Ruby-on-rails – Rails has_and_belongs_to_many migration

has-and-belongs-to-manymigrationruby-on-railsruby-on-rails-3

I have two models restaurant and user that I want to perform a has_and_belongs_to_many relationship.

I have already gone into the model files and added the has_and_belongs_to_many :restaurants and has_and_belongs_to_many :users

I assume at this point I should be able to do something like with Rails 3:

rails generate migration ....

but everything I have tried seems to fail. I'm sure this is something really simple I'm new to rails so I'm still learning.

Best Answer

You need to add a separate join table with only a restaurant_id and user_id (no primary key), in alphabetical order.

First run your migrations, then edit the generated migration file.

Rails 3

rails g migration create_restaurants_users_table

Rails 4:

rails g migration create_restaurants_users

Rails 5

rails g migration CreateJoinTableRestaurantUser restaurants users

From the docs:

There is also a generator which will produce join tables if JoinTable is part of the name:

Your migration file (note the :id => false; it's what prevents the creation of a primary key):

Rails 3

class CreateRestaurantsUsers < ActiveRecord::Migration
  def self.up
    create_table :restaurants_users, :id => false do |t|
        t.references :restaurant
        t.references :user
    end
    add_index :restaurants_users, [:restaurant_id, :user_id]
    add_index :restaurants_users, :user_id
  end

  def self.down
    drop_table :restaurants_users
  end
end

Rails 4

class CreateRestaurantsUsers < ActiveRecord::Migration
  def change
    create_table :restaurants_users, id: false do |t|
      t.belongs_to :restaurant
      t.belongs_to :user
    end
  end
end

t.belongs_to will automatically create the necessary indices. def change will auto detect a forward or rollback migration, no need for up/down.

Rails 5

create_join_table :restaurants, :users do |t|
  t.index [:restaurant_id, :user_id]
end

Note: There is also an option for a custom table name that can be passed as a parameter to create_join_table called table_name. From the docs

By default, the name of the join table comes from the union of the first two arguments provided to create_join_table, in alphabetical order. To customize the name of the table, provide a :table_name option:

Related Solutions

Ruby-on-rails – Can Rails Routing Helpers (i.e. themodel_path(model)) be Used in Models

In Rails 3 and higher:

Rails.application.routes.url_helpers

e.g.

Rails.application.routes.url_helpers.posts_path
Rails.application.routes.url_helpers.posts_url(:host => "example.com")

Ruby-on-rails – Rails: How to rename a database column in a Ruby on Rails migration

rename_column :table, :old_column, :new_column

You'll probably want to create a separate migration to do this. (Rename FixColumnName as you will.):

script/generate migration FixColumnName
# creates  db/migrate/xxxxxxxxxx_fix_column_name.rb

Then edit the migration to do your will:

# db/migrate/xxxxxxxxxx_fix_column_name.rb
class FixColumnName < ActiveRecord::Migration
  def self.up
    rename_column :table_name, :old_column, :new_column
  end

  def self.down
    # rename back if you need or do something else or do nothing
  end
end

For Rails 3.1 use:

While, the up and down methods still apply, Rails 3.1 receives a change method that "knows how to migrate your database and reverse it when the migration is rolled back without the need to write a separate down method".

See "Active Record Migrations" for more information.

rails g migration FixColumnName

class FixColumnName < ActiveRecord::Migration
  def change
    rename_column :table_name, :old_column, :new_column
  end
end

If you happen to have a whole bunch of columns to rename, or something that would have required repeating the table name over and over again:

rename_column :table_name, :old_column1, :new_column1
rename_column :table_name, :old_column2, :new_column2
...

You could use change_table to keep things a little neater:

class FixColumnNames < ActiveRecord::Migration
  def change
    change_table :table_name do |t|
      t.rename :old_column1, :new_column1
      t.rename :old_column2, :new_column2
      ...
    end
  end
end

Then just db:migrate as usual or however you go about your business.

For Rails 4:

While creating a Migration for renaming a column, Rails 4 generates a change method instead of up and down as mentioned in the above section. The generated change method is:

$ > rails g migration ChangeColumnName

which will create a migration file similar to:

class ChangeColumnName < ActiveRecord::Migration
  def change
    rename_column :table_name, :old_column, :new_column
  end
end

Best Answer

Related Solutions

Ruby-on-rails – Can Rails Routing Helpers (i.e. themodel_path(model)) be Used in Models

Ruby-on-rails – Rails: How to rename a database column in a Ruby on Rails migration

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