MongoDB查询结果转换：将find结果精简为ObjectId数组

Hey there! To convert your query results into an array containing only the ObjectIds, you can use one of these straightforward approaches:

Using Cursor.map() (Client-Side Transformation) #

The find() method returns a cursor, which has a built-in map() function to transform each document. This is simple and works well for most cases:

// Get array of ObjectId instances
db.abhishek.em.find({}).map(doc => doc._id)

If you're using an older MongoDB shell version that doesn't support arrow functions, use the traditional function syntax:

db.abhishek.em.find({}).map(function(doc) {
  return doc._id;
})

Output Example: #

[ObjectId("5ac62d35b075e574b3e7eeaa"), ObjectId("5ac62d3fb075e574b3e7eeab"), ObjectId("5ac62d4eb075e574b3e7eeac")]

If you need the string representations of the ObjectIds instead, modify the map function:

db.abhishek.em.find({}).map(doc => doc._id.toString())

String Output Example: #

["5ac62d35b075e574b3e7eeaa", "5ac62d3fb075e574b3e7eeab", "5ac62d4eb075e574b3e7eeac"]

Using Aggregation (Database-Side Transformation) #

For larger datasets, processing the transformation on the database server can reduce data transfer to the client. Use the aggregation pipeline:

db.abhishek.em.aggregate([
  { $project: { _id: 1 } }, // Keep only the _id field
  { $group: { _id: null, ids: { $push: "$_id" } } }, // Collect all _ids into an array
  { $project: { _id: 0, ids: 1 } } // Return just the ids array
]).toArray()[0].ids

This will give you the same array of ObjectIds as the cursor.map() method, but the work happens on the database side.

内容的提问来源于stack exchange，提问作者Abhishek Mathur