Ruby finding duplicates in MongoDB - ruby

Question

I am struggling to get this working efficiently I think map reduce is the answer but can't getting anything working, I know it is probably a simple answer hopefully someone can help
Entry Model looks like this:
field :var_name, type: String
field :var_data, type: String
field :var_date, type: DateTime
field :external_id, type: Integer
If the external data source malfunctions we get duplicate data. One way to stop this was when consuming the results we check if a record with the same external_id already exists, as one we have already consumed. However this is slowing down the process a lot. The plan now is to check for duplicates once a day. So we are looking get a list of Entries with the same external_id. Which we can then sort and delete those no longer needed.
I have tried adapting the snippet from here https://coderwall.com/p/96dp8g/find-duplicate-documents-in-mongoid-with-map-reduce as shown below but get
failed with error 0: "exception: assertion src/mongo/db/commands/mr.cpp:480"
def find_duplicates
map = %Q{
function() {
emit(this.external_id, 1);
}
}
reduce = %Q{
function(key, values) {
return Array.sum(values);
}
}
Entry.all.map_reduce(map, reduce).out(inline: true).each do |entry|
puts entry["_id"] if entry["value"] != 1
end
end
Am I way off? Could anyone suggest a solution? I am using Mongiod, Rails 4.1.6 and Ruby 2.1

Answer 1

I got it working using the suggestion in the comments of the question by Stennie using the Aggregation framework. It looks like this:
results = Entry.collection.aggregate([
{ "$group" => {
_id: { "external_id" => "$external_id"},
recordIds: {"$addToSet" => "$_id" },
count: { "$sum" => 1 }
}},
{ "$match" => {
count: { "$gt" => 1 }
}}
])
I then loop through the results and delete any unnecessary entries.