How do I use the map function for a meteor collection?

Using a tutorial, we have a collection called Posts.

Posts.find() returns a cursor which lets me iterate over all the Posts.find().fetch() will give me an array of all the posts, but this might be a lot of data.

Suppose I just want one element of Posts, like the titles, in an array: I can do this:

titles=Posts.find().map(function(a) {return a.title}); // works

Suppose I want the title and the ownerIds. I was debugging this and did:

a=Posts.find()
titles=a.map((function(a) {return a.title;}); // works
ownerIds=a.map((function(a) {return a.ownerId;}); //doesn't work, cursor already iterated over, returns empty array.

This does not work. Why?

How do I use the map function for a meteor collection?

http://docs.meteor./#map

Using a tutorial, we have a collection called Posts.

Posts.find() returns a cursor which lets me iterate over all the Posts.find().fetch() will give me an array of all the posts, but this might be a lot of data.

Suppose I just want one element of Posts, like the titles, in an array: I can do this:

titles=Posts.find().map(function(a) {return a.title}); // works

Suppose I want the title and the ownerIds. I was debugging this and did:

a=Posts.find()
titles=a.map((function(a) {return a.title;}); // works
ownerIds=a.map((function(a) {return a.ownerId;}); //doesn't work, cursor already iterated over, returns empty array.

This does not work. Why?

• javascript
• meteor

• you could also return an object like this 'titles=a.map((function(a) {return {title: a.title,ownerId: a.ownerId}}); – Kumar Ravi Commented Jul 14, 2015 at 9:45

2 Answers 2

You can use a cursor more than once by calling rewind on it. From the docs:

The forEach, map, or fetch methods can only be called once on a cursor. To access the data in a cursor more than once, use rewind to reset the cursor.

So this should work:

a=Posts.find()
titles=a.map((function(a) {return a.title;});
a.rewind();
ownerIds=a.map((function(a) {return a.ownerId;});

ANS: The reason it doesn't work is that a cursor can only be iterated over once. That is, a cursor cannot be used the same an an array.

(Other people may already know this, but it took me a half hour of fruitless debugging until I figured it out.)