```ruby
require_relative 'query_modifier'

module ConceptQL
  module QueryModifiers
    class DrugQueryModifier < QueryModifier
      attr :db

      def initialize(*args)
        super
        @db = query.db
      end

      def modified_query
        return query unless op.domain == :drug_exposure
        query.from_self(alias: :de)
          .left_join(micro_table.as(:mt), mt__drug_concept_id: :de__drug_concept_id)
          .select_all(:de)
          .select_append(:mt__amount_value___drug_amount)
          .select_append(:mt__amount_unit___drug_amount_units)
          .select_append(:mt__drug_name___drug_name)
          .from_self
      end

      private

      def micro_table
        # TODO: Does drug_strength only have RXNORM concept_ids?
        # TODO: What is vocabulary for units?  Can we shrink concept table to just that vocab before joining?
        db.from(:drug_strength___ds)
          .join(:concept___dc, ds__drug_concept_id: :dc__concept_id)
          .select(
            :ds__drug_concept_id___drug_concept_id,
            :ds__amount_value___amount_value,
            :ds__amount_unit___amount_unit,
            :dc__concept_name___drug_name,
          )
      end
    end
  end
end
```

Fix bug when domain not present on an operator
user
```ruby
require_relative 'query_modifier'
require 'facets/kernel/try'

module ConceptQL
  module QueryModifiers
    class DrugQueryModifier < QueryModifier
      attr :db

      def initialize(*args)
        super
        @db = query.db
      end

      def modified_query
        return query unless domain == :drug_exposure
        query.from_self(alias: :de)
          .left_join(micro_table.as(:mt), mt__drug_concept_id: :de__drug_concept_id)
          .select_all(:de)
          .select_append(:mt__amount_value___drug_amount)
          .select_append(:mt__amount_unit___drug_amount_units)
          .select_append(:mt__drug_name___drug_name)
          .from_self
      end

      private

      def domain
        op.try(:domain) rescue nil
      end

      def micro_table
        # TODO: Does drug_strength only have RXNORM concept_ids?
        # TODO: What is vocabulary for units?  Can we shrink concept table to just that vocab before joining?
        db.from(:drug_strength___ds)
          .join(:concept___dc, ds__drug_concept_id: :dc__concept_id)
          .select(
            :ds__drug_concept_id___drug_concept_id,
            :ds__amount_value___amount_value,
            :ds__amount_unit___amount_unit,
            :dc__concept_name___drug_name,
          )
      end
    end
  end
end
```
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