Merge MongoDB collections

merge_mongo_collections.js

/*******************************************************************************
* Merges matching documents from source collection into desination collection 
* Usage:
*        mergeCollections("srcCollName", "destCollName", null, ["f1","f3"])
*        mergeCollections("srcCollName", "destCollName", {xy:"z"}, ["f1","f4"])       
********************************************************************************/
function mergeCollections(sourceCollection, destCollection, sourceQuery, setFields) {
  var nMatched = 0;
  var nModified = 0;
  sourceQuery = sourceQuery || {}
  db[sourceCollection].find(sourceQuery).forEach(function(sourceDoc){
    var updateKeyPairs = {};
    if (setFields) {
      setFields.forEach(function(field){
        updateKeyPairs[field] = sourceDoc[field];  
      });  
    }
    else {
      updateKeyPairs = sourceDoc;
    }
    var updateRes = db[destCollection].update(
                                { 
                                  _id: sourceDoc._id
                                },
                                {
                                  $set: updateKeyPairs
                                }
                              )
    nMatched += updateRes.nMatched;
    nModified += updateRes.nModified;
  });
  print("Matched:", nMatched, "; Modified:", nModified);
}