how to push data into array mongodb

Question

Lookup with three collections re-manage data. I got confuse which to use. Please guide on this

db.post.aggregate([
  {
    $lookup: {
      from: "users",
      localField: "created_by",
      foreignField: "_id",
      as: "created_users"
    }
  },
  {
    $lookup: {
      from: "comments",
      let: {
        p_id: "$_id"
      },
      pipeline: [
        {
          $match: {
            $expr: {
              $eq: [
                "$post_id",
                "$$p_id"
              ]
            }
          }
        }
      ],
      as: "comments"
    }
  },
  {
    $lookup: {
      from: "users",
      localField: "comments.sender_id",
      foreignField: "_id",
      as: "commented_user"
    }
  }
])

There are three collection posts, user, and comments we would like to get who has comment for a post, Commented user should come under comments like this

[
  {
    "_id": ObjectId("5eeb02881982961ada625c7d"),
    "commented_user": [
      {
        "_id": ObjectId("5e4d0973babf2b74ca868f4d"),
        "first_name": "James",
        "last_name": "Smith",
        "timestamp": 1.582106995137e+12
      },
      {
        "_id": ObjectId("5e4d0973babf2b74ca868f6d"),
        "first_name": "Alex",
        "last_name": "Jimy",
        "timestamp": 1.582106995139e+12
      }
    ],
    "comments": [
      {
        "_id": ObjectId("5eeb08e26fb7f270e4077617"),
        "date": 1.592461538924e+12,
        "post_id": ObjectId("5eeb02881982961ada625c7d"),
        "sender_id": ObjectId("5e4d0973babf2b74ca868f4d"),
        "text": "Nice ",
        "commented_user": {
            "_id": ObjectId("5e4d0973babf2b74ca868f4d"),
            "first_name": "James",
            "last_name": "Smith",
            "timestamp": 1.582106995137e+12
        },
      },
      {
        "_id": ObjectId("5eeb08e26fb7f270e4077618"),
        "date": 1.592461538923e+12,
        "post_id": ObjectId("5eeb02881982961ada625c7d"),
        "sender_id": ObjectId("5e4d0973babf2b74ca868f6d"),
        "text": "Nice One",
        "commented_user": {
            "_id": ObjectId("5e4d0973babf2b74ca868f6d"),
            "first_name": "Alex",
            "last_name": "Jimy",
            "timestamp": 1.582106995137e+12
        },
      }
    ],
    "created_by": ObjectId("5e4e74eb380054797d9db623"),
    "created_users": [],
    "date": 1.589441206774e+12,
    "title": "Covid19"
  }
]

here is my attempt https://mongoplayground.net/p/UwRjj-er0K5

Please help on this thanks

Tom Slabbaert · Accepted Answer · 2020-06-18 13:54:51Z

1

You can do something like this:

The strategy is $unwinding the results so we can match per comment and then reconstruct the former structure.

db.post.aggregate([
 {
   $lookup: {
     from: "users",
     localField: "created_by",
     foreignField: "_id",
     as: "created_users"
   }
 },
 {
   $lookup: {
     from: "comments",
     let: {
       p_id: "$_id"
     },
     pipeline: [
       {
         $match: {
           $expr: {
             $eq: [
               "$post_id",
               "$$p_id"
             ]
           }
         }
       }
     ],
     as: "comments"
   }
 },
 {
   $unwind: "$comments"
 },
 {
   $lookup: {
     from: "users",
     localField: "comments.sender_id",
     foreignField: "_id",
     as: "commented_user"
   }
 },
 {
   $unwind: "$commented_user"
 },
 {
   $addFields: {
     comments: {
       $mergeObjects: [
         "$comments",
         {
           commented_user: "$commented_user"
         }
       ]
     }
   }
 },
 {
   $group: {
     _id: "$_id",
     comments: {
       $push: "$comments"
     },
     created_by: {
       $first: "$created_by"
     },
     created_users: {
       $first: "$created_users"
     },
     date: {
       $first: "$date"
     },
     title: {
       $first: "$title"
     },

   }
 }
])

Mongo Playground

answered Jun 18, 2020 at 13:54

Tom Slabbaert

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8 Comments

Sunil Dubey Over a year ago

can we do without $group or using $project ? if please help with different playground

Tom Slabbaert Over a year ago

the $group is required to restore the comments structure to an array, but if you don't mind it being unwinded you can replace it with project yes.

Sunil Dubey Over a year ago

why this failed ? mongoplayground.net/p/aKVO0Eo4bbX

Tom Slabbaert Over a year ago

Because if you don't group then $comments isn't an array: mongoplayground.net/p/8FTIbHv7fmQ

Tom Slabbaert Over a year ago

Yea, take a look here: mongoplayground.net/p/OA1s79voayX. instead of unwinding the comments i just selected the last one. (this assumes it's sorted by date in the collection)

|

thammada.ts · Accepted Answer · 2020-06-18 19:06:20Z

Starting with your the code you have attempted, you can just move your second $lookup into the first $lookup's pipeline, then $unwind it

db.post.aggregate([
  {
    $lookup: {
      from: "users",
      localField: "created_by",
      foreignField: "_id",
      as: "created_users"
    }
  },
  {
    $lookup: {
      from: "comments",
      let: { p_id: "$_id" },
      pipeline: [
        {
          $match: {
            $expr: { $eq: ["$post_id", "$$p_id"] }
          }
        },
        { // second level $lookup
          $lookup: {
            from: "users",
            localField: "sender_id",
            foreignField: "_id",
            as: "commented_user"
          }
        },
        { // $unwind to get single object instead of array
          $unwind: "$commented_user"
        }
      ],
      as: "comments"
    }
  },
])

Mongo Playground

Note for OP: There is discussion to get the latest comment instead. With this approach you can $sort and $limit in the outermost $lookup

db.post.aggregate([
  {
    $lookup: {
      from: "users",
      localField: "created_by",
      foreignField: "_id",
      as: "created_users"
    }
  },
  {
    $lookup: {
      from: "comments",
      let: { p_id: "$_id" },
      pipeline: [
        {
          $match: {
            $expr: { $eq: ["$post_id", "$$p_id"] }
          }
        },
        {
          $sort: { date: -1 } // or { _id: -1 }
        },
        {
          $limit: 1 // limit to only one element
        },
        {
          $lookup: {
            from: "users",
            localField: "sender_id",
            foreignField: "_id",
            as: "commented_user"
          }
        },
        {
          $unwind: "$commented_user"
        }
      ],
      as: "comments"
    }
  },
  { // // $unwind to get single object instead of array
    $unwind: "$comments"
  }
])

Mongo Playground

Awesome @thammada Thank you so much for this explanation. Your answer is really useful.

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