select distinct mongodb C#

Question

I have to select distinct records from my simple mongo db database. I have many simple records these records looks like this :

{"word":"some text"}

My code is very simple.

    const string connectionString = "mongodb://localhost";
    var client = new MongoClient(connectionString);

    MongoServer server = client.GetServer();
    MongoDatabase database = server.GetDatabase("text8");
    MongoCollection<Element> collection = database.GetCollection<Element>("text8");
    MongoCursor<Element> words = (MongoCursor<Element>)collection.FindAll();

But I have't idea how to select distinct word's from database. Could someone can give me some advice ?

Do you want to split your records into separate words and return these distinct words or simply return distinct values for word key? — Shad
– Shad, Commented Nov 3, 2013 at 20:03
No, my intention id to select unrepeatable words from collection. — Konrad
– Konrad, Commented Nov 4, 2013 at 13:40

Shad · Accepted Answer · 2016-11-02 08:35:25Z

12

MongoDB API has a distinct aggregation command, which returns distinct values found for a specified key in a collection. You can also use it from C# Driver:

var distinctWords = collection.Distinct("word");

where collection - is an instance from your example. This query will return all distinct values of word field in the collection.

Also, as @JohnnyHK mentioned in comment, you can use linq approach, since it is supported by C# driver:

var distinctWords = collection.AsQueryable<Element>().Select(e => e.Word).Distinct();

edited Nov 2, 2016 at 8:35

answered Nov 3, 2013 at 19:59

Shad

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2 Comments

JohnnyHK Over a year ago

The LINQ interface provided by the C# driver executes its queries via MongoDB, so it's not retrieving all records and then filtering them in code and is typically just as efficient.

Shad Over a year ago

@JohnnyHK, thanks for your correction. I actually thought about support of linq by C# driver but wasn't entirely sure. I've corrected answer.

D__ · Accepted Answer · 2020-01-17 16:09:02Z

3

this work´s for me

Collection.Distinct<string>("ColumnNameForDistinct", FilterDefinition<T>.Empty).ToListAsync()

answered Jan 17, 2020 at 16:09

D__

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Comments

Hylaean · Accepted Answer · 2013-11-03 17:52:03Z

1

My guess would be to make "word" an index on this db. Then using some linq to query it in a simple expression: var res = col.Query().Select(e => e.word).Distinct();

This would result in reading all words from the index.

answered Nov 3, 2013 at 17:52

Hylaean

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Comments

Stephen Rauch · Accepted Answer · 2018-06-13 01:37:40Z

1

The MongoCollection.Distinct Method (String) V2.0 is Legacy

for new version API like 2.4 use:

FieldDefinition<yueyun.land,string> field = "FirstName";
var bx = _yueyunlands.Distinct<string>(field, Builders<yueyun.land>.Filter.Empty).ToList();

edited Jun 13, 2018 at 1:37

Stephen Rauch♦

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answered Jun 13, 2018 at 1:14

billschen

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Comments

Taylan · Accepted Answer · 2019-09-16 11:49:08Z

1

If you want to filter first and get distinct afterwards and also do all of these at MongoDB side, you can use the following example. In this example I applied a filter, got distinct values and finally got count:

        var filter = Builders<Logs>.Filter.Ne(x => x.Id, null);
        var count = collection.Distinct(x => x.Id, filter).ToList().Count();

answered Sep 16, 2019 at 11:49

Taylan

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Comments

WiredPrairie · Accepted Answer · 2013-11-03 20:17:12Z

MongoDB doesn't have a built in operator to split a string of words from a query as there's not a way to split a string, then run a "distinct" operation on it.

One option would be to create a MapReduce and do the split in the MapReduce code and count each word. You can't do this with just C# code.

A second, and possibly simpler option would be to pre-split the field into words so that you could use one of the distinct operators:

{ "word": [ "some", "text"] }

Then:

dbCollection.Distinct("word");

This would of course work if you just want to treat the entire string as a "word" rather than each word separately . MapReduce's aren't real-time ... the pseudo-code would be:

map = function() {
  var splits = this.word.split(' ');
  for(var i = 0, l = splits.length; i < l; i++) {
     emit(splits[i], 1);
  }
}

reduce = function(word, vals) {
   var count = 0;
   for(var i=0, l=vals.length; i < l; i++) {
       count += vals[i];
   }
   return count;
}

When you run the MapReduce, it would be a collection of the number of occurrences of each word.

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