Removing duplicate records using MapReduce

Question

I'm using MongoDB and need to remove duplicate records. I have a listing collection that looks like so: (simplified)

[
  { "MlsId": "12345"" },
  { "MlsId": "12345" },
  { "MlsId": "23456" },
  { "MlsId": "23456" },
  { "MlsId": "0" },
  { "MlsId": "0" },
  { "MlsId": "" },
  { "MlsId": "" }
]

A listing is a duplicate if the MlsId is not "" or "0" and another listing has that same MlsId. So in the example above, the 2nd and 4th records would need to be removed.

How would I find all duplicate listings and remove them? I started looking at MapReduce but couldn't find an example that fit my case.

Here is what I have so far, but it doesn't check if the MlsId is "0" or "":

m = function () { 
    emit(this.MlsId, 1); 
} 

r = function (k, vals) { 
   return Array.sum(vals); 
} 

res = db.Listing.mapReduce(m,r); 
db[res.result].find({value: {$gt: 1}}); 
db[res.result].drop();

Answer 1

I have not used mongoDB but I have used mapreduce. I think you are on the right track in terms of the mapreduce functions. To exclude he 0 and empty strings, you can add a check in the map function itself.. something like

m = function () { 
  if(this.MlsId!=0 && this.MlsId!="") {    
    emit(this.MlsId, 1); 
  }
} 

And reduce should return key-value pairs. So it should be:

r = function(k, vals) {
  emit(k,Arrays.sum(vals);
}

After this, you should have a set of key-value pairs in output such that the key is MlsId and the value is the number of thimes this particular ID occurs. I am not sure about the db.drop() part. As you pointed out, it will most probably delete all MlsIds instead of removing only the duplicate ones. To get around this, maybe you can call drop() first and then recreate the MlsId once. Will that work for you?

Answer 2

In mongodb you can use a query to restrict documents that are passed in for mapping. You probably want to do that for the ones you don't care about. Then in the reduce function you can ignore the dups and only return one of the docs for each duplicate key.

I'm a little confused about your goal though. If you just want to find duplicates and remove all but one of them then you can just create a unique index on that field and use the dropDups option; the process of creating the index will drop duplicate docs. Keeping the index will ensure that it doesn't happen again.

http://www.mongodb.org/display/DOCS/Indexes#Indexes-DuplicateValues