ruby on rails - Reset PK number based on association -

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i have post , comments table.

post has many comments, , comment belongs post.

i want have primary keys start @ 1 when create comment post, can access comments in rest-ful manner, e.g:

/posts/1/comments/1 /posts/1/comments/2 /posts/2/comments/1 /posts/2/comments/2

how can achieve rails 3? using mysql database.

bonus: using sequel orm; approach compatible sequel, not activerecord, awesome.

well, can't use id this, id primary key here. can add field database table comment_number , make unique in scope of post:

#migration def change   add_column :comments, :comment_number, :integer, null: false   add_index :comments, [:post_id, :comment_number], unique: true end  #class class comment < activerecord::base   belongs_to :post    validates :post_id, presence: true   validates :comment_number, uniqueness: { scope: :post_id }  end

now in place need ensure column populated:

class comment < activerecord::base   #...    before_create :assign_comment_number    private    def assign_comment_number     self.comment_number = (self.class.max(:comment_number) || 0) + 1   end  end

last step tell rails use column instead of id. need override to_param method:

class comment < activerecord::base   #...   def to_param     comment_number   end end

update:

one more thing, useful make field read-only:

class comment < activerecord::base   attr_readonly :comment_id end

also after rethinking having uniqueness validation on comment_number makes little sense having assigned after validations run. should rid of , rely on database index.

even having validation, there still possible condition race. override save method handle constraint validation exception retry couple of time ensure won't break application flow. topic question.


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