Fix #4893: Add a FIELD_present parameter variation for text fields.

Usage:

* https://danbooru.donmai.us/wiki_pages.json?search[body_present]=true
* https://danbooru.donmai.us/wiki_pages.json?search[body_present]=false

app/logical/concerns/searchable.rb

@@ -307,6 +307,14 @@ module Searchable
       relation = relation.where(attr => params[attr])
     end
 
+    if params[:"#{attr}_present"].present? && params[:"#{attr}_present"].truthy?
+      relation = relation.where.not(attr => "")
+    end
+
+    if params[:"#{attr}_present"].present? && params[:"#{attr}_present"].falsy?
+      relation = relation.where(attr => "")
+    end
+
     if params[:"#{attr}_eq"].present?
       relation = relation.where(attr => params[:"#{attr}_eq"])
     end

test/unit/concerns/searchable.rb

@@ -82,6 +82,9 @@ class SearchableTest < ActiveSupport::TestCase
         assert_search_equals([@p3, @p2], source_not_like: "a*")
         assert_search_equals([@p3, @p2], source_not_ilike: "A*")
         assert_search_equals([@p3, @p2], source_not_regex: "^a.*")
+
+        assert_search_equals([], source_present: "false")
+        assert_search_equals([@p3, @p2, @p1], source_present: "true")
       end
 
       should "support multiple operators on the same attribute" do