c68c706f1f52047f08862b3")
      }
    },
    {
      "$limit": 1
    },

    { 
      $unwind: {
        path: "$fragments"
      }
    },
    {
      $project: {
        "fragments.number": 1,
        "fragments.assignment_history": {"$slice": ["$fragments.assignment_history", -1]}
      }
    }

This gets me this result...

{
  "_id": ObjectId("5c68c706f1f52047f08862b6"),
  "fragments": {
    "number": 1,
    "assignment_history": [{
      "to": ObjectId("5c68c706f1f52047f08862b4"),
      "on": 1550370567067
    }]
  }
}
{
  "_id": ObjectId("5c68c706f1f52047f08862b6"),
  "fragments": {
    "number": 2,
    "assignment_history": []
  }
}
{
  "_id": ObjectId("5c68c706f1f52047f08862b6"),
  "fragments": {
    "number": 3,
    "assignment_history": [{
      "to": null,
      "on": 1550370567067
    }]
  }
}

I need to end up with 2 objects, where assignment_history.to is null, and where assignment_history has no items/length.

can you show an example object of this territories?

RichieK
– RichieK

2019-02-17 16:35:23 +00:00
Commented Feb 17, 2019 at 16:35 — RichieK
– RichieK, Commented Feb 17, 2019 at 16:35

Ashh · Accepted Answer · 2019-02-17 16:45:03Z

2

You can use one more $match condition after the $unwind

db.getCollection('territories').aggregate([
  { "$match": { "congregation": ObjectId("5c68c706f1f52047f08862b3") }},
  { "$limit": 1 },
  { "$unwind": { "path": "$fragments" }},
  { "$match": {
    "$or": [
      { "fragments.assignment_history.to": null },
      { "fragments.assignment_history.to": { "$exists": false }}
    ]
  }},
  { "$project": {
    "fragments.number": 1,
    "fragments.assignment_history": { "$slice": ["$fragments.assignment_history", -1] }
  }}
])

answered Feb 17, 2019 at 16:45

Ashh

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Comments

elad frizi · Accepted Answer · 2019-02-17 16:55:04Z

0

Try adding this stage before the project. Its mean give me those with empty fragments.assignment_history, or those that have no fragments.assignment_history.to

{
  $match: {
     $or : [
            { "fragments.assignment_history" : {$size: 0}},
            { 
              "fragments.assignment_history" :{
                        $not: { $elemMatch: { to : {$exists: true} } 
              }
            }
           ]
  }
}

answered Feb 17, 2019 at 16:55

elad frizi

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