How to index a collection by unique values?

Question

MongoDB

There is a collection, for example -
[
{userID:"111111", content:"text"}, {userID:"222222", content:"text"},
{userID:"111111", content:"text"}, { userID:"111111", content:"text"},
{userID:"222222", content:"text"}, {userID:"333333", content:"text"},
{userID:"333333", content: "text"}, {userID:"111111", content:"text"}
]
Elements with the same userID need to be indexed separately. Those. the result will be -
[
{userID:"111111", content:"text", index:0}, {userID:"222222", content:"text", index:0},
{userID:"111111", content: "text"
{userID:"333333", content:"text", index:1}, {userID:"111111", content:"text", index:3}
]
I know there is a .createIndex function, but how do I compose a filter/ configure it so that it indexes all userIDs separately?
You can, of course, use
.createIndex({userID:"111111"})
.createIndex({userID:"222222"})
.createIndex({userID:"333333"})
We will get the desired result, but there are about 2000 unique id in the collection and doing it this way is not the most rational solution.

Answer 1

The thing is that I need to get three randomly selected "content" values, for a certain userID

So you need a regular index, not a unique one.
And to choose N random documents in Monge there is a special. command: https://stackoverflow.com/questions/24806721/mongo...

Answer 2

Even I do not understand what version of mongo? The createIndex method is used to create an index. In your case it would look like this..createIndex("userID: 1")