如何创建迁移以仅在索引存在时删除索引，而不是在不存在时抛出异常? [英] How to create a migration to remove an index only if it exists, rather than throwing an exception if it doesn't?

问题描述

现在，如果 books 表没有 created_at 或 updated_at 字段，当前迁移可能会失败:

Right now, the current migration might fail, if the books table doesn't have created_at or updated_at fields:

class AddTimestampIndexes < ActiveRecord::Migration
  def up
    remove_index :books, :created_at
    remove_index :books, :updated_at

    add_index  :books, :created_at
    add_index  :books, :updated_at
  end

  def down
    remove_index :books, :created_at
    remove_index :books, :updated_at
  end
end

如果 remove_index 无法删除索引而不是引发错误，它是否会采取任何静默方式继续进行?

Does remove_index take any options to silently proceed if it fails to remove the index rather than raising an error?

推荐答案

您可以在迁移中使用 index_exists? 方法来测试您需要删除的索引是否确实存在.

You can use the index_exists? method within your migration to test whether the index you need to remove is actually there.

看看这里的文档:http://apidock.com/rails/ActiveRecord/ConnectionAdapters/SchemaStatements/index_exists%3F

我没有测试过，但你应该可以使用这样的东西:

I've not tested it, but you should be able to use something like this:

class AddTimestampIndexes < ActiveRecord::Migration
  def up
    remove_index :books, :created_at if index_exists?(:books, :created_at)
    remove_index :books, :updated_at if index_exists?(:books, :updated_at)

    add_index  :books, :created_at
    add_index  :books, :updated_at
  end

  def down
    remove_index :books, :created_at
    remove_index :books, :updated_at
  end
end

虽然，从表面上看，如果它们不存在，您真的只想创建它们吗?这可能更适合您的迁移:

Although, by the looks of things, you really only want to create them if they don't exist? This might be more appropriate for your migration:

class AddTimestampIndexes < ActiveRecord::Migration
  def up
    add_index  :books, :created_at unless index_exists?(:books, :created_at)
    add_index  :books, :updated_at unless index_exists?(:books, :updated_at)
  end

  def down
    remove_index :books, :created_at
    remove_index :books, :updated_at
  end
end

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