Add search_meta key param support to ES_Query - see #17

[tlovett1]

Should close #17 . This PR allows you to pass "search_meta" to ES_Query that supports an array of post meta keys to be searched. @AaronHolbrook #reviewmerge

[tlovett1]

I don't think a filter makes sense. Filters are difficult to use. The purpose of the query object is to be able to customize which posts your receive. Empowering the user with the query object is much more valuable than forcing them to use a filter.

Regarding using meta_query, I'm undecided. The intention of meta_query is different from that of search_meta. meta_query let's you specify keys and values. search_meta does not let you specify a value but rather relies on 's'. If you created a WP_Query object with arguments 's' and 'meta_query', the meta query part would not use 's'. The intention of my pull request is to allow 's' to be used in a meta query thus necessitating a new argument, search_meta.

[AaronHolbrook]

Instead of defining values in the meta_query you could simply define keys / terms to include.

I was referring to this for the filter instead of an action prior to the search request firing: 3357674#classes-class-ep-api-php-P4

I don't think it makes sense to only offer an action in that spot, why provide the ability to execute functionality but not modify what is about to be executed by the plugin? I think the filter makes more sense in this case. Also - not trying to force anyone to use a filter, simply providing a way to modify behavior in unforeseen and unplanned ways.

[tlovett1]

I 100% agree on only adding an action. The only reason I added that was to make unit testing easier. I'm totally open to adding a filter in another PR.

[tlovett1]

@AaronHolbrook anymore issues with this? Would be nice to get this merged

[AaronHolbrook]

Any concerns over whether or not we should sanitize the $key of the meta field being passed in?