Sorting and Deduplicating MongoDB Data with Reduce

Question

Sorting and Deduplicating MongoDB Data with Reduce

I'm currently using the Reduce function to generate a combined String of fields from an array.

For instance, suppose I have an array of subdocuments named children - and each individual child contains a name field.

For example:

[
   {name:"Zak"}, {name:"Bob"}, {name:"Sharon"}, {name:"Zak"}, {name:"Cindy"}, {name:"Bob"}, {name:"Peter"}
]

By using the following expression, I can produce a "csv" of

"Zak, Bob, Sharon, Zak, Cindy, Bob, Peter,"

:

(Yes, I am aware that I could potentially utilize $cond to identify the last iteration and eliminate the trailing ", ".

  uniqueCsv: {
   $reduce: {
     input: "$children",
     initialValue: '',
     in: {
       $concat: ["$$this.name", ", ", "$$value"]
     }
   }
  }

Is there a way to use this reduce function to remove duplicates and alphabetically sort the names so that we end up with:

"Bob, Cindy, Peter, Sharon, Zak" ?

Thank you!

javascript mongodb aggregation-framework reduce

Answer 1

Answer №1

$setUnion is utilized here to extract unique elements from children.name and it also ensures that the strings are sorted in ascending order
$concat is used to combine strings with specific conditions. The first parameter is set as $$value, the second parameter involves a condition to check if the value is empty or not, and the third parameter is set as $$this which denotes the current string

db.collection.aggregate([
  {
    $project: {
      uniqueCsv: {
        $reduce: {
          input: { $setUnion: "$children.name" },
          initialValue: "",
          in: {
            $concat: [
              "$$value",
              { $cond: [{ $eq: ["$$value", ""] }, "", ", "] },
              "$$this"
            ]
          }
        }
      }
    }
  }
])

Check Playground

Another approach involves using $substr instead of $cond

$substr is used to manipulate the current reduce operation where the result will be , Bob, Cindy, Peter, Sharon, Zak
The objective is to remove the first 2 characters from the string. The second parameter signifies starting from the second position, and the third parameter indicates the length of the string - 1, which returns all strings

db.collection.aggregate([
  {
    $project: {
      uniqueCsv: {
        $substr: [
          {
            $reduce: {
              input: { $setUnion: "$children.name" },
              initialValue: "",
              in: {
                $concat: ["$$value", ", ", "$$this"]
              }
            }
          },
          2,
          -1
        ]
      }
    }
  }
])

Explore Playground

Important Note:

According to MongoDB, "$setUnion: The order of the elements in the output array is unspecified."

Although based on my personal experience, the output array is usually in ascending order. I have extensively used it in my projects and have consistently noticed an ascending order of strings as output.

Ultimately, it is your decision whether to incorporate this technique into your project without official confirmation from MongoDB.

Answer 2

$setUnion is utilized here to extract unique elements from children.name and it also ensures that the strings are sorted in ascending order
$concat is used to combine strings with specific conditions. The first parameter is set as $$value, the second parameter involves a condition to check if the value is empty or not, and the third parameter is set as $$this which denotes the current string

db.collection.aggregate([
  {
    $project: {
      uniqueCsv: {
        $reduce: {
          input: { $setUnion: "$children.name" },
          initialValue: "",
          in: {
            $concat: [
              "$$value",
              { $cond: [{ $eq: ["$$value", ""] }, "", ", "] },
              "$$this"
            ]
          }
        }
      }
    }
  }
])

Check Playground

Another approach involves using $substr instead of $cond

$substr is used to manipulate the current reduce operation where the result will be , Bob, Cindy, Peter, Sharon, Zak
The objective is to remove the first 2 characters from the string. The second parameter signifies starting from the second position, and the third parameter indicates the length of the string - 1, which returns all strings

db.collection.aggregate([
  {
    $project: {
      uniqueCsv: {
        $substr: [
          {
            $reduce: {
              input: { $setUnion: "$children.name" },
              initialValue: "",
              in: {
                $concat: ["$$value", ", ", "$$this"]
              }
            }
          },
          2,
          -1
        ]
      }
    }
  }
])

Explore Playground

Important Note:

According to MongoDB, "$setUnion: The order of the elements in the output array is unspecified."

Although based on my personal experience, the output array is usually in ascending order. I have extensively used it in my projects and have consistently noticed an ascending order of strings as output.

Ultimately, it is your decision whether to incorporate this technique into your project without official confirmation from MongoDB.

Sorting and Deduplicating MongoDB Data with Reduce

Answer №1

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