Rails Migrations: Check Existence and Keep Going?

ruby-on-rails migration
Dan Rosenstark picture Dan Rosenstark · Oct 29, 2009 · Viewed 30.9k times · Source

I was doing this kind of thing in my migrations:

add_column :statuses, :hold_reason, :string rescue puts "column already added"

but it turns out that, while this works for SQLite, it does not work for PostgreSQL. It seems like if the add_column blows up, even if the Exception is caught, the transaction is dead and so the Migration can't do any additional work.

Is there any non-DB sepecific ways to check if a column or table already exist? Failing that, is there any way to get my rescue block to really work?

Answer

Tobias Cohen picture Tobias Cohen · May 11, 2011

As of Rails 3.0 and later, you can use column_exists? to check for the existance of a column.

unless column_exists? :statuses, :hold_reason
  add_column :statuses, :hold_reason, :string
end

There's also a table_exists? function, which goes as far back as Rails 2.1.

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