Display distinct list from MongoDB using c#

Question

I have the following document structure in mongoDb:

I want to query it from an asp.net C# app using linq. I would like a distinct list of all the t values. If I run this code I get a list of t values for all of my documents and the value of t is repeated over documents.

var query = from m in collection.AsQueryable()
                        select m.t;

I therefore want a distinct list of t. I amended my code to the following but nothing is returned and there is no error message.

var query = (from p in collection.AsQueryable()
                        select p.t).Distinct();

What am I doing wrong?

mickl · Accepted Answer · 2018-12-18 17:10:52Z

4

The thing is that Distinct() returns IQueryable<T> which represents the database operation. To materialize that you need to run .ToList(), the rest of your code is fine, try:

var query = (from p in collection.AsQueryable()
                    select p.t).Distinct();
var data = query.ToList();

answered Dec 18, 2018 at 17:10

mickl

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5 Comments

MrVoid Over a year ago

this answer is actually right, BsonDocument already implements IEquatable<BsonDocument>

Silentbob Over a year ago

@mickl - thanks. Is there anyway to speed it up as its been well over a minute and its still populating data. My db has over 3 million documents and there are approx 3000 distinct values of t.

mickl Over a year ago

@Silentbob do you have an index on "t" field ?

Silentbob Over a year ago

@mickl yes i do

mickl Over a year ago

@Silentbob then I suppose you need to cache this distinct list somehow, maybe in-memory, maybe in a separate collection that can be recalculated periodically

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