MongoDB 3.2 now allows one to combine data from multiple collections into one through the $lookup aggregation stage. As a practical example, lets say that you have data about books split into two different collections. First collection, called books, having the following data: { “isbn”: “978-3-16-148410-0”, “title”: “Some cool book”, “author”: “John Doe” } { … Read more
This would be the easier way to do it using aggregate: db.contest.aggregate([ {“$group” : {_id:”$province”, count:{$sum:1}}} ])
MongoDB 2.2’s new $elemMatch projection operator provides another way to alter the returned document to contain only the first matched shapes element: db.test.find( {“shapes.color”: “red”}, {_id: 0, shapes: {$elemMatch: {color: “red”}}}); Returns: {“shapes” : [{“shape”: “circle”, “color”: “red”}]} In 2.2 you can also do this using the $ projection operator, where the $ in a … Read more
The best way to do this is in version 4.2+ which allows using the aggregation pipeline in the update document and the updateOne, updateMany, or update(deprecated in most if not all languages drivers) collection methods. MongoDB 4.2+ Version 4.2 also introduced the $set pipeline stage operator, which is an alias for $addFields. I will use … Read more