Return the last “true” value for each group

Question

I have collection in which documents are like:

{
    _id: ObjectId(),
     user: ObjectId(),  
     studentName: Str         


        

Answer 1

The best way to do this is using the aggregation framework. You need to $group your documents by "user" and return the last document for each user using the $last accumulator operator but for this to work, you need a preliminary sorting stage using the $sort aggregation pipeline operator. To sort your documents, you need to consider both the "createdAt" field and the "user" field.

The last stage in the pipeline is the $match stage where you select only those last documents where "isAbandoned" equals true.

db.students.aggregate([
    { "$sort": { "user": 1, "createdAt": 1 } }, 
    { "$group": { 
        "_id": "$user", 
        "last": { "$last": "$$ROOT" }
    }}, 
    { "$match": { "last.isAbandoned": true } }
])

which returns something like this:

{ 
    "_id" : ObjectId("56c85244bd5f92cd78ae4bc1"),
    "last" : {
        "_id" : ObjectId("56cee51503b7cb7b0eda9c4c"),
        "user" : ObjectId("56c85244bd5f92cd78ae4bc1"),
        "studentName" : "Rajeev",
        "createdAt" : ISODate("2016-02-25T11:27:17.281Z"),
        "isAbandoned" : true
    }
}

To get the expected result, we need to use the $replaceRoot pipeline operator starting from verion 3.4 to promote the embedded document to the top level

{
    $replaceRoot: { newRoot: "$last" }
}

In older version, you need to use the $project aggregation pipeline operation to reshape our documents. So if we extend our pipeline with the following stage:

{ 
    "$project": { 
        "_id": "$last._id", 
        "user": "$last.user", 
        "studentName": "$last.studentName", 
        "createdAt": "$last.createdAt", 
        "isAbandoned": "$last.isAbandoned"
}}

it produces the expected output:

{
    "_id" : ObjectId("56cee51503b7cb7b0eda9c4c"),
    "user" : ObjectId("56c85244bd5f92cd78ae4bc1"),
    "studentName" : "Rajeev",
    "createdAt" : ISODate("2016-02-25T11:27:17.281Z"),
    "isAbandoned" : true
}