ElasticSearch boolean operators issue: OR behaves like AND if NOT is presented

• Tested with the latest Haystack release
• Tested with the current Haystack master branch

Expected behaviour

SearchQuerySet().filter(~SQ(field='value') | SQ(field='value'))

Should return all index records

Actual behaviour

Returns 0 records

Investigation

The specified code generates the next elasticsearch query:

{
    "query": {
        "query_string" : {
            "query" : "(NOT (field:(value)) OR field:(value))"
            ...
        }
    }
}

I found if to enclose NOT expression in parentheses ((NOT (filed:(value))) OR filed:(value)) the issue will be resolved.

• Operating system version: Ubuntu 16.04
• Search engine version: ElasticSearch 2.4
• Python version: 2.7.12
• Django version: 1.8.17
• Haystack version: 2.6.0

Start by reproducing the SearchQuerySet filter expression against Elasticsearch 2.4 and inspect the generated query string shown in the issue. Trace the query-building entry point responsible for NOT and OR grouping; the issue is done when the expression returns all index records and a regression test covers this case.