选择不同的 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 ?

Answer 1

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();

Answer 2

这项工作适合我

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

Answer 3

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.

Answer 4

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();

Answer 5

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();

Answer 6

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.