如何在 MongoDB 中创建“触发器”

Question

I would like of create a trigger where, to each subdocument inserted would increment in other collection a field, for generate a count of subdocuments that collection.

I tried create a search using MapReduce, but for Milions of the Registries is very slow.

Note: I use C#, but if you like show how to do in Bson, no problem.

Extructure my collection

public class Header
{
    public Header()
    {
        Operation= new List<Operation>();
    }

    public ObjectId Id { get; set; }
    public Int64 Code1 {get; set;}
    public Int64 Code2 {get; set;}
    public string Name { get; set; }
    public List<Operation> Operations { get; set; }
}

public class Operation
{
    public Operation()
    {
        Itens = new List<Item>();
    }

    public string Value { get; set; }
    public List<Item> Item { get; set; }
}

public class Item
{
    public string Value { get; set; }
}

Answer 1

MongoDB has no triggers. You will have to implement this in your application by inserting the document and when the insert was successful, you use the $add operator to increment the field in the other document.

Update: If you happen to rent a MongoDB Atlas instance from a service provider, then you can use triggers . But if you want to run MongoDB on your own servers, then this feature is not available.

Answer 2

MongoDB does in 2020 since July 2019, have triggers. https://docs.mongodb.com/stitch/triggers/database-triggers/

Answer 3

You can use change streams , specifically the collection.watch method available in drivers.

Database triggers from MongoB Atlas use these under the hood.

Answer 4

Script example using MongoDB Atlas Triggers :

exports = function(changeEvent) {
  const { updateDescription, fullDocument, ns } = changeEvent;
  const updatedFields = Object.keys(updateDescription.updatedFields);
  
  // For debug
  //console.log('changeEvent', JSON.stringify(changeEvent));

  const isUpdated = updatedFields.some(field =>
    field.match(/updatedAt/)
  );
  const updatedAt = fullDocument.updatedAt;

  // Prevent update again after the update
  if (!isUpdated || !updatedAt) {
    const { _id } = fullDocument;
    
    console.log(`Triggered! ${ns.db}:${ns.coll}:${_id}, isUpdated:${isUpdated ? 'true' : 'false'}, updatedAt:${updatedAt}`);
    
    const mongodb = context.services.get(ns.db /* Cluster Name, like the DB name */);
    const collection = mongodb.db(ns.db).collection(ns.coll);
    
    collection.updateOne({
      _id: _id,
    }, {
      $set: {
        updatedAt: new Date(),
      }
    });
  }
};

Source: https://stackoverflow.com/a/73310825/717267