Ruby on Rails - Create a migration in ruby on rail- ruby on rails tutorial - rails guides - rails tutorial - ruby rails

Add/remove fields in existing tables Add/remove fields in existing tables

Create a migration by running:

rails generate migration AddTitleToCategories title:string

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This will create a migration that adds a title column to a categories table:

class AddTitleToCategories < ActiveRecord::Migration[5.0]
  def change
    add_column :categories, :title, :string
  end
end

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Similarly, you can generate a migration to remove a column: rails generate migration RemoveTitleFromCategories title:string This will create a migration that removes a title column from the categories table:

class RemoveTitleFromCategories < ActiveRecord::Migration[5.0]
  def change
    remove_column :categories, :title, :string
  end
end

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While, strictly speaking, specifying type (:string in this case) is not necessary for removing a column, it's helpful, since it provides the information necessary for rolling it back.

Create a table

Create a migration by running:

rails g CreateUsers name bio

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Rails detects the intent to create a join table by finding JoinTable in migration name. Everything else is determined from the names of the fields you give after the name.

class CreateJoinTableParticipation < ActiveRecord::Migration
  def change
    create_join_table :users, :groups do |t|
      # t.index [:user_id, :group_id]
      # t.index [:group_id, :user_id]
    end
  end
end

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Uncomment the necessary index statements and delete the rest.

Precedence

Notice that the example migration name CreateJoinTableParticipation matches the rule for table creation: it has a Create prefix. But it did not generate a simple create_table. This is because migration generator (source code) uses a first match of the following list: