add post

app/models/post.rb

@@ -0,0 +1,27 @@
+class Post < ApplicationRecord
+  belongs_to :user
+  belongs_to :quoted_post, optional: true, class_name: 'Post'
+
+  validates :content, length: { maximum: 777 }
+  validates :content, presence: true, if: :quoted_post?
+
+  def kind
+    if quoted_post?
+      :quoted_post
+    elsif repost?
+      :repost
+    else
+      :post
+    end
+  end
+
+  private
+
+  def quoted_post?
+    content.present? && quoted_post_id
+  end
+
+  def repost?
+    content.blank? && quoted_post_id
+  end
+end