Adding a column to an existing table in a Rails mi

问题:

I have a Users model which needs an :email column (I forgot to add that column during the initial scaffold).

I opened the migration file and added t.string :email, did rake db:migrate, and got a NoMethodError. Then I added the line

add_column :users, :email, :string

again rake db:migrate, again NoMethodError. Am I missing a step here?

Edit: here's the migration file.

class CreateUsers < ActiveRecord::Migration  
  def self.up  
    add_column :users, :email, :string  
    create_table :users do |t|  
      t.string :username  
      t.string :email  
      t.string :crypted_password  
      t.string :password_salt  
      t.string :persistence_token  

      t.timestamps  
    end  
  end  

  def self.down  
    drop_table :users  
  end  
end

回答1:

If you have already run your original migration (before editing it), then you need to generate a new migration (rails generate migration add_email_to_users email:string will do the trick). Then do a rake db:migrate and it'll run the new migration.

If you have not yet run the original migration you can just edit it, like you're trying to do. Your migration code is almost perfect: you just need to remove the add_column line completely (that code is trying to add a column to a table, before the table has been created, and your table creation code has already been updated to include a t.string :email anyway).

回答2:

Use this command at rails console

rails generate migration add_fieldname_to_tablename fieldname:string

and

rake db:migrate

to run this migration

回答3:

You can also do

rake db:rollback

if you have not added any data to the tables.Then edit the migration file by adding the email column to it and then call

rake db:migrate

This will work if you have rails 3.1 onwards installed in your system.

Much simpler way of doing it is change let the change in migration file be as it is. use

$rake db:migrate:redo

This will roll back the last migration and migrate it again.

回答4:

Sometimes rails generate migration add_email_to_users email:string produces a migration like this

class AddEmailToUsers < ActiveRecord::Migration[5.0]
  def change
  end
end

In that case you have to manually an add_column to change:

class AddEmailToUsers < ActiveRecord::Migration[5.0]
  def change
    add_column :users, :email, :string
  end
end

And then run rake db:migrate

回答5:

To add a column I just had to follow these steps :

1. rails generate migration add_fieldname_to_tablename fieldname:string

   Alternative

   rails generate migration addFieldnameToTablename

   Once the migration is generated, then edit the migration and define all the attributes you want that column added to have.

   Note: Table names in Rails are always plural (to match DB conventions). Example using one of the steps mentioned previously-

   rails generate migration addEmailToUsers

2. rake db:migrate

Or

1. You can change the schema in from db/schema.rb, Add the columns you want in the SQL query.

2. Run this command: rake db:schema:load

   Warning/Note

   Bear in mind that, running rake db:schema:load automatically wipes all data in your tables.

回答6:

When I've done this, rather than fiddling the original migration, I create a new one with just the add column in the up section and a drop column in the down section.

You can change the original and rerun it if you migrate down between, but in this case I think that's made a migration that won't work properly.

As currently posted, you're adding the column and then creating the table.

If you change the order it might work. Or, as you're modifying an existing migration, just add it to the create table instead of doing a separate add column.

回答7:

You can also force to table columns in table using force: true, if you table is already exist.

example:

ActiveRecord::Schema.define(version: 20080906171750) do
  create_table "authors", force: true do |t|
    t.string   "name"
    t.datetime "created_at"
    t.datetime "updated_at"
  end
end

回答8:

You could rollback the last migration by

rake db:rollback STEP=1

or rollback this specific migration by

rake db:migrate:down VERSION=<YYYYMMDDHHMMSS>

and edit the file, then run rake db:mirgate again.

回答9:

You can also do this .. rails g migration add_column_to_users email:string

then rake db:migrate also add :email attribute in your user controller ;

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