如何在 Rails 中编写条件迁移? [英] How to write conditional migrations in rails?

问题描述

I am looking for ways to write migrations in rails that can be executed against the database many times without failing.

For instance let say I have this migration:

class AddUrlToProfile < ActiveRecord::Migration
  def self.up
    add_column :profile, :url, :string
  end

  def self.down
    remove_column :profile, :url
  end
end

If the url column already exists in the Profile table (if the schema.rb has been modified unexpectedly for instance), my migration will fail saying that it's a duplicate!

So how to execute this migration only if it has to?

Thanks

解决方案

You can do something like this:

class AddUrlToProfile < ActiveRecord::Migration
  def self.up
    Profile.reset_column_information
    add_column(:profile, :url, :string) unless Profile.column_names.include?('url')

  end

  def self.down
    Profile.reset_column_information
    remove_column(:profile, :url) if Profile.column_names.include?('url')
  end
end

This will reset the column information before it begins - making sure that the Profile model has the up-to-date column information from the actual table. It will then only add the column if it doesn't exist. The same thing happens for the down function, but it only removes the column if it exists.

If you have multiple use cases for this you could factor the code out into a function and re-use that in your migrations.

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