Get latest value in mongodb nested documents

Question

0.00/5 (No votes)

See more:

Hi everyone.
I'm trying to figure out how can I extract the latest value of say a variable in a set of documents.
Let's say I have 2 documents:

{ 
   "Name": "doc1", 
   "Date": ISODate("2019-12-31T00:00:00.000Z"),
   "Values" : [
         {
            "VariableName": "VAR1",
            "VariableValue": "VAL1"
         },
         {
            "VariableName": "VAR2",
            "VariableValue": "VAL2"
         } ]
},
{ 
   "Name": "doc2", 
   "Date": ISODate("2020-01-31T00:00:00.000Z"),
   "Values" : [
         {
            "VariableName": "VAR1",
            "VariableValue": "VAL3"
         },
         {
            "VariableName": "VAR3",
            "VariableValue": "VAL3"
         } ]
}

I'ld like to write a "query" that extract only the latest value of each variable:

{ 
   "Document": "doc2", 
   "Date": ISODate("2020-01-31T00:00:00.000Z"),
   "VariableName", "VAR1",
   "VariableValue", "VAL3"
},
{
   "Document": "doc1", 
   "Date": ISODate("2019-12-31T00:00:00.000Z"),
   "VariableName", "VAR2",
   "VariableValue", "VAL2"
},
{ 
   "Document": "doc2", 
   "Date": ISODate("2020-01-31T00:00:00.000Z"),
   "VariableName", "VAR3",
   "VariableValue", "VAL3"
}

I'm stuck at the join stage of the pipeline when I have to join the original table with the greatest date values.

Can somebody help please?

What I have tried:

//Prepare documents
var o = [
{ 
   "Name": "doc1", 
   "Date": ISODate("2019-12-31T00:00:00.000Z"),
   "Values" : [
         {
            "VariableName": "VAR1",
            "VariableValue": "VAL1"
         },
         {
            "VariableName": "VAR2",
            "VariableValue": "VAL2"
         } ]
},
{ 
   "Name": "doc2", 
   "Date": ISODate("2020-01-31T00:00:00.000Z"),
   "Values" : [
         {
            "VariableName": "VAR1",
            "VariableValue": "VAL3"
         },
         {
            "VariableName": "VAR3",
            "VariableValue": "VAL3"
         } ]
}
];

//Insert objects
db.getCollection("docs").insertMany(o);

//Flattening I get
db.docs.aggregate(
[
   { "$unwind" : "$Values" },
   { "$project": { "Name": 1, "Date" : 1, "VariableName": "$Values.VariableName", "VariableValue": "$Values.VariableValue" } }
]
).pretty();

{
        "_id" : ObjectId("5e96d8632f441e3738a0ff2f"),
        "Name" : "doc1",
        "Date" : ISODate("2019-12-31T00:00:00Z"),
        "VariableName" : "VAR1",
        "VariableValue" : "VAL1"
}
{
        "_id" : ObjectId("5e96d8632f441e3738a0ff2f"),
        "Name" : "doc1",
        "Date" : ISODate("2019-12-31T00:00:00Z"),
        "VariableName" : "VAR2",
        "VariableValue" : "VAL2"
}
{
        "_id" : ObjectId("5e96d8632f441e3738a0ff30"),
        "Name" : "doc2",
        "Date" : ISODate("2020-01-31T00:00:00Z"),
        "VariableName" : "VAR1",
        "VariableValue" : "VAL3"
}
{
        "_id" : ObjectId("5e96d8632f441e3738a0ff30"),
        "Name" : "doc2",
        "Date" : ISODate("2020-01-31T00:00:00Z"),
        "VariableName" : "VAR3",
        "VariableValue" : "VAL3"
}

//Then I need to find latest value bye date and VariableName
db.docs.aggregate(
[
   { "$unwind" : "$Values" },
   { "$project": { "Name": 1, "Date" : 1, "VariableName": "$Values.VariableName", "VariableValue": "$Values.VariableValue" } },
   { "$group": { _id : { "VariableName": "$VariableName"}, latest: {"$max" : "$Date"} } }
]
).pretty();

{
        "_id" : {
                "VariableName" : "VAR3"
        },
        "latest" : ISODate("2020-01-31T00:00:00Z")
}
{
        "_id" : {
                "VariableName" : "VAR2"
        },
        "latest" : ISODate("2019-12-31T00:00:00Z")
}
{
        "_id" : {
                "VariableName" : "VAR1"
        },
        "latest" : ISODate("2020-01-31T00:00:00Z")
}

Posted 15-Apr-20 0:09am

coccide

Updated 15-Apr-20 3:05am

Add a Solution

1 solution

Add a Solution

Add your solution here

Treat my content as plain text, not as HTML

Preview 0

…

Existing Members

Sign in to your account

...or Join us

Download, Vote, Comment, Publish.

Your Email
Password
Forgot your password?

Your Email
This email is in use. Do you need your password?
Optional Password

I have read and agree to the Terms of Service and Privacy Policy
Please subscribe me to the CodeProject newsletters

When answering a question please:

Read the question carefully.
Understand that English isn't everyone's first language so be lenient of bad spelling and grammar.
If a question is poorly phrased then either ask for clarification, ignore it, or edit the question and fix the problem. Insults are not welcome.
Don't tell someone to read the manual. Chances are they have and don't get it. Provide an answer or move on to the next question.

Let's work to help developers, not make them feel stupid.

This content, along with any associated source code and files, is licensed under The Code Project Open License (CPOL)

coccide · Answer 1 · 2020-04-15T03:06:00

Ok I managed to get what I want using some intermediate view objects but I think there should be a more elegant way to do that:

var tutti = db.createView(
"tutti",
"docs",
[
   { "$unwind" : "$Values" },
   { "$project": { "Name": 1, "Date" : 1, "VariableName": "$Values.VariableName", "VariableValue": "$Values.VariableValue" } }
]
);

db.ultimeModifiche.drop();
var ultimeModifiche = db.createView(
"ultimeModifiche",
"tutti",
[
   { "$group": { _id : { "VariableName": "$VariableName"}, latest: {"$max" : "$Date"} } },
   { "$project": { "VariableName": "$_id.VariableName", "latest": 1, _id:0 } }
]
);


db.ultimiValori.drop();
var ultimiValori = db.createView(
"ultimiValori",
"ultimeModifiche",
[
   { 
      "$lookup": 
      { 
	     from : "tutti",
		 let: {
                variableName: "$VariableName",
                latest: "$latest"
             },
		 pipeline: [
                {
                   $match: {
                      $expr: {
                         $and: [
                            {
                               $eq: [
                                  "$VariableName",
                                  "$$variableName"
                               ]
                            },
                            {
                               $eq: [
                                  "$Date",
                                  "$$latest"
                               ]
                            }
                         ]
                      }
                   }
                }
             ],
		 as: "output"
	   }
   },   
    {
      $replaceRoot: { newRoot: { $mergeObjects: [ { $arrayElemAt: [ "$output", 0 ] }, "$$ROOT" ] } }
   },
   { $project: { output: 0 } }
   
]
);
db.ultimiValori.find();