Combining arrays within single mongo document with aggregate

Question

I am trying to create a call that combines all images between 'images' and 'posts.images' and sorts by createdAt. So far I have:

output = [Array(1), Array(3), {…}, {…}, {…}, {…}, {…}, {…}]

So it appears to be combining the arrays into a parent array instead of merging into a single array.

What I want:

output = [{…},{…},{…},{…},{…},{…},{…},{…},{…},{…},{…},{…}]

Here is my schema:

const deviceSchema = new Schema({
owner: {
    type: mongoose.Schema.Types.ObjectId,
    ref: 'User',
},
images: [{
    url: {
        type: String,
    },
    createdAt: {
        type: Date,
    }
}],
posts: [{
    description: {
        type: String,
    },
    images: [{
        url: {
            type: String,
        },
        createdAt: {
            type: Date,
        }
    }],
    createdAt: {
        type: Date,
    }
}],
});

Here is my aggregate call:

const images = await Device.aggregate([
        { $project: {
            combinedImages: { $concatArrays: ["$images", "$posts.images"] }
        }},
        { $match: { _id: id }},
        { $unwind: '$combinedImages' },
        { $sort: { 'combinedImages.createdAt': -1 }},
        { $skip: (page-1)*limit },
        { $limit: limit },
        { $group: { _id: '$_id', images: { $push: '$combinedImages'}}}
    ])

Answer 1

You can use a combination of $concatArrays and $reduce to combine all images into a single array, e.g.:

db.collection.aggregate([
  {
    $set: {
      combinedImages: {
        "$concatArrays": [
          "$images",
          {
            $reduce: {
              input: "$posts",
              initialValue: [],
              in: {
                "$concatArrays": [
                  "$$value",
                  "$$this.images"
                ]
              }
            }
          }
        ]
      }
    }
  }
])

This leads to the following output:

{
    // ...
    "combinedImages": [
      {
        "url": "img1"
      },
      {
        "url": "img2"
      },
      {
        "url": "post_img11"
      },
      {
        "url": "post_img12"
      },
      {
        "url": "post_img21"
      },
      {
        "url": "post_img22"
      }
    ],
    // ...
}

See this mongoplayground to test.