Retrieve all elements in an array in mongoDB

Question

How can I assign an array to a variable. I am able to return the documents that I want but not able to specifically get all the elements in the array inside a particular field. This field is called 'likes.'

My query to filter documents I want:

Posts.find({}, {owner: Meteor.user()})

I would like to retrieve all the elements inside an array field called 'likes' from the Posts collection. (Each element in the 'likes' field is an object ID)

I tried with various operators such as $all and $in of the 'likes' field and tested within console.log, but I cant manage to get the Ids. According to the mongo docs, I need to specify an element within the operator but I don't want that. I simply want anything that is inside.

var likers = Posts.find({}, {owner: Meteor.user()}, {likes: {$in: [] }})

Basically, Im trying to retrieve all the elements from the 'likes' field so I can make another query to return results.

I am running meteor 0.9+

David Weldon · Accepted Answer · 2014-10-09 17:00:31Z

5

Give this a try:

var myPosts = Posts.find(
  {owner: Meteor.user()},
  {fields: {likes: 1}}
).fetch()

var myLikes = _.chain(myPosts)
  .pluck('likes')
  .flatten()
  .uniq()
  .value();

First we fetch all of the posts where the current user is the owner. Each post will contain an _id and a likes array. Next we extract all of the likes by:

Plucking out each likes from each document. This gets us an array of arrays.
Flatten the array of arrays into a single array of likes.
Run uniq so we have only the unique likes (not necessary but could be an optimization).

You can then use myLikes in a subsequent query. For example:

Likes.find({_id: {$in: myLikes}})

Here is some test data in case myPosts is empty:

var myPosts = [
  {_id: 1, likes: [1, 2, 3]},
  {_id: 2, likes: [2, 3, 4]},
  {_id: 3, likes: [4, 5, 6]}
];

In this case myLikes will be: [ 1, 2, 3, 4, 5, 6 ]

edited Oct 9, 2014 at 17:00

answered Oct 9, 2014 at 16:17

David Weldon

64.4k11 gold badges152 silver badges146 bronze badges

Sign up to request clarification or add additional context in comments.

3 Comments

meteorBuzz Over a year ago

Hi David, I have tried this. After putting the myLikes variable into the console.log and into another query, there is no value returned.

David Weldon Over a year ago

It may be that myPosts is an empty array for you because the documents don't exist or they were not published. Assuming I got the right data structure, the myLikes code is correct. I added some test data to the bottom of my answer to verify.

Shingai Munyuki Over a year ago

@DavidWeldon THANKS! I have been searching for this for months!

Hubert OG · Accepted Answer · 2014-10-09 14:59:34Z

1

You can specify which fields you want your cursor to contain, so you can limit it to the likes field:

var allLikes = Posts.find({
  owner: Meteor.user()
}, {
  fields: {likes: 1},
}).fetch();

This will give you an array of all posts, where each post contain only the likes array:

[{
  likes: ['A', 'B', 'C'],
}, {
  likes: ['D', 'E'],
}];

etc. Now, I understand you want a single array with all the objectIDs listed there. For this you can transform the array with underscore. There are numerous ways to obtain the result, for example:

var likes = _.reduce(allLikes, function(memo, post) {
  return memo.concat(post.likes);
}, []);

answered Oct 9, 2014 at 14:59

Hubert OG

19.6k7 gold badges48 silver badges73 bronze badges

2 Comments

meteorBuzz Over a year ago

Each post will only have one array. Inside it is a list of object ids. So the underscore method is not necessary but thanks for pointing it out I can keep in mind for future. From a single array such as the one you wrote ['A', 'B', 'C'] I would like this in a variable AllLikes. Then, query another collection Forms.find({_id: allLikes});

meteorBuzz Over a year ago

Essentially, I am trying to retrieve all the values inside the array of the 'likes' field. Then use these values to query another collection.

meteorBuzz · Accepted Answer · 2014-10-09 22:34:21Z

0

I just realised, in the Posts.find query, I had to insert an empty criteria for it to work. So the change resulted to the following.

var myPosts = Posts.find(
{},
{owner: Meteor.user()},
{fields: {likes: 1}}
).fetch()

Either way, I could not have come to this result without your input so plus one for you David!

edited Oct 9, 2014 at 22:34

answered Oct 9, 2014 at 22:29

meteorBuzz

3,2105 gold badges37 silver badges62 bronze badges

Collectives™ on Stack Overflow

Retrieve all elements in an array in mongoDB

3 Answers 3

3 Comments

2 Comments

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

3 Answers 3

3 Comments

2 Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Related