Let’s see how to use elemMatch aggregate in MongoDB in this article. Bobcares, as a part of our Server Management Service offers solutions to every query that comes our way.

Using elemMatch Aggregate in MongoDB

The $elemMatch operator in MongoDB is a powerful tool for querying documents that contain arrays and retrieving specific elements from those arrays based on specified criteria. When we have documents with arrays, sometimes we need to find elements within those arrays that meet certain conditions. This is where we use $elemMatch:

Usage: $elemMatch filters the contents of an array field in query results. It ensures that only the first array element that matches a specific condition is included in the query result.

Returned Element: It works similarly to the $ operator, both projecting the first matching array element from each document based on a condition. However, $elemMatch offers more explicit condition arguments. This is useful when projecting based on multiple fields in array documents or conditions not present in the query itself.

Field Order: One notable feature of $elemMatch is that regardless of the field order in the document, the projected field appears after other existing field inclusions. This means that even if the field is listed before others in the document, it will appear after them in the projection results.

Version Consideration: In MongoDB 4.2 and earlier versions, $elemMatch projection of an existing field maintains the ordering in the document.

Restrictions: There are a couple of restrictions to be aware of. $elemMatch projection is not supported in find() operations on views, and we cannot use a $text query expression within $elemMatch.

Example

Let’s say we have a collection called players with documents containing an array field called games. We can use $elemMatch to project only the games where the score is greater than 5. Even if games appears before other fields like joined and lastLogin in the document, it will appear after them in the projection results.

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Conclusion

In summary, the article offers a detailed explanation of using the elemMatch in aggregation with MongoDB.