CODEWITHSUNDEEP
pythonmongodbpymongo
LaSanton
LaSanton

Reputation: 157

Get list of values of last N match Documents Pymongo

I have a MongoDB collection where the last 4 documents look like this:

{'_id': ObjectId('60eb507f9b35c6271434be28'),
  'user_id': '123',
  'name': 'abc',
  'weight': 80}

{'_id': ObjectId('60eb507f9b35c6271434be29'),
  'user_id': '123',
  'name': 'abc',
  'weight': 75}

{'_id': ObjectId('60eb507f9b35c6271434be30'),
  'user_id': '789',
  'name': 'ghi',
  'weight': 88}

{'_id': ObjectId('60eb507f9b35c6271434be31'),
  'user_id': '123',
  'name': 'abc',
  'weight': 80}

Using Pymongo, I want to get a list, that will contain the last 3(most recent) "weight" values of the user_id="123" and name="abc" of my collection. In the above case, the wanted result would be

[80,75,80]

I think one condition in my query would be:

{"$match": {"$and" : [{"user_id": "123"},{"name": abc}]}}

but i am not sure about the rest.

Thank you in advance.

Upvotes: 0

Views: 219

Answers (1)

HandsomeCoder
HandsomeCoder

Reputation: 756

If you have timestamp value stored in tr_timestamp then you can use .sort() and .limit() to get you expected output like this:

db.collection.find({
      "user_id": "123"
    }).sort({
      "tr_timestamp": -1
    }).limit(3)

If you still want to use the aggregate pipeline then your query will look like this

db.collection.aggregate([
  {
    "$match": {
      "user_id": "123",
      "name": "abc"
    }
  },
  {
    "$sort": {
      "tr_timestamp": -1
    }
  },
  {
    "$limit": 3
  }
])

Here is the link to the playground to test your use-case: Mongo Playground

Also, In $match stage there is no need to combine query explicitly in $and as by default behaviour will be the same as $and only so I have removed that part in my query

OR

If you just want a list of weights then your query will look like this:

db.collection.aggregate([
  {
    "$match": {
      "user_id": "123"
    }
  },
  {
    "$sort": {
      "tr_timestamp": -1
    }
  },
  {
    "$limit": 3
  },
  {
    "$group": {
      "_id": "$user_id",
      "weights": {
        "$push": "$weight"
      }
    }
  }
])

Here is the link to the playground to test your use-case: Mongo Playground

Upvotes: 1

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