Ruby on Rails: adding columns to existing database

问题:

I'm getting an error:

SQLite3::SQLException: no such column: ideas.list_id: 
SELECT "ideas".* FROM "ideas"  
WHERE "ideas"."list_id" = 2

But I added

t.integer :list_id

to my db migration file:

class CreateIdeas < ActiveRecord::Migration
  def change
    create_table :ideas do |t|
      t.string :name
      t.text :description
      t.string :picture

      t.timestamps
    end
    add_foreign_key :ideas, :lists
  end
end

which gave me this:

class CreateIdeas < ActiveRecord::Migration
  def change
    create_table :ideas do |t|
      t.string :name
      t.text :description
      t.string :picture
      t.integer :list_id
      t.timestamps
    end
    add_foreign_key :ideas, :lists
  end
end

and then I typed

rake db:migrate

Any idea why I would be getting an error saying there's no column? I'm still new to RoRs. Do I have to add a column some other way?

Thanks

回答1:

As Speransky suggested, you should never modify old migration files. Rather you should create a new migration that adds the desired column. For instance, in this case you would run the following command in your app to create the new migration:

rails generate migration AddListIdColumnToIdeas list_id:integer

And Rails would generate the migration file automatically and the only thing left to do is run rake db:migrate.

If you insist on modifying the old migration file, you can add the column as you did and run the following:

rake db:drop
rake db:create
rake db:migrate

Which will destroy your current database, create a new one and run all the migrations (which will include your new column).

回答2:

If you want to add a new column to an exist database, you should use rails generate migration. So you can try rails generate migration add_list_id_to_ideas list_id:integer and then use rake db:migrate to commit this change.

回答3:

You should not add new rows to old migrations. Migration is a step of building database. And number of last executed migration is stored in schema, and it will not be run or redone if you use will use rake db:migrate. If you run the migration with creating the table before, then you should create new migration where you may use add_column method.

回答4:

migration file name has the datetime encoded in its name so rails run this migration one and do not run it again unless you do a rollback

and here come the magic of migration to build you db with small steps so no need to update a migration after run rake db:migrate , you should make a new migration to do the change you want to your db schema

and remember to

remove the added line form the old migration file as it might raise errors if you decided to rollback this migration

回答5:

Rails 4.0 easy way to add single or multiple column https://gist.github.com/pyk/8569812

回答6:

You can also do this ..

rails g migration add_column_to_users list_id:string

then rake db:migrate

also add :list_id attribute in your user controller ;

for more detail check out http://guides.rubyonrails.org/active_record_migrations.html

回答7:

If you already have files in your migrate folder, you could just add column you want there(just type the code), delete development.sqlite or whatever represents your db file, and run rake db:migrate. It will then create a new sqlite file with new column in table, and you can check it in schema.rb

So, basically, everything you did seems good, except you didn't delete your database file. Doing this seems the easiest for me, all though you will lose all the files in your database. If you're just testing and developing Rails app, this works. Can anyone comment if there is something wrong with this approach, besides what i wrote?

Edit: I actually found an answer about that here Editing Existing Rails Migrations is a good idea?