"uninitialized constant" error during migration

Question

I'm a newbie in Ruby on Rails and I want to generate a mysql database using migrations.

I tried this command

ruby bin/rake db:drop db:create db:migrate --trace

but I always get this error:

    rake aborted!
    StandardError: An error has occurred, all later migrations canceled:

    uninitialized constant CreateOrganisation::Referential
C:/Sites/blog/db/migrate/20120823053740_create_organisation.rb:13:in `up'
    C:/RailsInstaller/Ruby2.0.0/lib/ruby/gems/2.0.0/gems/activerecord-4.1.8/lib/active_record/migration.rb:598:in `exec_migration'
    C:/RailsInstaller/Ruby2.0.0/lib/ruby/gems/2.0.0/gems/activerecord-4.1.8/lib/active_record/migration.rb:579:in `block (2 levels) in migrate'
    C:/RailsInstaller/Ruby2.0.0/lib/ruby/2.0.0/benchmark.rb:281:in `measure'
    C:/RailsInstaller/Ruby2.0.0/lib/ruby/gems/2.0.0/gems/activerecord-4.1.8/lib/active_record/migration.rb:578:in `block in migrate'
    C:/RailsInstaller/Ruby2.0.0/lib/ruby/gems/2.0.0/gems/activerecord-4.1.8/lib/active_record/connection_adapters/abstract/connection_pool.rb:294:in `with
    _connection'
    C:/RailsInstaller/Ruby2.0.0/lib/ruby/gems/2.0.0/gems/activerecord-4.1.8/lib/active_record/migration.rb:577:in `migrate'
    C:in `migrate'
    C:/RailsInstaller/Ruby2.0.0/lib/ruby/gems/2.0.0/gems/activerecord-4.1.8/lib/active_record/migration.rb:991:in `block in execute_migration_in_transacti
    on'
    C:/RailsInstaller/Ruby2.0.0/lib/ruby/gems/2.0.0/gems/activerecord-4.1.8/lib/active_record/migration.rb:1039:in `ddl_transaction'
    C:/RailsInstaller/Ruby2.0.0/lib/ruby/gems/2.0.0/gems/activerecord-4.1.8/lib/active_record/migration.rb:990:in `execute_migration_in_transaction'
    C:/RailsInstaller/Ruby2.0.0/lib/ruby/gems/2.0.0/gems/activerecord-4.1.8/lib/active_record/migration.rb:952:in `block in migrate'
    C:/RailsInstaller/Ruby2.0.0/lib/ruby/gems/2.0.0/gems/activerecord-4.1.8/lib/active_record/migration.rb:948:in `each'
    C:/RailsInstaller/Ruby2.0.0/lib/ruby/gems/2.0.0/gems/activerecord-4.1.8/lib/active_record/migration.rb:948:in `migrate'
    C:/RailsInstaller/Ruby2.0.0/lib/ruby/gems/2.0.0/gems/activerecord-4.1.8/lib/active_record/migration.rb:807:in `up'
    C:/RailsInstaller/Ruby2.0.0/lib/ruby/gems/2.0.0/gems/activerecord-4.1.8/lib/active_record/migration.rb:785:in `migrate'
    C:/RailsInstaller/Ruby2.0.0/lib/ruby/gems/2.0.0/gems/activerecord-4.1.8/lib/active_record/railties/databases.rake:34:in `block (2 levels) in <top (req
    uired)>'
    C:/RailsInstaller/Ruby2.0.0/lib/ruby/gems/2.0.0/gems/rake-10.4.2/lib/rake/task.rb:240:in `call'
    C:/RailsInstaller/Ruby2.0.0/lib/ruby/gems/2.0.0/gems/rake-10.4.2/lib/rake/task.rb:240:in `block in execute'
    C:/RailsInstaller/Ruby2.0.0/lib/ruby/gems/2.0.0/gems/rake-10.4.2/lib/rake/task.rb:235:in `each'
    C:/RailsInstaller/Ruby2.0.0/lib/ruby/gems/2.0.0/gems/rake-10.4.2/lib/rake/task.rb:235:in `execute'
    C:/RailsInstaller/Ruby2.0.0/lib/ruby/gems/2.0.0/gems/rake-10.4.2/lib/rake/task.rb:179:in `block in invoke_with_call_chain'
    C:/RailsInstaller/Ruby2.0.0/lib/ruby/2.0.0/monitor.rb:211:in `mon_synchronize'
    C:/RailsInstaller/Ruby2.0.0/lib/ruby/gems/2.0.0/gems/rake-10.4.2/lib/rake/task.rb:172:in `invoke_with_call_chain'
    C:/RailsInstaller/Ruby2.0.0/lib/ruby/gems/2.0.0/gems/rake-10.4.2/lib/rake/task.rb:165:in `invoke'
    C:/RailsInstaller/Ruby2.0.0/lib/ruby/gems/2.0.0/gems/rake-10.4.2/lib/rake/application.rb:150:in `invoke_task'
    C:/RailsInstaller/Ruby2.0.0/lib/ruby/gems/2.0.0/gems/rake-10.4.2/lib/rake/application.rb:106:in `block (2 levels) in top_level'
    C:/RailsInstaller/Ruby2.0.0/lib/ruby/gems/2.0.0/gems/rake-10.4.2/lib/rake/application.rb:106:in `each'
    C:/RailsInstaller/Ruby2.0.0/lib/ruby/gems/2.0.0/gems/rake-10.4.2/lib/rake/application.rb:106:in `block in top_level'
    C:/RailsInstaller/Ruby2.0.0/lib/ruby/gems/2.0.0/gems/rake-10.4.2/lib/rake/application.rb:115:in `run_with_threads'
    C:/RailsInstaller/Ruby2.0.0/lib/ruby/gems/2.0.0/gems/rake-10.4.2/lib/rake/application.rb:100:in `top_level'
    C:/RailsInstaller/Ruby2.0.0/lib/ruby/gems/2.0.0/gems/rake-10.4.2/lib/rake/application.rb:78:in `block in run'
    C:/RailsInstaller/Ruby2.0.0/lib/ruby/gems/2.0.0/gems/rake-10.4.2/lib/rake/application.rb:176:in `standard_exception_handling'
    C:/RailsInstaller/Ruby2.0.0/lib/ruby/gems/2.0.0/gems/rake-10.4.2/lib/rake/application.rb:75:in `run'
    bin/rake:4:in `<main>'
    Tasks: TOP => db:migrate

I tried the commands:

rake db:rollback
rake db:reset
rake db:schema:dump  
rake db:schema:load

but in vain.

The file that causes the error (I guess) is:

class CreateOrganisation < ActiveRecord::Migration
  def up
    create_table :organisations do |t|
      t.string :name
      t.timestamps
    end
    change_table :referentials do |n|
      n.belongs_to :organisation
    end
    change_table :users do |u|
      u.belongs_to :organisation
    end
    Referential.reset_column_information
    User.reset_column_information

    organisation = Organisation.find_or_create_by_name!("Chouette")
    Referential.update_all :organisation_id => organisation.id
    User.update_all :organisation_id => organisation.id
  end

  def down
    drop_table :organisations
  end
end

I tried putting def change..end instead of def up..end/def down..end but I still get the same error

I can't see what I'm missing here.

Answer 1

You're trying to change the table referential and it doesn't exist:

change_table :referentials do |n|
  n.belongs_to :organisation
end

The error refers to an uninitialized constant ":referentials", which refers to that particular table.

Consider using a generator to create your migrations until you understand how they work; you can always tweak the generated migrations or write your own from scratch later.

Example

rails generate migration CreateFoo name:string amount:integer owner:string:index

And that will create

class CreateFoo < ActiveRecord::Migration
  def change
    create_table :foos do |t|
      t.string :name
      t.integer :amount
      t.string :owner
    end
    add_index :foos, :owner
  end
end

Reference

I suggest you read the Rails Migration Guide, which should help you understand how to use migrations.