class MyDocuments
  include Mongoid::Document

  scope :text_search, ->(search) {
    where(
      {
        '$text' => { '$search' => search }
      }
    )
  }

  field :category, type: Symbol, default: nil

  field :body, type: String, default: ''

  index({
    body: 'text'
  })
end

::Mongoid::Tasks::Database.create_indexes

MyDocuments.delete_all

MyDocuments.create(category: :fruit, body: 'Apple - I love apples from Washington state')
MyDocuments.create(category: :fruit, body: 'Eggplant - Eggplants are batonically a fruit')

batonically = 'batonically'

puts "Criteria 1"
puts MyDocuments.where(category: :fruit).text_search(batonically).criteria.inspect
puts
puts "Criteria 2"
puts MyDocuments.text_search(batonically).where(category: :fruit).criteria.inspect
puts
puts "Criteria 3"
puts MyDocuments.where(category: :fruit).where('$text' => { '$search' => batonically }).criteria.inspect
puts
puts "Criteria 4"
puts MyDocuments.where('$text' => { '$search' => batonically }).where(category: :fruit).criteria.inspect

Criteria 1
#<Mongoid::Criteria
  selector: {:$text=>{:$search=>"batonically"}}
  options:  {}
  class:    MyDocuments
  embedded: false>

Criteria 2
#<Mongoid::Criteria
  selector: {"$text"=>{"$search"=>"batonically"}, "category"=>:fruit}
  options:  {}
  class:    MyDocuments
  embedded: false>

Criteria 3
#<Mongoid::Criteria
  selector: {"category"=>:fruit, "$text"=>{"$search"=>"batonically"}}
  options:  {}
  class:    MyDocuments
  embedded: false>

Criteria 4
#<Mongoid::Criteria
  selector: {"$text"=>{"$search"=>"batonically"}, "category"=>:fruit}
  options:  {}
  class:    MyDocuments
  embedded: false>