$reduce and $setUnion for nested arrays in MongoDB

Question

I am looking to merge nested arrays using $reduce and $setUnion in MongoDB. Following is the sample input -

levels: [
 [[80,100,120]],[[100,150]],[[200,80,10         


        

Answer 1

You can use below aggregation with single $project stage

db.collection.aggregate([
  { "$project": {
    "levels": {
      "$reduce": {
        "input": {
          "$reduce": {
            "input": "$levels",
            "initialValue": [],
            "in": { "$setUnion": ["$$this", "$$value"] }
          }
        },
        "initialValue": [],
        "in": { "$setUnion": ["$$this", "$$value"] }
      }
    }
  }}
])