'like' or $regex query inside $cond in MongoDB - regex

Question

Please go through this question of mine:
MongoDB $group and explicit group formation with computed column
But this time, I need to compare strings, not numbers.The CASE query must have a LIKE:
CASE WHEN source LIKE '%Web%' THEN 'Web'
I then need to group by source. How to write this in Mongo? I am trying the following but not sure if $regex is supported inside $cond. By the way, is there a list of valid operators inside $cond somewhere? Looks like $cond isn't very fond of me :)
db.Twitter.aggregate(
{ $project: {
"_id":0,
"Source": {
$cond: [
{ $regex:['$source','/.* Android.*/'] },
'Android',
{ $cond: [
{ $eq: ['$source', 'web'] }, 'Web', 'Others'
] }
]
}
} }
);
There're many other values that I need to write in there, doing a deeper nesting. This is just an example with just 'Android' and 'Web' for the sake of brevity. I have tried both with $eq and $regex. Using $regex gives error of invalid operator whereas using $eq doesn't understand the regex expression and puts everything under 'Others'. If this is possible with regex, kindly let me know how to write it for case-insensitive match.
Thanks for any help :-)

Answer 1

Well, it still seems to be not even scheduled to be implemented :(
https://jira.mongodb.org/browse/SERVER-8892
I'm using 2.6 and took a peek on 3.0, but it's just not there.
There's one workaround though, if you can project your problem onto a stable substring. Then you can $substr the field and use multiple nested $cond. It's awkward, but it works.

Answer 2

Maybe you can try it with MapReduce.
var map = function()
{
var reg1=new RegExp("(Android)+");
var reg2=new RegExp("(web)+");
if (reg1.test(this.source)){
emit(this._id,'Android');
}
else if (reg2.test(this.source))
{
emit(this._id,'web');
}
}
var reduce = function (key,value){
var reduced = {
id:key,
source:value
}
return reduced;
}
db.Twitter.mapReduce(map,reduce,{out:'map_reduce_result'});
db.map_reduce_result.find();
You can use JavaScript regular expresions instead of MongoDB $regex.