Scaffold actually provides a way to generate relationships, you should use the :references data type
rails g scaffold Comment body:string author:string post:references
This will generate a migration for the comments table with a post_id field and index for it. The generator will also add belongs_to :post to the Comment model.
It will not however generate the reverse side of the relationship so you’ll need to add
has_many :comments
to the Post model yourself. You will also need to add nested resource routing if this is something you need as the generator can not handle this.