How do I perform the SQL Join equivalent in MongoDB

Question

How do I use MongoDB to perform the SQL Join equivalent?

If you have two collections (users and comments), let's suppose that I want to retrieve all the comments with pid=444 and the user information for each.

comments
  { uid:12345, pid:444, comment="blah" }
  { uid:12345, pid:888, comment="asdf" }
  { uid:99999, pid:444, comment="qwer" }

users
  { uid:12345, name:"john" }
  { uid:99999, name:"mia"  }

Is it possible to gather all the comments with a specific field (like...find(pid:444)) and the user details connected to each remark at once?

I now gather the comments that meet my requirements first, then I identify all the uids in that result set, obtain the user objects, and combine them with the results of the comments. Things seem like I'm doing it incorrectly.

Answer 1

The majority of the responses to this question are incorrect as of Mongo 3.2. A left outer join and the new $lookup operator that was added to the aggregate pipeline are nearly identical:

https://docs.mongodb.org/master/reference/operator/aggregation/lookup/#pipe._S_lookup

From the docs:

{
   $lookup:
     {
       from: <collection to join>,
       localField: <field from the input documents>,
       foreignField: <field from the documents of the "from" collection>,
       as: <output array field>
     }
}

Of course, Mongo is not a relational database, and the developers are cautious in their recommendations for $lookup's use cases, but at least as of version 3.2, joining is now supported by MongoDB.

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