How to combine all documents' field of array together in MongoDB?-CodePudding

How to combine the field of the array of all documents together in MongoDB?

For example:

My collection contains these documents:

[
  {
    _id: 0,
    categories: ['a', 'b', 'c'],
  },
  {
    _id: 1,
    categories: ['d', 'e', 'f'],
  },
  {
    _id: 2,
    categories: ['g', 'h', 'i'],
  },
];

And the result must be:

['a', 'b', 'c', 'd', 'e', 'f', 'g', 'h', 'i']

CodePudding user response：

Best way to do this is to use distinct

db.collection.distinct('categories')

If the results are more than 16mb then you can only use $unwind and $group in an aggregation:

db.collection.aggregate([
    {
        $unwind: "$categories"
    },
    {
        $group: {
            _id: "$categories"
        }
    }
])

CodePudding user response：

You can $unwind and $group like this:

db.collection.aggregate([
  {
    "$unwind": "$categories"
  },
  {
    "$group": {
      "_id": null,
      "categories": {
        "$push": "$categories"
      }
    }
  }
])

Example here

CodePudding user response：

  const list = [
    {
      _id: 0,
      categories: ["a", "b", "c"],
    },
    {
      _id: 1,
      categories: ["d", "e", "f"],
    },
    {
      _id: 2,
      categories: ["g", "h", "i"],
    },
  ];
  let newList = list.map((item) => [...item.categories]);
  let finalList = newList.reduce((a, b) => [...a, ...b]);

first make a new containing all the categories lists from your list. Then use a reducer like shown with the help of spread operators to get them separated

CodePudding user response：

If you are asking about JS, I would use reduce method:

const res = [
  {
    _id: 0,
    categories: ['a', 'b', 'c'],
  },
  {
    _id: 1,
    categories: ['d', 'e', 'f'],
  },
  {
    _id: 2,
    categories: ['g', 'h', 'i'],
  },
].reduce((acc, current) => ([...acc, ...current.categories]), [])

console.log(res)