CSHARP-4141: Clustered Indexes for all Collections.

[JamesKovacs]

The API to create a clustered index is:

var options = new CreateCollectionOptions<Product>
{
    ClusteredIndex = new ClusteredIndexOptions<Product>
    {
        Key = Builders<Product>.IndexKeys.Ascending(x => x.Id),
        Unique = true,
        Name = "custom clustered index key name"
    }
};
db.CreateCollection("products", options);

The Key must be { _id: 1 } and Unique must be true at the moment, but the spec was written in such a way that the server can remove these requirements in the future and not require additional work by drivers. If you create a new ClusteredIndexOptions<T>(), these options will be defaulted automatically.

Ideally I wanted to support non-generic CreateCollectionsOptions.ClusteredIndex, but CreateCollectionOptions<TDocument> derives from CreateCollectionOptions. I couldn't think of a way to define a CreateIndexOptions.ClusteredIndex property with type ClusteredIndexOptions<BsonDocument> but then change the type of the property on the derived type to CreateCollectionOptions<TDocument>.ClusteredIndex to ClusteredIndexOptions<TDocument>. I opted to only support the ClusteredIndex property on CreateCollectionOptions<TDocument>. If anyone has ideas, I'm all ears.

[rstam]

I think the ClusteredIndex is being "rendered" at the wrong level here. I suggest:

var clusteredIndex = options.ClusteredIndex?.Render(documentSerializer, serializerRegistry);

This is similar to how the Validator is rendered just below.

[rstam]

And adding a Render method to ClusteredIndexOptions:

internal BsonDocument Render(IBsonSerializer<TDocument> documentSerializer, IBsonSerializerRegistry serializerRegistry)
{
    return new BsonDocument
    {
        { "key", _key.Render(documentSerializer, serializerRegistry) },
        { "unique", _unique },
        { "name", _name, _name != null }
    };
}

[rstam]

Note: lots of our Render methods are public. Perhaps this one should be also?

[JamesKovacs]

You're right. Moved. It's a grab bag whether our Render methods are internal or public. I'll stick with internal for now.

[rstam]

I think adding ClusteredIndexOptions<TDocument> to CreateCollectionOptions<TDocument> is fine.

The non-generic CreateCollectionOptions only exists for backward compatibility. If we were starting from a clean slate the non-generic base class would not exist and we would only have CreateCollectionOptions<TDocument>.

[rstam]

LGTM