Concatenate with condition in MongoDB?

To concatenate with condition in MongoDB, use $cond and in that, work with $concat. Let us create a collection with documents −

> db.demo745.insertOne({Value1:"100",Value2:"100"});
{
   "acknowledged" : true,
   "insertedId" : ObjectId("5eae6419a930c785c834e554")
}
> db.demo745.insertOne({Value1:"40",Value2:"50"});
{
   "acknowledged" : true,
   "insertedId" : ObjectId("5eae6421a930c785c834e555")
}
> db.demo745.insertOne({Value1:"13",Value2:"45"});
{
   "acknowledged" : true,
   "insertedId" : ObjectId("5eae6429a930c785c834e556")
}

Display all documents from a collection with the help of find() method −

> db.demo745.find();

This will produce the following output −

{ "_id" : ObjectId("5eae6419a930c785c834e554"), "Value1" : "100", "Value2" : "100" }
{ "_id" : ObjectId("5eae6421a930c785c834e555"), "Value1" : "40", "Value2" : "50" }
{ "_id" : ObjectId("5eae6429a930c785c834e556"), "Value1" : "13", "Value2" : "45" }

Followings is the query to concatenate with condition −

> db.demo745.aggregate(
...    [
...       { "$redact": {
...          "$cond": [
...             { "$eq": [
...                { "$concat": [ "$Value1","$Value2" ] },
...                   "1345"
...                ]},
...                "$$KEEP",
...                "$$PRUNE"
...          ]
...       }}
...    ]
... )

This will produce the following output −

{ "_id" : ObjectId("5eae6429a930c785c834e556"), "Value1" : "13", "Value2" : "45" }