#99

backend/app/models/material.rb

@@ -12,9 +12,17 @@ class Material < ApplicationRecord
 
   has_one_attached :file, dependent: :purge
 
+  validates :tag_id, presence: true, uniqueness: true
+
   validate :file_must_be_attached
   validate :tag_must_be_material_category
 
+  def content_type
+    return nil unless file&.attached?
+
+    file.blob.content_type
+  end
+
   private
 
   def file_must_be_attached
@@ -24,7 +32,7 @@ class Material < ApplicationRecord
   end
 
   def tag_must_be_material_category
-    return if tag.blank? || tag.material?
+    return if tag.blank? || tag.character? || tag.material?
 
     errors.add(:tag, '素材カテゴリのタグを指定してください．')
   end

backend/app/models/tag.rb

@@ -31,6 +31,7 @@ class Tag < ApplicationRecord
            class_name: 'TagSimilarity', foreign_key: :target_tag_id, dependent: :delete_all
 
   has_many :deerjikists, dependent: :delete_all
+  has_many :materials
 
   belongs_to :tag_name
   delegate :wiki_page, to: :tag_name