Q: Mongo db aggregation multiple conditions I want to project a collection applying exporting a value only if a field is inside a range.
Sort of:
db.workouts.aggregate({
 $match: { user_id: ObjectId(".....") }
},
{
 $project: { 
        '20': { $cond: [ {$gt: [ "$avg_intensity", 20]} , '$total_volume', 0] }
    }    
 })

I need to get the value only if the avg_intensity is inside a certain range. I will then group and sum on the projection result.
What I am trying to do is applying a $gt and $lt filter but with no much success.
db.workouts.aggregate(
{
   $match: { user_id: ObjectId("....") }
},
{
$project: { 
        '20': { $cond: [ [{$gt: [ "$avg_intensity", 20]}, {$lt: [ "$avg_intensity", 25]}] ,    '$total_volume', 0] }
    }    
 })

How may I apply both $gt and $lt conditions?

A: To  combine logical conditions under a $cond operator then wrap the conditions with an $and operator:
db.workouts.aggregate([
    { "$match": { "user_id": ObjectId("....") }},
    { "$project": { 
       "20": { "$cond": [
           { "$and": [ 
               { "$gt": [ "$avg_intensity", 20 ] },
               { "$lt": [ "$avg_intensity", 25 ] }
           ]},    
           "$total_volume", 
           0
       ]}
   }}
])


A: filter with multiple condition:
let booking = await Booking.findOne(
    { userId: req.currentUser?.id },
    {
      id: 1,
      status: 1,
      cart: {
        $filter: {
          input: '$cart',
          as: 'c',
          cond: {
            $and: [
              { $eq: ['$$c.date', cDate] },
              { $eq: ['$$c.product', new mongoose.Types.ObjectId(productId)] },
            ],
          },
        },
      },
      version: 1,
    }
  ).populate('cart.product', {
    id: 1,
    name: 1,
    thumbnail: 1,
    photos: 1,
    mrp: 1,
    minPrice: 1,
  });


A: If I got your requirements right you should put the filter in the match part of the pipeline:
db.workouts.aggregate( [
  { $match: { user_id: ObjectId("...."), "avg_intensity": { $gt: 20, $lte: 25 } } },
  { $group: { _id: ..., count: ... } }
] );


A: In the following way we can aggregate, Here i using two conditions which are city,year 
db.getCollection('geolocations').aggregate(
   [{$unwind : '$geolocation'},
     {
       $group:
         {
           _id: {city:"$geolocation.city",year: { $year: "$geolocation.date" }},
           count: { $sum: 1 }
         }
     }
   ]
)

Output:
/* 1 */
{
    "_id" : {
        "city" : "Noida",
        "year" : 2020
    },
    "count" : 1.0
}

/* 2 */
{
    "_id" : {
        "city" : "Villupuram",
        "year" : 2020
    },
    "count" : 1.0
}

/* 3 */
{
    "_id" : {
        "city" : "Chennai",
        "year" : 2020
    },
    "count" : 2.0
}

