Return primary form

main.rb

@@ -26,6 +26,7 @@ get '/words/:id' do
     word_id = Integer params[:id]
 
     json({
+      primary_form: context.primary_form(word_id),
       part_of_speech: context.part_of_speech_name(word_id),
       examples: context.examples(word_id),
     })
@@ -67,6 +68,21 @@ private
 
 public
 
+  ##
+  # @return [String, nil]
+  #
+  def primary_form(word_id)
+    word_id = Integer word_id
+
+    column = @db_conn.exec_params(
+      'SELECT primary_form FROM words WHERE id = $1',
+      [word_id],
+    ).values.first&.first
+
+    str = String(column).strip.freeze
+    str unless str.empty?
+  end
+
   ##
   # @return [String, nil]
   #