mongodb push values into array with $reduce

Question

I have an document structure like this:

_id: 123,
posts: [
   {
      _id: 234,
      likesUser: [345, 456, 567, 678]
   }
]

I want to use $reduce here so my expected output should look like this:

   output: {
      likes: [23, 31, 12, 32],  //each element is the size of the "likesUser" array
      ids: ["14312", "2342", "2312", "235234"]  //each element is the "_id" in the posts array
   }

I have try it like this:

  let result = await Post.aggregate([
    {
      $match: {
        _id: USER
      }
    },
    {
      $project: {
        posts: {
          $reduce: {
            input: "$posts",
            initialValue: { likes: [], ids: [] },
            in: {
              likes: {
                $push: { $size: "$$this.likesUser" }
              },
              ids: {
                $push: "$$this._id"
              }
            }
          }
        }
      }
    }
  ]);

In my frontend i use chart.js. The yAxsis are the likes and the xAxsis are the ID's. So for every element in my posts array i need the length of likesUser and _id But it doenst work. Has anybody an idea how to achive this?

Tom Slabbaert · Accepted Answer · 2020-06-01 14:11:50Z

6

You are very close, you just have a minor syntax error in your $reduce, Try this:

let result = await Post.aggregate([
    {
        $match: {
            _id: USER
        }
    },
    {
        $project: {
            posts: {
                $reduce: {
                    input: "$posts",
                    initialValue: {likes: [], ids: []},
                    in: {
                        likes: {
                            $concatArrays: ["$$value.likes", [{$size: "$$this.likesUser"}]]
                        },
                        ids: {
                            $concatArrays: ["$$value.ids", ["$$this._id"]]
                        }
                    }
                }
            }
        }
    }
]);

answered Jun 1, 2020 at 14:11

Tom Slabbaert

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Comments

ngShravil.py · Accepted Answer · 2020-06-01 14:25:53Z

You are on the right way. You need to use $mergeObjects operator to achieve this. Following query will be helpful:

db.collection.aggregate([
  {
    $match: {
      _id: USER
    }
  },
  {
    $project: {
      post: {
        $reduce: {
          input: "$posts",
          initialValue: {
            likes: [],
            ids: []
          },
          in: {
            $mergeObjects: [
              "$$value",
              {
                likes: {
                  $concatArrays: [
                    "$$value.likes",
                    [
                      {
                        $size: "$$this.likesUser"
                      }
                    ]
                  ]
                },
                ids: {
                  $concatArrays: [
                    "$$value.ids",
                    [
                      "$$this._id"
                    ]
                  ]
                }
              }
            ]
          }
        }
      }
    }
  }
])

MongoPlayGroundLink

I hope the above is helpful.

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