How to filter array elements

问题

I have the following MongoDB document:

{
 _id: ObjectId(5), 
 items: [1,2,3,45,4,67,9,4]
}

I need to fetch that document with filtered items (1, 9, 4)

result:

{
 _id: ObjectId(5), 
 items: [1,9,4]
}

I've tried an $elemMatch projection but it returns only one item:

A.findById(ObjectId(5))
   .select({ items: { $elemMatch: {$in: [1, 9, 4]}}})
   .exec(function (err, docs) {
      console.log(doc); // { _id: ObjectId(5), items: [ 1 ] }
      done(err);
});

How can I get the document with items: 1, 9, 4 only?

回答1:

In modern versions of MongoDB (3.2+) you can use the $filter operator to select a subset of an array field to return based on a specified condition. Returned elements will be in the original order from the field array.

Example in the mongo shell:

db.items.aggregate([
    { $match : {
        _id: 5
    }},
    { $project: {
        items: {
            $filter: {
                input: "$items",
                cond: {
                    "$in": ["$$this", [1, 9, 4]]
                }
            }
        }
     }
}])

Note: because the original array in this question has the value 4 twice, the $filter command will return both occurrences:

{ "_id" : 5, "items" : [ 1, 4, 9, 4 ] }

For an alternative approach which will only return the unique matching items, the $setIntersection operator could be used:

db.items.aggregate([
    { $match : {
        _id: 5
    }},        
    { $project: {
        items: {
            $setIntersection: ['$items', [1,4,9]] 
        }
    }}
])

This will return: { "_id" : 5, "items" : [ 1, 4, 9 ] }.

(original answer from September, 2012 below)

If you want the document manipulation to happen on the server side, you can use the Aggregation Framework in MongoDB 2.2:

db.items.aggregate(

  // Match the document(s) of interest
  { $match : {
     _id: 5
  }},

  // Separate the items array into a stream of documents
  { $unwind : "$items" },

  // Filter the array
  { $match : {
    items: { $in: [1, 9, 4] }
  }},

  // Group the results back into a result document
  { $group : {
     _id: "$_id",
     items: { $addToSet : "$items" }
  }}
)

Result:

{
    "result" : [
        {
            "_id" : 5,
            "items" : [
                9,
                4,
                1
            ]
        }
    ],
    "ok" : 1
}

回答2:

A.items = A.items.filter( function(i) {return i == 1 || i == 9 || i == 4} );

回答3:

Use underscore in your node.js app:

npm install underscore

var _ = require('underscore');

You can use the intersection function of arrays:

 intersection_.intersection(*arrays)

Computes the list of values that are the intersection of all the arrays. Each value in the result is present in each of the arrays.

_.intersection([1, 2, 3], [101, 2, 1, 10], [2, 1]);
=> [1, 2]

http://documentcloud.github.com/underscore/#intersection

来源：https://stackoverflow.com/questions/12593998/how-to-filter-array-elements