Minimize MongoDB aggregation using specific criteria

I am facing a challenge with my array of objects named "extra" that have different properties: some objects include the property "plus" while others do not. I am attempting to split this "extra" array into two separate arrays - one called "cheap" containing objects without the "plus" property, and another named "exp" which includes only objects with the "plus" property. I believe I can achieve this using the $reduce method in MongoDB aggregate along with $concatArrays by using $cond to check for the existence of the "plus" property. Here is an example of what I have tried:

Example data:

{
    extra: [
        {
            description: "laces",
            type: "exterior",
            plus: '200'
        },
        {
            description: "sole",
            type: "interior"
        },
        {
            description: "logo",
            type: "exterior"
        },
        {
            description: "stud",
            type: "exterior",
            plus: '450'
        }
    ],
}

 {
    $project: {
        extra: {
            $reduce: {
                input: ['$extra'],
                initialValue: {cheap: [], exp: []},
                $cond: {
                    if: {$eq: ['$$this.plus', null]},
                    then: {
                        in: {
                            cheap: {
                                $concatArrays: ['$$value.cheap', '$$this'],
                            },
                        },
                    },
                    else: {
                        in: {
                            exp: {
                                $concatArrays: ['$$value.exp', '$$this'],
                            },
                        },
                    },
                },
            },
        },
    },
}

Despite trying several variations of the $cond section, I have been unsuccessful in getting it to work. Any assistance would be greatly appreciated. Thank you, K.

javascriptmongodbaggregation-frameworkreduce

Answer №1

Aside from some minor syntax errors, there is another issue with your understanding of the $ne operator.

In this scenario, you are assuming that a missing value should be treated as equal to null. However, this is not how MongoDB operates. For example, given a document:

{ name: "my name" }

The aggregation query:

{ $cond: { $eq: ["$missingField", null] } }

Will not result in true as anticipated because a missing field is not equivalent to null. I have taken the initiative to correct the syntax errors and provide a functional pipeline:

db.collection.aggregate([
  {
    $project: {
      extra: {
        $reduce: {
          input: "$extra",
          initialValue: {
            cheap: [],
            exp: []
          },
          in: {
            cheap: {
              "$concatArrays": [
                "$$value.cheap",
                {
                  $cond: [
                    "$$this.plus",
                    [],
                    [
                      "$$this"
                    ],
                    
                  ]
                }
              ]
            },
            exp: {
              "$concatArrays": [
                "$$value.exp",
                {
                  $cond: [
                    "$$this.plus",
                    [
                      "$$this"
                    ],
                    []
                  ]
                }
              ]
            }
          }
        },
        
      },
      
    },
    
  }
])

Mongo Playground

It's important to note that the $cond function evaluates the plus field. This means if the field exists but has a null value or a value of 0, it will still consider the document as a match for the cheap array. Keep this in mind and make adjustments accordingly.

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