group by day/month and take average of rating on that day/month in mongo

To group documents by day/month and return the month key in your output, you need to first $project the key fields to appropriate formats using the Date operators, in particular the $dateToString and $month operators.

This can be done in a $project stage prior to the $group step but not necessary since the $group pipeline accommodates mostly the accumulator operators.

In the preceding $group pipeline, you can the group the documents by the formatted date key, aggregate using the $avg operator and return the month as an integer from the previous pipeline using $first accumulator operator.

Running the following aggregation pipeline should give you the desired result:

db.collection.aggregate([  { "$group": {    "_id": {         "$dateToString": { "format": "%Y-%m-%d", "date": "$ceatedAt" }     },    "average": { "$avg": "$rating" },    "month": { "$first": { "$month": "$ceatedAt" } },  } }]) 

For the collection, use an aggregate() to $group, with the grouping _id of the $month from the ceatedAt (sic) field of your documents, and make the month field in the outputted document equal that value.

Then, create a field named ceatedAt that uses $dateToString to format the $date field of the inputted document as "%Y-%m-%d".

Finally, create a field named average, that is the $avg of the $rating field of the inputted document, in your grouped documents.

db.collection.aggregate([  {    $group: {      _id: { month: { $month: "$ceatedAt" } },      date: { first: "$ceatedAt" },      average: { $avg: "$rating" }    }  },  {    $project: {      month: "$month",      ceatedAt: { $dateToString: { format: "%Y-%m-%d", date: "$date" } },      average: "$average"    }  }])