Add the ability to define how models are mapped to hits.

[AaronRustad]

This PR addresses #421

When searching against aliases, results from Elasticsearch are returned with the actual _index name rather than the alias name. The current implementation of Multiple Model Search uses the _index to determine which model to use. Unfortunately, when working with an aliased index, this mapping fails.

I don't think there is a 'correct' way to get around this, other than to give the user of the library the option of setting the logic for determining how to map indices to aliased models.

This PR gives the option of overriding the default model to hit selection code.

For example, in production, I'll use a timestamped index like `content-20150515164511'.

# INITIALIZER
Elasticsearch::Model.model_to_hit_selector = lambda do |model,hit|
  /#{model.index_name}-.*/ =~ hit[:_index] && model.document_type == hit[:_type]
end

[AaronRustad]

Is there any interest in seeing this merged... I know I am! ;-)

[hongsheng-razoo]

I am facing the exact same problem ... can we merge this solution?

[karmi]

Hi, ugh, this is one of the PRs which has slipped by... I was concentrated on other things for a long strecth of time, and in the last weeks was more focused on the Ruby client (preparing for the 5.0 release and similar).

I like the idea of a lambda, @AaronRustad, you know I do :) I'll look into this today or tomorrow, please bug me until we sort this out. Sorry to put the strain on you, but it would help me...

[gareth]

This error has just bitten us too, I'm definitely interested in getting a solution merged

[AaronRustad]

Lol, I haven't bugged you for a while @karmi .... I've since moved on and I'm not sure there is any interest in this any more. Shall we close it?

[ryansch]

We just hit our heads on this while shipping a bunch of stuff into production.

@karmi The default behaviour should allow the index name followed by anything to match.

Elasticsearch::Model.model_to_hit_selector = lambda do |model, hit|
  /#{model.index_name}.*/ =~ hit[:_index] && model.document_type == hit[:_type]
end