Is it possible to use MongoDB to query for entries that have a particular value in a field in an object in an array?

Yes, to query for a field in an object in the array with MongoDB, use $elemMatch operator. This operator allows you to match documents where at least one array element satisfies all specified query criteria.

Syntax

db.collectionName.find({
    "arrayFieldName": { 
        $elemMatch: { 
            "fieldName": "value" 
        } 
    }
});

Sample Data

Let us create a collection with sample documents ?

db.objectInAnArrayDemo.insertMany([
    {
        "StudentDetails": [
            {"StudentName": "John", "StudentMessage": "Hi"},
            {"StudentName": "Larry", "StudentMessage": "Hello"}
        ]
    },
    {
        "StudentDetails": [
            {"StudentName": "Carol", "StudentMessage": "Hello"},
            {"StudentName": "David", "StudentMessage": "Good Morning"}
        ]
    }
]);

{
    "acknowledged": true,
    "insertedIds": [
        ObjectId("5c92635d36de59bd9de06381"),
        ObjectId("5c92637936de59bd9de06382")
    ]
}

Display All Documents

db.objectInAnArrayDemo.find().pretty();

{
    "_id": ObjectId("5c92635d36de59bd9de06381"),
    "StudentDetails": [
        {
            "StudentName": "John",
            "StudentMessage": "Hi"
        },
        {
            "StudentName": "Larry",
            "StudentMessage": "Hello"
        }
    ]
}
{
    "_id": ObjectId("5c92637936de59bd9de06382"),
    "StudentDetails": [
        {
            "StudentName": "Carol",
            "StudentMessage": "Hello"
        },
        {
            "StudentName": "David",
            "StudentMessage": "Good Morning"
        }
    ]
}

Query for Field in Array Object

Find documents where any student has the message "Good Morning" ?

db.objectInAnArrayDemo.find({
    "StudentDetails": { 
        $elemMatch: { 
            "StudentMessage": "Good Morning" 
        } 
    }
});

{
    "_id": ObjectId("5c92637936de59bd9de06382"),
    "StudentDetails": [
        {
            "StudentName": "Carol",
            "StudentMessage": "Hello"
        },
        {
            "StudentName": "David",
            "StudentMessage": "Good Morning"
        }
    ]
}

Conclusion

The $elemMatch operator is essential for querying nested objects within arrays. It matches documents where at least one array element satisfies the specified criteria, making it perfect for complex array queries.