[NEST] GetMapping does not work like the _mapping API

[pickypg]

GetMapping is currently limited to having knowledge of the index and type in advance, which somewhat defeats the purpose of requesting the mapping in some dynamic cases.

From the documentation:

var mapping = this.ConnectedClient.GetMapping<ElasticSearchProject>();

This returns an IGetMappingResponse based on the request using details extracted from ElasticSearchProject, but it only handles the first index and type (if it managed to return more than one).

However, the _mapping REST API allows you to request the mapping for an optional list of indices and an optional list of types.

# All index mappings with all types
GET /_mapping
# Same as above
GET /_all/_mapping
# Prefix matches with all types
GET /logstash-*/_mapping
# Only the same as above, but only the logs and [made up] "special-logs" types
GET /logstash-*/_mapping/logs,special-logs
# Only indices that contain the specified type(s)
GET /_mapping/type

Each of those are valid requests, but the current NEST GetMapping request would at best handle the first index and its type, thus dropping information.

[pickypg]

Speaking with @gmarz about it, he suggested that it should probably work more similarly to the Multi-Get API.

[Mpdreamz]

Argh! this is a relic from pre 1.0 and should have been tackled as part of that release :(, totally forgot about it. Thanks for pointing it out again @pickypg !

I think we could support it rather easily without introducing breaking changes:

public interface IGetMappingResponse : IResponse
{
    // add this so we can expose all the mappings returned.
    IEnumerable<RootObjectMapping> Mappings { get; }
    //obsolete this or keep it (could be handy for cases when you are pretty
    // sure you are only getting back one mapping
    RootObjectMapping Mapping { get; }
    void Accept(IMappingVisitor visitor);
}

[gmarz]

@Mpdreamz sounds good to me, but I don't think we can do this easily without breaking the API.

In order to support multiple indices and types to match the spec, we'd have to change the base classes of GetMappingRequest and GetMappingDescriptor.

Also, we'll need a non-typed GetMappingDescriptor.