Setting keyword field in elasticsearch

I upgraded Rails from 4 to 5.2 and have also updated elasticsearch from 2.4 to 6.8.

The following is the error I'm getting.

[400] {"error":{"root_cause":[{"type":"illegal_argument_exception","reason":"Fielddata is disabled on text fields by default. Set fielddata=true on [activity_type] in order to load fielddata in memory by uninverting the inverted index. Note that this can however use significant memory. Alternatively use a keyword field instead."}]

This is what the request* looks like:

activity_types=>{:terms=>{:field=>"activity_type"}}

Take a look at the activity_search.rb:

class ActivitySearch < Search::Model
  include DateSearch
  apply_date_filters
  apply_date_aggregations

  filters :activity_types, type: :terms, field: :activity_type, aggregate: false

  aggregates :activity_types, :subject_ids, :levels

  ....

Take a look at the model.rb:

module Search
  class Model
    def add_aggregation(name, opts={})
      self._aggregations = _aggregations.deep_merge({name.to_sym => opts})
      return if method_defined? "aggregate_#{name}" 
      define_method "aggregate_#{name}" do |config={}|
      type = config.delete(:type){ :terms }
      field = config.delete(:field){ name.to_s.singularize }
      Search::Aggregation.new(type, field, opts)
    end

    def response
      @response ||= Search::Response.new(search.response)
    end

    def request
      @request ||= build_request
    end

    def search
      @search ||= run_search
    end

    private

    def run_search
      search_class.search(request.params)
    end

    def search_class
      self.class.name.demodulize.gsub('Search', '').safe_constantize
    end

    def build_request
      request = Search::Request.new
      request.query(*queries)
      request.filter(*filters)
      request.aggregate(aggregations)
      request.sort(*sorts)
      request
    end
  end

The activity_search_facade.rb:

class Companies::ActivitySearchFacade < SimpleDelegator
  delegate :aggregations, to: :response
end

Can someone help solve this error?

Please note the elasticsearch-rails gem version is 6.0.0.

Let me know if you need any more code.

* How do I change this request so that it runs correctly with version 6?

UPDATE

Could it be something around this based on the error I'm getting?

curl -X GET 'localhost:9200/development_activities'

  "activity_type":{"type":"text" }

How do I change this to type: "keyword"? ^

So the activity model looked like:

class Activity < ApplicationRecord
  include SearchCommon

this is what search_common.rb looked like:

module SearchCommon
  extend ActiveSupport::Concern

  included do
    include Elasticsearch::Model

    after_commit on: [:create, :update] do
      __elasticsearch__.index_document
    end

    after_commit on: [:destroy] do
      __elasticsearch__.delete_document
    end

    index_name [Rails.env, model_name.collection].join('_')
  end
end

So I just added:

module SearchCommon
  extend ActiveSupport::Concern

  included do
    include Elasticsearch::Model

    after_commit on: [:create, :update] do
      __elasticsearch__.index_document
    end

    after_commit on: [:destroy] do
      __elasticsearch__.delete_document
    end

    mapping do # ADDITION 
     indexes :activity_type, type: 'keyword' # ADDITION
    end # ADDITION 

    index_name [Rails.env, model_name.collection].join('_')
  end
end