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How to compare attributes of different objects in MongoDB object array?
To compare attributes, use $let along with $indexOfArray. Let us first create a collection with documents −
> db.demo366.insertOne( ... { ... ... "Name" : "Chris", ... "details" : [ ... { ... "Id" : "John1", ... "value" : "test" ... }, ... { ... "Id" : "John2", ... "value" : 18 ... }, ... { ... "Id" : "John3", ... "value" : 20 ... } ... ]} ... ); { "acknowledged" : true, "insertedId" : ObjectId("5e57ddd92ae06a1609a00ae7") }
Display all documents from a collection with the help of find() method −
> db.demo366.find();
This will produce the following output −
{ "_id" : ObjectId("5e57ddd92ae06a1609a00ae7"), "Name" : "Chris", "details" : [ { "Id" : "John1", "value" : "test" }, { "Id" : "John2", "value" : 18 }, { "Id" : "John3", "value" : 20 } ] }
Following is the query to compare attributes of different objects in the MongoDB object array −
> db.demo366.find( ... {"$expr":{ ... "$let":{ ... "vars":{ ... "john2":{"$arrayElemAt":["$details",{"$indexOfArray":["$details.Id","John2"]}]}, ... "john3":{"$arrayElemAt":["$details",{"$indexOfArray":["$details.Id","John3"]}]} ... }, ... "in":{"$lt":["$$john2.value","$$john3.value"]}} ... }})
This will produce the following output −
{ "_id" : ObjectId("5e57ddd92ae06a1609a00ae7"), "Name" : "Chris", "details" : [ { "Id" : "John1", "value" : "test" }, { "Id" : "John2", "value" : 18 }, { "Id" : "John3", "value" : 20 } ] }
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