MongoDB Aggregate $project

Question

I store our web server logs in MongoDB and the schema looks similar to as follows:

[
  {  
    "_id" : 12345,
    "url" : "http://www.mydomain.com/xyz/abc.html",
    ....
  },
  ....
]

I am trying to use the $project operator to reshape this schema a little bit before I start passing my collection through an aggregation pipeline. Basically, I need to add a new field called "type" that will later be used to perform group-by. The logic for the new field is pretty simple.

if "url" contains "pattern_A" then set "type" = "sales lead";
else if "url" contains "pattern_B" then set "type" = "existing client";
...

I'm thinking it would have to be something like this:

db.weblog.aggregate(
  { 
    $project : {
      type : { /* how to implement the logic??? */ }
    }
  }
);

I know how to do this using map-reduce (by setting the "keyf" attribute to a custom JS function that implements the above logic) but am now trying to use the new aggregation framework to do this. I tried to implement the logic using the expression operators but so far couldn't get it to work. Any help/suggestion would be greatly appreciated!

Answer 1

I am sharing my "solution" in case others encounter the same needs like mine.

After researching for a couple of weeks, as @asya-kamsky suggested in one of his comments, I've decided to add a computed field to my original MongoDB schema. It's not ideal because whenever the logic for the computed field changes I would have to do bulk updates to update all documents in my collection but it was either that or rewrite my code to use MapReduce. I chose the former for now. In looking at MongoDB Jira board, it would appear that many people have asked for more diverse operators to be added for the $project operator and I certainly hope that the MongoDB dev team gets around to adding them sooner than later

Operator for splitting string based on a separator.

New projection operator $elemMatch

Allow $slice operator in $project

add a $inOrder operator to $project