Get Slice of objects form mongoose Array of Objects

Question

I have the following model in the mongoose schema. In this model Messages is the array of objects.

{
"_id" : ObjectId("5a4e6e0f003db94a2c6e52d3"),
"IsGroup" : false,
"GroupName" : "Group1",
"Messages" : [ 
    ObjectId("5a4e6e53003db94a2c6e52d4"), 
    ObjectId("5a4e6e56003db94a2c6e52d5"), 
    ObjectId("5a4e6e58003db94a2c6e52d6"), 
    ObjectId("5a4e6e5b003db94a2c6e52d7"), 
    ObjectId("5a4e6e60003db94a2c6e52d8"), 
    ObjectId("5a4e6e63003db94a2c6e52d9"), 
    ObjectId("5a4e6e65003db94a2c6e52da"), 
    ObjectId("5a4e6e68003db94a2c6e52db"), 
    ObjectId("5a4e6e72003db94a2c6e52dc"), 
    ObjectId("5a4e6e77003db94a2c6e52dd"), 
    ObjectId("5a4e6e7d003db94a2c6e52de"), 
    ObjectId("5a4e6e80003db94a2c6e52df")
 ]
 "__v" : 0
}

I am trying to get the last 5 messages form the array. I achieved this by the following code. Model is my mongo model.

Model.UserGroupModel.findById(GroupId)
    .select({ Messages: { '$slice':-5}})
    .exec(function (err, GroupResult) {
}

But my challenge is to get the next previous 5 elements which are need to start form previous item.

ex: In first result I have to get following result.

ObjectId("5a4e6e68003db94a2c6e52db"), 
ObjectId("5a4e6e72003db94a2c6e52dc"), 
ObjectId("5a4e6e77003db94a2c6e52dd"), 
ObjectId("5a4e6e7d003db94a2c6e52de"), 
ObjectId("5a4e6e80003db94a2c6e52df")

In next request I have to get. I will pass the input of previously returned objectId("5a4e6e68003db94a2c6e52db") for where the next result start point to get.

ObjectId("5a4e6e58003db94a2c6e52d6"), 
ObjectId("5a4e6e5b003db94a2c6e52d7"), 
ObjectId("5a4e6e60003db94a2c6e52d8"), 
ObjectId("5a4e6e63003db94a2c6e52d9"), 
ObjectId("5a4e6e65003db94a2c6e52da"),

How to query this one in mongoose?

s7vr · Accepted Answer · 2018-01-04 22:21:39Z

2

You can try below aggregation function for fetching next records in 3.4 version.

Model.UserGroupModel.aggregate([
  {"$match":{"_id":GroupId,"Messages":MessageId}},
  {"$project":{
    "Messages":{
      "$slice":[
        "$Messages",
        {"$subtract":[{"$indexOfArray":["$Messages",MessageId]},5]}
        ,5
      ]
    }
  }}
]).exec(function (err, GroupResult) {}

answered Jan 4, 2018 at 22:21

s7vr

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