Rails:How to fix ActiveRecord::Irreversible Order Error

Question

I get the following error:

2.4.1 :001 > Blogger.last
ActiveRecord::IrreversibleOrderError: Relation has no current order and table has no primary key to be used as default order from (irb):1

What is the problem and how do I fix it?

Further information:

Blogger.first seems to work:

Blogger.first
Blogger Load (3.6ms)  SELECT  "bloggers".* FROM "bloggers" LIMIT $1  [["LIMIT", 1]]

#<Blogger id: 1, blogger_id: 502, deadline: nil, reminded: nil, needsreminder: nil, created_at: "2018-03-13 12:10:09", updated_at: "2018-03-13 12:10:09", interests: "Ethik, Gesellschaft, Kultur, Logik, Wissen, Wissen..."> 

Bloggers were created with a seed:

Blogger.create!([
  {id: 1, blogger_id: 502, deadline: nil, reminded: nil, needsreminder: nil, interests: "Ethik, Gesellschaft, Kultur, Logik, Wissen, Wissenschaft"},
  {id: 2, blogger_id: 453, deadline: nil, reminded: nil, needsreminder: nil, interests: "Ethik, Gesellschaft, Kultur"},
  {id: 3, blogger_id: 434, deadline: nil, reminded: nil, needsreminder: nil, interests: "Menschsein und Sprache"},
  {id: 4, blogger_id: 370, deadline: nil, reminded: nil, needsreminder: nil, interests: "Ethik, Gesellschaft, Kultur"},
  {id: 5, blogger_id: 485, deadline: nil, reminded: nil, needsreminder: nil, interests: "Unbekannt"}
])

These are the migrations for Blogger:

class CreateBloggers < ActiveRecord::Migration[5.1]
  def change
    create_table :bloggers do |t|
      t.integer :blogger_id
      t.date :deadline
      t.date :reminded
      t.boolean :needsreminder

      t.timestamps
    end
  end
end


class AddInterestsToBloggers < ActiveRecord::Migration[5.1]
  def change
    add_column :bloggers, :interests, :string
  end
end

Blogger.rb

class Blogger < ApplicationRecord


end

Rails: 5.1.4

Ruby: 2.4.1

Postgresql: PostgreSQL 9.5.12 on x86_64-pc-linux-gnu, compiled by gcc (Ubuntu 5.4.0-6ubuntu1~16.04.9) 5.4.0 20160609, 64-bit

Answer 1

I've just added primary_key and it works:

class MyModel < ApplicationRecord
  self.primary_key = :id
  # ...
end

Answer 2

I rolled back my migrations and then migrated them again.

rails db:rollback
rails db:migrate
rails db:seed

After that, the error does not occur again:

2.4.1 :002 > Blogger.last
  Blogger Load (1.5ms)  SELECT  "bloggers".* FROM "bloggers" ORDER BY "bloggers"."id" DESC LIMIT $1  [["LIMIT", 1]]
 => #<Blogger id: 5, blogger_id: 485, deadline: nil, reminded: nil, needsreminder: nil, created_at: "2018-03-13 19:58:33", updated_at: "2018-03-13 19:58:33", interests: "Unbekannt"> 
2.4.1 :003 > 

However, I'd still be happy if someone could explain what to do if this kind of error happens.