PG :: UndefinedColumn：错误：列comment.post_id不存在

Question

I cant seem to figure out why I keep getting this error. I am trying to add comment forms to a blog I am making and it wont work. Here is what the full error says.

ActiveRecord::StatementInvalid in Posts#show

Showing /Users/ipbyrne/FirstRailsApp/blog/app/views/posts/show.html.erb where line #17 raised:

PG::UndefinedColumn: ERROR: column comments.post_id does not exist LINE 1: SELECT COUNT( ) FROM "comments" WHERE "comments"."post_id" ... ^ : SELECT COUNT( ) FROM "comments" WHERE "comments"."post_id" = $1

</p>

   <div id="comments">
       <h2><%= @post.comments.count %> Comments</h2> <-- Says it breaks here
       <%= render @post.comments %>

       <h3>Add a comment:</h3>

From what i can understand there is a column missing in one of the tables in the schema.rb file? Incase this the case here is what mine looks like

# encoding: UTF-8
# This file is auto-generated from the current state of the database. Instead
# of editing this file, please use the migrations feature of Active Record to
# incrementally modify your database, and then regenerate this schema definition.
#
# Note that this schema.rb definition is the authoritative source for your
# database schema. If you need to create the application database on another
# system, you should be using db:schema:load, not running all the migrations
# from scratch. The latter is a flawed and unsustainable approach (the more migrations
# you'll amass, the slower it'll run and the greater likelihood for issues).
#
# It's strongly recommended that you check this file into your version control system.

ActiveRecord::Schema.define(version: 20140911230918) do

  create_table "comments", force: true do |t|
    t.string   "name"
    t.text     "body"
    t.integer  "post_id"
    t.datetime "created_at"
    t.datetime "updated_at"
  end

  add_index "comments", ["post_id"], name: "index_comments_on_post_id"

  create_table "posts", force: true do |t|
    t.string   "title"
    t.text     "body"
    t.datetime "created_at"
    t.datetime "updated_at"
  end

  create_table "users", force: true do |t|
    t.string   "email",                  default: "", null: false
    t.string   "encrypted_password",     default: "", null: false
    t.string   "reset_password_token"
    t.datetime "reset_password_sent_at"
    t.datetime "remember_created_at"
    t.integer  "sign_in_count",          default: 0,  null: false
    t.datetime "current_sign_in_at"
    t.datetime "last_sign_in_at"
    t.string   "current_sign_in_ip"
    t.string   "last_sign_in_ip"
    t.datetime "created_at"
    t.datetime "updated_at"
  end

  add_index "users", ["email"], name: "index_users_on_email", unique: true
  add_index "users", ["reset_password_token"], name: "index_users_on_reset_password_token", unique: true

end

Answer 1

You probably just need to add the column into your comments table.

You can do that by creating a migration via the terminal to add the column in:

$ rails g migration add_post_id_column_to_comments post:belongs_to
$ rake db:migrate

The reason you want to use post:belongs_to is that Rails will automatically append the _id suffix and create a foreign key in the comments table to refer back to each other.

So essentially this part of the migration post:belongs_to will add the column post_id to your comments table. (Same thing if you did for example cars:belongs_to , you would get cars_id , etc)

That way you can be able to refer to the post's comments like this:

@post.comments

The reason why your @post.comments is failing now is that it is looking for that post_id column that you have not made yet which is probably also because you may not have defined the relationship between your Post and Comment models.

---

If you haven't done so already you just need to quickly define the relationship in each model:

• A post has many comments.
• A comment belongs to a post.

Get the lingo?

In your Post model, just add

class Post < ActiveRecord::Base
    has_many :comments   # make sure it's pluralized
end

and in your Comment model, add

class Comment < ActiveRecord::Base
    belongs_to :post  # and this one is singularized
end

Then try running your app again. Let me know how it goes.

Answer 2

Yes, you're missing the field post_id on the Comment model, which would tell Rails which comments belong to which posts.

You can add it by generating a migration from the command line like so:

rails generate migration AddPostIdToComments post:references
rake db:migrate