term_parser.rb

require 'parslet'

module TermParser
  # This is a simple parser that matches a sequence of non-whitespace characters
  # and converts it to an Elasticsearch match query.
  class QueryParser < Parslet::Parser
    rule(:term) { match('[^\s]').repeat(1).as(:term) }
    rule(:space) { match('\s').repeat(1) }
    rule(:query) { (term >> space.maybe).repeat.as(:query) }
    root(:query)
  end

  class QueryTransformer < Parslet::Transform
    rule(:term => simple(:term)) { term.to_s }
    rule(:query => sequence(:terms)) { Query.new(terms) }
  end

  # A query represented by a list of parsed user terms
  class Query
    attr_accessor :terms

    def initialize(terms)
      self.terms = terms
    end

    def to_elasticsearch
      {
        :query => {
          :match => {
            :title => {
              :query => terms.join(" "),
              :operator => "or"
            }
          }
        }
      }
    end
  end
end