MongoDB C# - Index field in array

Question

With the following structure:

class A
{
    ...
    List<B> L;
}

class B
{
    ...
    string S;
}

and a collection of objects A, is it possible to create an index on the S fields?

Update: The solution proposed in the comments does NOT work:

public class A
{
    public List<B> L;
    public A()
    {
        L = new List<B>();
    }
}

public class B
{
    public string S;
}

and index creation:

MongoCollection.Indexes.CreateOne(new BsonDocument {{ "B.S", 1}});

yields:

only the _id field gets indexed, but B.S is not.

Possible duplicate of How to create MongoDB MultiKey index on attribute of items in an array .NET Driver — ProgrammingLlama
– ProgrammingLlama, Commented Mar 5, 2018 at 4:05
I saw this one, but I don't see how my question relates to multi-key — Thomas
– Thomas, Commented Mar 5, 2018 at 4:06
I assume you plan to ignore the fact that it shows you how to index an array item's value then?: await collection.Indexes.CreateOneAsync(new BsonDocument {{"name", 1}, {"bars.key", 1}}); — ProgrammingLlama
– ProgrammingLlama, Commented Mar 5, 2018 at 4:07
Or this also shows the same thing albeit for an older version. — ProgrammingLlama
– ProgrammingLlama, Commented Mar 5, 2018 at 4:14

ProgrammingLlama · Accepted Answer · 2018-03-05 05:44:45Z

2

The last time I checked (mid-late last year), a => a.L[-1].S syntax did not work to create an index, so I do not believe there is a LINQ way of achieving this at the moment.

In my own project, I'm using code similar to this:

myCollection.Indexes
    .CreateOne(Builders<A>.IndexKeys
        .Ascending("L.S"));

This will create an index using the array items.

answered Mar 5, 2018 at 5:44

ProgrammingLlama

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3 Comments

Thomas Over a year ago

Can you check that the index is really built? on my end when I look I don't see an index created on that collection (using Studio 3T)

ProgrammingLlama Over a year ago

You tried B.S, not L.S - B is your class name, not your property name.

ProgrammingLlama Over a year ago

I just found my own answer from a Google search whilst looking for something else.

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