¿Necesita un campo para la agregación en MongoDB?

Do you need an on field for aggregation in MongoDB?

The aggregation results documents must contain the field (s) specified in the on, unless the on field is the _id field. If the _id field is missing from a results document, MongoDB adds it automatically. The specified field or fields cannot contain a null or an array value.

For some collection with a field { wins: Number }, how could I use MongoDB Aggregation Framework to get the total number of wins across all documents in a collection? If I have 3 documents with wins: 5, wins: 8, wins: 12 respectively, how could I use MongoDB Aggregation Framework to return the total number, i.e. total: 25.

If documents to merge include the same field name, the field, in the resulting document, has the value from the last document merged for the field. Create a collection sales with the following documents: The following operation uses $mergeObjects as a accumulator in a $group stage that groups documents by the item field:

Grouping the documents by the item field, the following operation uses the $max accumulator to compute the maximum total amount and maximum quantity for each group of documents. A collection students contains the following documents:

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: And the second collection, called books_selling_data, having the following data:

mergeObjects overwrites the field values as it merges the documents. If documents to merge include the same field name, the field, in the resulting document, has the value from the last document merged for the field. Create a collection orders with the following documents: Create another collection items with the following documents:

It has its own operator, we can get the field of the current document by $ symbol + field name. To understand the MongoDB group by multiple fields first, let’s have a look at a list of all operators that can be used in $ group: $ sum – Returns the sum of all numeric fields. $ avg – Calculates the average between numeric fields.