Index time boost is deprecated and it is no longer supported by elasticsearch

[johnyoonh]

Found it the after a long testing
elastic/elasticsearch#4664
The test in the repo is most likely to fail. Here is more relevant information with a possible workaround:
https://stackoverflow.com/questions/25313611/elasticsearch-boost-deprecated-any-alternative

• Tested with the latest haystack-elasticsearch release
• Tested with the current haystack-elasticsearch master branch

Expected behaviour

The boost should affect the output score when the value is set in def prepare

Actual behaviour

No change in the search result. This works in ES 1.X with '_boost'

Steps to reproduce the behaviour

1. Add prepare
   def prepare(self, obj):
   data = super(ArticleIndex, self).prepare(obj)
   data['boost'] = obj.cached_hotness
   return data

Configuration

Typical

HAYSTACK_CONNECTIONS = {
"default": {
    "ENGINE": "haystack_elasticsearch.elasticsearch5.Elasticsearch5SearchEngine",
    "URL": "http://127.0.0.1:9200/",
    "INDEX_NAME": "haystack",
    "INCLUDE_SPELLING": True,
    },
}

• Operating system version: Ubuntu 16.04 LTS
• Search engine version: Lucene 6.6.1
• Python version: Python 2.7.12
• Django version: 1.10.8
• Haystack version: 5.6.7 (Also tried on 2.4.6)
• haystack-elasticsearch version: 0.1.0