How do I retroactively add a primary key to my table in rails?

问题

I've created a table without a primary key (:id => false), but now it has come back to bite my ass.

My app is already in production and I can't just drop it and recreate another one.

Is there a way to run a migration to add another auto increment primary key column to my table?

回答1:

The command to add a primary key in a migration is:

add_column :my_table, :id, :primary_key

However, the wording of your question suggests that your table already has an auto-increment column. Unless I'm mistaken, there are several DBMS that do not allow more than one auto-increment column on a table.

If you DO already have an auto-increment column and you actually want to use that column as your primary key, just add the following to your model:

set_primary_key "my_existing_column"

Or in more recent versions of Rails:

self.primary_key = "my_existing_column"

In the case that you already have an auto-increment column and you can't use that as the primary key, you may be out of luck.

回答2:

If for some reason you created a table with a custom id field, but forgot to set id as the primary key, you need to run a migration to create the primary key constraint. The following was tested against a PostgreSQL database:

class AddPrimaryKeyConstraintToFoobars < ActiveRecord::Migration
  def up
    execute "ALTER TABLE foobars ADD PRIMARY KEY (id);"
  end

  def down
    execute "ALTER TABLE foobars DROP CONSTRAINT foobars_pkey;"
  end
end

回答3:

I know in mySQl that you can add a column that has a default increment value. If you add that then each row will have a unique int value (and any new row will get a int value 1 greater then the last row added)

You could add this column and set it as a primary key.

回答4:

Was this a join table that now needs to become a real model with a primary key? If so, your best bet is to create the new table and copy the data into it.

来源：https://stackoverflow.com/questions/5328528/how-do-i-retroactively-add-a-primary-key-to-my-table-in-rails

标签

ruby-on-rails

migration

primary-key

key